Term |
Short Definition |
References |
Alarm |
Signal, device, or software that alerts users of an important condition, event, or emergency condition. |
http://www.indiamart.com/ |
Aliasing Error |
Occurs when a signal is digitized at less than twice its highest frequency component. Chemical sensors are often low frequency devices, but it is important to know their bandwidth and the bandwidth of the system they are embedded into. It is possible for high frequency noise from power supplies, pump diaphragms, or other sources to fold back into the sensor signal greatly degrading detector performance. |
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Algorithm |
The group of procedures including logical testing and mathematical operations used to extract information from a sensor signal or reading. In modern times often encoded in microcomputer software but might be as simple as a recipe for estimating the value of a variable from a series of readings. |
http://www.ll.mit.edu/publications |
Alpha |
Parameter for Type I error in statistics or misdetection in a chemical detector. |
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Ammonia Slip Monitor |
Refers to a monitor used in the Selective Catalytic Reduction (SCR) technique to monitor excess ammonia exhausted by the stack. |
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Analog to Digital (A/D) Converter |
An electronic device used to convert an analog signal to a stream of digital data. Choosing the right A/D converter characteristics including analog signal range, resolution, and speed for the converter is critical to the success of the overall chemical detector system. |
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Anti-aliasing Filter |
An analog filter placed in front of an analog-to-digital converter to ensure that frequencies above the Nyquist frequency (1/2 the sampling frequency) are greatly attenuated. This should avoid the problems associated with aliasing errors. |
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Area Monitor |
Chemical detectors designed to monitor a specific area or areas in a plant (if using multipoint sampling) usually for worker health and safety, explosive vapors, etc. |
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Automatic Control System |
A system for controlling chemical, power or other plant processes using a computer. Historically the computer may have been mechanical or an electronic analog computer but modern systems use a digital process control computer. When designing a chemical sensor system for a particular plant design it is helpful to know something about the control strategy and process control system being used. Rarely in a plant are detector signals simply displayed remotely with no control system involved. |
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Averaging |
The sum of a number of readings divided by the total. In statistics, the average assumes random noise around a mean. In many cases this is a very poor assumption, especially in real time signal processing, and a simple average gives grossly misleading results. A better approach is to apply a digital signal processing algorithm but this assumes some knowledge of the underlying measurement process. |
Term |
Short Definition |
References |
Background |
Background is the chemical matrix in which the chemical detector is expected to operate less the target chemical. The Minimum Level of Detection can be determined, for example, by showing statistically that the detector can differentiate between the target chemical and background. In order to determine the Minimum Limit of Detection a number of background samples and a number of low concentration samples are analyzed in order to calculate these statistics. |
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Back flush |
In chromatography, back flushing is a technique for cleaning up columns of residual material by reversing the carrier flow. Back flush techniques extend to other detectors where contaminant material can "hang up" in the sampling system or detection cell causing hysteresis and reversing the carrier flow can be part of a strategy for ridding the detector of the material. |
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Band Pass Filter |
Analog or digital filter implemented in hardware or software that passes a band of frequencies non attenuated while attenuating both lower and higher frequencies. |
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Band Stop Filter |
A filter that attenuates a band of frequencies. One application, common in chemical instruments, is the need to suppress noise from a switching power supply which provides power to the unit but may introduce noise at specific frequencies due to its design. |
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Bandwidth |
The width of frequency spectrum required to accurately represent the sensor or detector signal. Many chemical sensors are low frequency devices but spectrometers can have a large bandwidth. |
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Baseline |
The detector output with no target chemical present. Can compare sensor or detector readings against the baseline to get Minimum Level of Detection. |
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Barometer |
Many chemical sensors have output signals dependent on atmospheric or sample gas pressure. A barometer, pressure, or vacuum sensor is often required to correct the signal. This is a particular problem when dealing with an atmospheric monitor given changes in atmospheric pressure over the day. |
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Basic Language |
A computer language popular in the 1970’s that gave rise to Visual Basic, a Microsoft product, that is used to implement Proportional Integration Derivative (PID) control systems and signal data processing. |
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Batch |
A group of samples processed together rather than continuously in real-time. |
http://prosensus.ca/solutions |
Beta (Electronics) |
Current gain in a junction (PNP NPN) transistor circuit which is approximately linear. |
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Beta (Statistics) |
Parameter for Type II -false negative error in statistics or false alarm in chemical detectors. |
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Beta Alumina |
A material sometimes used as a solid electrolyte in gas sensors. |
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Black Box |
An electronic device with only the input and output specified and their relation to one another (transfer function). The internal workings of the black box may change arbitrarily as long as the external parameters remain the same. |
Signal Processing for Intelligent |
Bode Plot |
A Bode plot is a graph of the transfer function (which is a complex variable and is represented in the polar coordinates magnitude and phase) of an electronic amplifier illustrating both the amplitude and phase shift as a function of frequency. Many chemical sensors produce Gaussian peaks, for example, and the detection information (the information in the signal) is contained in the amplitude (used as a surogate for area) and relative time of occurrence of the center of the peak. It is important to have a zero phase shift in the amplifier from direct current (DC), or zero frequency, through the pass band so that peaks are not distorted in shape making these measurements difficult or impossible. Since this can be hard to approximate in hardware filter, digital filters implemented in software have greatly improved instrument accuracy. |
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Bourdon Tube |
Measures pressure based on the tendency of a tube to straighten out with increasing pressure. |
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Bimetallic switch |
A temperature sensitive switch that uses dissimilar metals with different coefficients of expansion. Commonly used in thermostatic controls for laboratory and field equipment. |
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Blocking |
Blocking is a disabling event that occurs when an interfering gas overwhelms a detector making it insensitive to the target gas. For example in an instrument that detects ions as the result of ion-molecule reactions a chemical component that steals all the ionic charge making the instrument insensitive but is not the desired target. |
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Boxcar Averaging |
Averaging the output from a Gated Integrator (see below) over many cycles to reduce noise. A risky business since it assumes random variations around a mean. |
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Bus |
A computer input/output design where all of the peripheral devices including memory and input/output devices are connected to the central processing unit through a common group of wires called a bus. |
Term |
Short Definition |
References |
Calibration |
Challenging a chemical detector with a known standard and setting the zero point and span or other detector parameters to maintain an accurate readout of chemical concentration. Calibration can be a significant cost driver for customers who need to maintain a device in the field. No or rare calibration required is highly desirable. |
http://www.owlstonenanotech.com/ |
Conducting Polymer Chemicl Sensors |
Conductive Polymer chemical sensors respond to a variety of gases including H2S and NO2. |
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Corona Discharge |
A corona discharge, often formed from a sharp needle and a ring, can be used to generate ions for mass spectrometry or ion mobility spectrometry (IMS) detectors. It is very difficult to design a corona discharge source for ambient air because it will become unstable and break down into a spark burning up the needle electrode. It is possible to use a pulsed corona for atmospheric pressure ionization. |
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Catalytic Bead |
Catalytic beads consist of a metal oxide bead formed around a platinum wire. Catalytic bead detectors are used in detecting explosive gases. The metal oxide making up the catalytic bead causes oxidation of the explosive gas at lower temperature than normal which heats up the platinum wire changing its resistance which is read out with a Wheatstone Bridge. |
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Chemical Monitor |
A device for monitoring chemical presence and/or concentration in a variety of applications. A chemical monitor continuously monitors one chemical. Unlike an analyzer it is for quantitative not qualitative measurements. |
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Chemometrics |
Using signal processing and complex computer algorithms to accurately measure chemicals in a background matrix. Frequently uses pattern recognition, Least Squares Analysis, Principle Components Analysis, and other complex statistical techniques. |
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Chemiresistive Sensor |
Chemical sensors based on metal oxides that change their resistance with exposure to the target chemical. Tin Oxide and Zinc Oxide are common. |
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Chopper |
A mechanical device consisting of a wheel with wedges removed from it which breaks up light from a light source as it is rotated in front of a light source (usually infrared) projected into a gas cell in older infrared instruments. The result of using a chopper is a pulse response from the detector which, until recently, was more easily processed by the electronics. Choppers are used in non-dispersive IR instruments and in photo acoustic spectroscopy (PAS). |
http://www.cambustion.com |
Chopper Amplifier |
Until recently it was difficult to amplify direct current in a stable very high gain amplifier. It was easier to use capacitive coupling and amplify an alternating current (AC) signal. A chopper amplifier relied on a mechanical chopper to turn the detector into an alternating current source and then amplify the AC signal. |
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Class and Division |
ANSI(NFPA) method of classifying hazardous environments such as would be found in a chemical process plant. Detector equipment must be enclosed in boxes that are appropriate for the Class and Division required by the plant environment. |
http://en.wikipedia.org/wiki/ |
Closed Loop System |
|
http://en.wikipedia.org/wiki |
Column |
Refers to a gas (or liquid) chromatograph column which is used to separate chemical components from a mixture prior to detection. Individual components elute from the column at a known time after injection. Using a column as a "front end" the chemical detector can more reliably detect target chemicals in a complex or unpredictable background. This is often referred to as a two dimensional analysis or detection because the chromatograph separation provides one axis of a two dimensional plot and the detector output spectra or signal provides the other dimension. They are orthogonal if the two dimensions are uncorrelated. Thus a much more powerful detection process can be provided. One good example of this approach in the field is the transportable GC-MS used by the military in reconnaissance vehicles. |
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Color wheel |
A color wheel is sometimes used with a spectrophotometer for colorimetric identification of compounds. |
http://www.lagoonsonline.com |
Colorimetric Detector |
Detecting a chemical through a colorimetric change. Can be as simple as a sensitive paper which changes color or a complex automatic detector system. One of the earliest chemical warfare detectors consisted of M8 paper attached to the end of a stick. |
http://www.inl.gov/ |
Cone Penetrometer |
A long rod that is pushed into the ground at a suspected contamination site to analyze soil in order to detect the presence of soil or groundwater contamination. Chemical as well as radiation sensors are frequently installed inside the tip of a cone penetrometer. |
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Continuous Emissions Monitor (CEM) |
A chemical measuring device designed to measure specific chemicals in an effluent stream such as exhaust from a power plant. The device is required in order for the plant owner to obtain a license to operate and is usually required to meet strict accuracy and reliability standards. |
http://www.epa.gov/airmarkets/ |
Term |
Short Definition |
References |
Deconvolution |
The effect of measuring a chemical “signal” is to modify the signal because of the limited bandwidth and gain of the measuring instrument. This process is modeled mathematically by the convolution of the chemical signal with an instrument function which is turn is a model of the detection process. A deconvolution algorithm may be used to restore the ideal chemical signal. Deconvolution may be accomplished with real time hardware and software or offline after the data has been captured. Offline deconvolution software is available in order to enhance the resolution of chromatography peaks for example. |
Deconvolution of overlapping chromatographic peaks, Richard F. Lacey |
Detector Flooding |
Some detectors may lose their ability to respond to a chemical challenge when an overwhelming amount of the target chemical is present. The response of the human nose to hydrogen sulfide is a great example although many detectors exhibit this problem. |
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Detector Saturation |
Most detectors will lose their ability to accurately measure chemicals above a certain value often specified by the span of the detector which is the response region. Especially in a health and safety application it is important to know how the detector will respond to very high concentrations above its specified maximum reading. |
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Detector Sensitivity |
Smallest change from one reading to another reading that the chemical detector can detect, which may be expressed as a percentage of the range, not to be confused with Minimum Level of Detection which is the lowest concentration of target chemical that the detector can detect. |
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Detection Algorithm |
A predefined mathematical method of analyzing chemical detector data to determine the presence and concentration of a target chemical. In modern times algorithms are most frequently implemented as microprocessor software. |
http://ieeexplore.ieee.org/ |
Digital Filter |
A signal processing filter used to process signal after the signal from a detector has been digitized. At frequencies typically used in chemical instruments the digital filter can easily be implemented in microprocessor or lap top software. |
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Digization Error |
Sensor signal or data error caused either by not having enough digital bits to accurately represent the signal (quantization error) or if the signal has frequency components more than one half the sampling frequency (Nyquist error). |
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Distributed Control System |
A control system consisting of a hierarchy of controllers where some of the control algorithms may be implemented by satellite controllers with a supervisory controller interfacing with the operator. |
http://www.slideshare.net |
Drift |
The tendency of a chemical sensor to depart or drift away from its accurate measurement of the chemical over time. This may be caused by changing environmental variables such as temperature or humidity or simply some instability in the sensor. |
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Drift tube |
The separation portion of an Ion Mobility Spectrometer (IMS) where clouds of ions undergo collisions with neutrals and separate themselves out. |
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Dynamic Error |
Error caused by the inability of the chemical detector to track the target chemical as it varies in time. If you are measuring the amount of target chemical in stack exhaust, dynamic error is the difference between the actual concentration and the chemical detector output in real time. |
http://en.wikipedia.org/wiki/ |
Dynamic versus Static |
Chemical detector response to a changing chemical concentration over time as opposed to static or steady state measurements. Since it is most easy to measure static response extra care must be taken to measure dynamic response and not just assume it. |
Term |
Short Definition |
References |
Evanescent wave detector |
A fiber optic chemical sensor base on a quantum mechanical principle that allows the detection of chemicals outside of a fiber by measuring the refraction of light in the fiber. Special case of a fiber optic detector. |
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Electrochemical cells |
Detects chemicals diffusing into a liquid electrolyte by their electrochemical potential. Sometimes referred to as a starved fuel cell. Very common gas and vapor detector that must expose vapor to a liquid electrolyte. |
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Electro spray Ionization |
Method of ionizing large macromolecules for mass spectrometry analysis while minimizing fragmentation. Somewhat similar to the geometry of a coronal discharge source but squirts the sample dissolved in a solvent out the end of a hallow electrode into an electric field. The solvent evaporates leaving the molecules in an ionized state. Electro spray sources have also been adapted to Ion Mobility Spectrometry. |
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Electron Capture Detector |
Highly sensitive, although not terribly specific, detector used to detect nitro compounds such as explosive vapors. Originally used only as a GC detector later stand-alone detectors have been used. |
http://chromatographyonline.findanalytichem.com/ |
End of Service Life Indicator |
A chemical detector used to detect the breakthrough of a target chemical through an activated carbon bed or other filter media. Chemical detectors used for this application save money for a customer by allowing for the replacement of filter media only when completely used rather than on a routine time. |
http://www.safetyonline.com/article.mvc/Real-Time-End-of-Service-Life-Indicators-will-0002 |
ERPG-3 |
Maximum concentration in air below which nearly all people could be exposed for one hour without life threatening consequences. |
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Explosive Gas Detector |
A detector designed to detect the presence of an explosive gas hazard. The detector does not necessarily have the ability identify the specific gas but measures the oxidation reaction in a Wheatstone bridge. |
http://www.nidcd.nih.gov/health/ |
Explosive Limits and Range |
Explosive gases will only cause an explosion within a concentration range between the lower explosive limit (LEL) and the Upper Explosive Limit (UEL). A detector needs to determine if the gas is in this range. |
http://www.engineeringtoolbox.com/explosive-concentration-limits-d_423.html |
Term |
Short Definition |
References |
False Alarm Rate |
There are two ways in which a detector can fail at its detection task: false alarm (type 2 error), or signal that a chemical is there when it is not, or misdetection (Type 1 error), miss the chemical when it is there and not alarm. |
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Fast Fourier Transform |
The Fourier Transform is an Integral Transform and uses continuous functions. The Fast Fourier Transform (FFT) is an approximation to the Fourier Transform that can be implemented very efficiently on a digital computer. The FFT does have some limitations in its ability to approximate the Fourier Transform but for real world instrumentation such as Fourier Transform Infrared (FTIR) Instruments the limitations have been easily overcome. The Fourier Transform is a fundamental mathematical operator that also allows an analyst to move between the time domain model of a real time sensor signal and the frequency domain where amplifiers and electronics can be designed. |
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Faraday Cup |
A type of ion detector used in a Mass Spectrometer (MS) or Ion Mobility Spectrometer (IMS) consisting of an electrode surrounded by a conductive metal cup which acts as a Faraday shield. If the electrode ion detector moves in the cup it sees a constant potential and does not create a noise signal. |
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Fence Line Monitor |
A monitor designed to detect chemicals passing over the boundary (fence line) of a plant site which may endanger the community. Fence line monitors frequently use an open path infrared system which projects a beam of infrared radiation around a plant perimeter. The monitor detects chemical vapors based on the absorption spectra. |
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Fiber Optic Chemical Detector |
Conventional Fiber Optic Chemical Detectors use a dye system or sol-gel coating (Ocean Optics) for detection. They can be used for gas or liquid detection. |
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Field Effect Transistor (FET) Chemical Sensor |
Unlike a junction transistor which controls the current through a junction, FETs use a gate to modulate the flow of current from a source to a drain through a semiconductor material by changing the electrical field in the semiconductor material. FETs can control a large current with a small voltage variation as opposed to conventional junction transistors the control a large current with a small current. FETs have been adapted as chemical sensors and one version is the Ion Selective Field Effect Transistor (ISFET) which can be used in electrochemical detectors. |
http://en.wikipedia.org/wiki |
Field Screening Technology |
Field instruments that can be used to screen for chemical contamination, for example at EPA superfund sites, to reduce the need for offsite laboratory analysis. Field Screening Technology saves money by quickly surveying a site for contaminants while only sending some samples back to a laboratory for detailed analysis or directing the remediation or excavation process in real time. |
http://www.osti.gov/bridge/ |
Figaro Gas Detector |
A very commonly used Japanese semiconductor gas detector that responds to a broad class of VOCs and hydrocarbons. |
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Filter Breakthrough |
Point at which an activated carbon filter or other filter media has reached saturation and begins to bleed through into its output. |
http://www.sciencedirect.com/ |
Flame Ionization Detector (FID) |
A chemical detector usually used after a gas chromatograph separation stage for that is commonly used to detect hydrocarbons. The detector uses a hydrogen flame for the pyrolysis of the target chemical and detects liberated ions. |
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Flame Photometric Detector (FPD) |
A chemical detector that uses pyrolysis of the sample in a hydrogen flame and measures the optical spectra rather than ions. The FPD is good for measuring sulfur and phosphorous compounds depending on an optical filter in front of the Photomultiplier tube. |
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Flange Survey |
EPA method calling for survey of process plant pipe flanges using a portable instrument in order to minimize air pollution. A potential market for portable detectors. |
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Four to twenty milliamp communications |
Communications technique frequently used to send data from remote processing plant sensors to a process control computer. In many laboratory instruments it is common to provide a signal voltage proportional to the measured variable but in remote chemical sensors it is most common to provide a 4 to 20 mA current loop because this technique minimizes vulnerability to noise in a potentially high noise environment requiring long cable runs. |
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Fourier Transform Infrared (FTIR) Spectrometer |
An infrared spectrometer that captures data across a wavelength band and processes it to form a spectra which can be used to identify a number of chemicals. Much more powerful technique than Non dispersive Infrared which detects one chemical. |
http://en.wikipedia.org/wiki |
Term |
Short Definition |
References |
Gas Chromatograph Detector |
A gas chromatograph is a separation device which separates materials in a column. It requires a detector at the end of the column to sense the chemicals in the effluent gas stream which have been separated out in packets. |
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Gas detector |
A detector that detects toxic or explosive gases or vapors. |
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Gas sniffer |
Another name for a portable gas detector. |
http://www.gassniffer.com/bw-honeywell-gas-detectors.html? |
Gated Integration |
A device to reduce noise from a chemical sensor or other instrument by integrating the signal measured in a narrow delay window is collected over many cycles and integrated. |
https://www.cfa.harvard.edu/~jbattat/ |
Gausian Peak |
An amazing variety of chemical separation and detection techniques produce Gaussian peaks which are the same shape and have the same mathematical properties as the Normal or Bell shaped curve in statistics. Typically the time of arrival of a Gaussian peak and its area are the important variables in a measuring system and the job of signal processing is to measure these variables as accurately as possible. An instrument is said to have adequate resolution if it can resolve separate peaks without overlap and poor resolution if the peaks overlap. Very sophisticated software programs are available to resolve overlapped peaks. |
http://en.wikipedia.org/wiki/Gaussian_function |
Goldfish Hydrogen Fluoride (HF) release experiment |
The Goldfish release experiment was a joint experiment conducted by the U.S. Department of Energy and a consortium of hydrogen fluoride manufactures and users to study the problem of containing a major release of HF. It was found that the HF polymerized and traveled a much longer distance than thought likely. Goldfish resulted in the development of complex remediation systems including the Shur-Shot detector developed by Exxon Mobil described on this web site. |
Term |
Short Definition |
References |
HAZOP |
A structured method of evaluating hazards at a chemical process plant. |
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Headspace Sampling Technique |
Using a chemical vapor detector to estimate the concentration of a chemical in a solvent or water such as sampling the headspace above wastewater. |
http://www.sigmaaldrich.com/analytical-chromatography/video/spme-video/headspace-sampling-video.html |
Hysteresis in a chemical detector |
A situation when a chemical detector can't clear itself fast enough to continuously track a chemical concentration reading. For example it responds to a high concentration and gets stuck there. Can be caused by chemical vapor or condensate getting stuck inside the detector including plating out on walls. Frequently detectors are provided with a back flush to clear down the detector as quickly as possible. Other strategies include a high heat mode to bake out the contaminant prior to the next challenge. |
Term |
Short Definition |
References |
Immunoassay detector |
Chemical Detection system commonly proposed for chemical and biological warfare agents based on immunoassay. |
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IDHL |
Immediately Dangerous to Health and Life. A toxic chemical concentration high enough to pose an immediate health hazard to exposed people. |
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Inlet |
The chemical detector inlet is the device designed to interface with sample stream which may be ambient air, a process gas stream, or a liquid. Inlet design is extremely important because it has to draw a representative sample and avoid becoming contaminated with background materials such as objects, dirt, or dust in a frequently harsh environment. |
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Instrumentation Amplifier |
An instrumentation amplifier is an integrated circuit or modular device specifically designed for sensor signal collection system. As such it has extremely high input impedance and can have extremely high gain with wide bandwidth and minimum signal (phase) distortion. |
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In situ gas analyzer |
A gas analyzer that measures a gas directly in the stack or exhaust stream. For example, an infrared source may be placed on one side of the stack and a receiver on the other side projecting an infrared beam through the stack gas. No inlet or sampling system is required in this case. |
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Intrinsic Safety |
Designing equipment so that it cannot cause ignition of an explosive gas or vapor. Ensuring that there is not enough energy to cause ignition. |
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Instrument engineer |
An engineer who is specially trained to design instruments. Instruments almost always require a multidisciplinary approach drawing from mechanical engineering, chemistry, electronics, and software. |
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Instrument Technician |
An Instrument Technician is typically the person at a plant responsible for maintaining, calibrating, and troubleshooting the instruments including the chemical detectors. The Instrumentation Technician is probably the closest person to any problem that arises with an instrument in the field and training should be directed to him or her. |
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Ion Guage |
A device for measuring vacuum, for example in a mass spectrometer, which is quite similar to Ion Mobility Spectrometry (IMS) in that ions are formed by a flow of electrons and drawn to a collector. |
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Ion Mobility Spectrometer |
A chemical detection technique based on sorting ions at atmospheric pressure in a drift field. IMS received a great deal of funding in the 1980'2 and 90's as the basis of a chemical warfare detector and was later adapted to industrial chemicals. |
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Ion Selective Electrode |
An electrochemical method of detections ions by their electrode potential. Used for measuring chlorine and fluorine. |
Term |
Short Definition |
References |
Johnson Noise |
Thermal noise in an electrolyte or conductor. Exploited in Fluctuation Spectrometry for example. |
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Joule |
Unit of energy defined as passing one ampere of current through a one ohm resistor for one second. Unit used in specifying intrinsically safe equipment used in chemical processing plants or oil refineries. Also used to specify flash lamps sometimes used in photo ionization. |
Classifying Explosion Prone Areas for the Petroleum, Chemical and Related Industries,W.O.E.Korver |
Junction |
Junction can be defines as a transition layer or boundary between two materials. In chemical detection the most common junction is between two doped semiconductors or P and N materials. Thermocouple junctions are also common. Semiconductor detectors rely on reactions at the junction in order detect chemicals. |
http://en.wikipedia.org/wiki/Heterojunction |
Junction Potential |
The voltage that exists across a junction of two dissimilar materials. Both semiconductor junctions and thermocouple junctions exhibit a Junction Potential. |
Term |
Short Definition |
References |
None at this time |
Please help out by identifying any terms beginnng with K. |
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Term |
Short Definition |
References |
Leak Detector |
Chemical detetors including mass spectrometers and ion mobility spectrometers require either a vacuum or a gas flow devoid of ambient air. A leak detector is a device used to find leaks in the fittings and pipes used in the instrument. For gross leaks the plumbing can often be pressurized and the rate of decrease in the pressure measured with a manometer. For hard vacuums and critical seals a helium leak detector can be used. The leak detector in this case is a special purpose mass spectrometer with a probe which can be placed in proximity with seals and fittings. The plumbing is pressurized with helium and the leak detector probe is brought close to critical areas. If there is even a tiny leak the detector will alarm thus helping to identify the leaking component. |
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Life Cycle Cost |
Sometimes called "cradle to grave" cost includes the purchase price, cost to install and maintain, and cost to dispose of the item. In the case of chemical detectors should also include the cost of misdetections and false alarms. |
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Limit of Detection (LOD) |
Lowest quantity of material that can be distinguished from background (blanks or zero air) |
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Linear Range |
Range of outputs where the output is a constant times the input chemical concentration |
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Liquid chromatograph |
Analogous to gas chromatography but forces a liquid sample through a column and uses different detectors. |
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Low Noise Electronics Design |
Often the performance of an instrument including a chemical detector is limited by the noise floor of the electronics. There is always an ultimate noise floor related to Johnson and Shot noise but with careful design the noise floor can be greatly reduced. In relatively low frequency instruments common noise sources consist of electrical components, especially switching power supplies, mechanical coupling between mechanically noisy components including pumps and solenoids, and pressure variations in the gas flows within the inlet and sensor. The traditional technique for overcoming these noise sources was to average the signal either directly or through a Box Car Integrator system. The best way to deal with the problem is not to average it out but to carefully troubleshoot the design in the first place. |
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Lower Explosive Limit |
According to EPA the concentration of a substance in air below which it will not catch fire. For practical purposes Lower Explosive Limit and Lower Flammable Limit are synonymous |
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Life Cycle Cost |
Sometimes called "cradle to grave" cost includes the purchase price, cost to install and maintain, and cost to dispose of the item. In the case of chemical detectors should also include the cost of misdetections and false alarms. |
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Liquid Column Gauge |
A very simple manometer which can be used to evaluate the leak rate of a closed system of pipes and fittings by pressurizing the system and then using the Liquid Column to measure the loss of pressure over time and establish a leak rate. |
Term |
Short Definition |
References |
Mass Spectrometer |
A chemical instrument that ionizes a gas or vapor, sorts the ions out by various methods such as and electric field (quadrupole) or time of flight. Ions may be molecules or fragments. Based on mass spectra (data) complex chemical mixtures can be identified. |
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Manometer |
A manometer is a pressure monitoring instrument and can be used to establish leak rates in closed systems such as a mass spectrometer. |
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Membrane Inlet |
Permeation membranes are frequently used as the inlet for chemical detectors, especially electrochemical cells, solid state detectors, and spectrometers such as IMS. A membrane inlet provides a certain amount of separation and blocks water and foreign objects from entering the inlet. Common membrane materials are Gor-Tex© and dimethmysiloxane (DMS) rubber. |
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Microphonics |
Microphonics is the creation of an electrical signal through the mechanical vibration of vulnerable components that produce an electrical signal in response to the vibration. Microphonics were a serious concern for most electronics design during the vacuum tube era but much of this concern was eliminated with the introduction of semiconductors and more so with digital processing. Microphonics is still a big problem, however for chemical detectors and instruments that use electrodes and high impedance amplifiers. Electrodes in spectrometers and electrochemical cells are especially vulnerable and great care needs to taken to mechanically support electrodes and troubleshoot a new design to ensure that do not cause microphonics. |
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Minium Level of Detection |
The minimum level of detection is the lowest level of a chemical that can be detected by a detector usually determined by running blanks and very low concentrations of the chemical. If both the responses to blanks and chemicals form Normal curves the Minimum Level of Detection should be 10 standard deviations away from the blank |
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Metal Oxide Semiconductor (MOS) chemical sensor |
MOS devices are a subcategory of Field Effect Transistors and MOS gas sensors are used for the detection of CO, CO2, NOx, and NH3. |
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Multipoint Monitoring |
Frequently area monitors and process monitors are complex expensive instruments and rather than supply one instrument for each area or monitoring point samples can be drawn and multiplexed through an arrangement of valves and hoses. A computer usually controls the sampling schedule and coordinates it with the instrument. |
http://www.chemicalonline.com |
Term |
Short Definition |
References |
Noise |
Noise is variation in detector output reading around the true signal caused by a variety of sources. It can be random (white noise) which can be averaged out, come from power supplies or mechanical pumps, or from chemical materials in the background. It's important to understand the noise sources as random noise should be dealt with separately from other noise sources. Common noise sources that should not be averaged out include switching power supplies, electromechanical pumps, or improperly shielded amplifiers that are vulnerable to stray pick up from other electric circuits. Probably the most common is 60 Hz hum from the surrounding wiring. These noise sources can be dealt with through better design or specific filters. |
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Non-Dispersive Infrared Detector |
A Chemical detector that passes infrared light through a gas cell and measures light absorption in the gas at one or several wavelengths. |
http://www.ecs.umass.edu |
Nonlinearity |
Deviation of detector output from a linear response Output= Constant x Input + Offset |
Term |
Short Definition |
References |
Open Loop System |
A chemical detection system which does not recirculate internal gas flows but uses an external supply of carrier gas, for example. |
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Operational Amplifier |
Operational Amplifiers were originally designed to support mathematical operations in analog computers where they were used as the active components in summing circuits, multipliers, integrators, and differentiators. They later found broad use in instrumentation because they have very high input impedance, very high gain, and very low output impedance. They also have a differential input which can be used to adjust a circuit of a constant bias. They have been suplanted by instrumentation amplifiers which provide more features specific to instrument applications, perform better in instrument applications, and require less design and external components. |
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Oxygen Depeltion Monitor |
A specialized chemical detector that measures the amount of oxygen in an area to ensure that workers have enough oxygen to conduct their work safely. |
http://www.aoi-corp.com/products |
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Short Definition |
References |
Paper Tape Chemical Detector |
Paper tape systems, especially Zellweger's Chemcassette technology, were the standard fixed point industrial hygiene detector for many years and relied on a chemically impregnated tape which would advance in steps placing sample on a chemically treated paper tape which changes color in response to a detection. |
http://www.honeywellanalytics.com/ |
Permeation Tube Generator |
A device capable of generating very low vapor concentrations of volatile chemicals. Used to calibrate detectors or test them. Permeations tubes are filled with the target chemical and periodically weighed to calculate the permeation rate in a controlled environment. The concentration of target chemical in carrier gas can accurately be determined from the permeation rate. |
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Photoionization Detector (PID) |
A gas detector that ionizes a target gas or vapor in a gas stream and detects it by measuring the ionization current between two electrodes. Selectivity in a PID is achieved by choosing the proper lamp with an appropriate wavelength for detecting the chemical of interest. A table of chemicals with required lamp energies can be obtained from the reference. |
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PIC Microcomputer |
A very common microcontroller that forms the basis of several inexpensive data acquisition systems. |
http://www.dataq.com/ |
Piezeoelectric gas sensors |
The two types of piezoelectric gas sensors are: Surface Acoustic Wave (SAW) devices, and Quartz Crystal Microbalance. Quartz Crystal Microbalances are larger, less easy to design into an array for multiple gas detection, but can be used in water. Both technologies rely on a coating for sensitivity and specificity which can be problematic. |
http://www.google.com/patents/about/6196052_ |
Pink Noise |
Many chemical detectors are low frequency devices. If the sensor output is a set of Gaussian peaks displaced in time, for example, the frequency spectrum is also Gaussian with it's maximum at direct current or zero frequency. Unlike white noise which is flat across the spectrum, Pink noise (1/f noise) has more power at lower frequencies and may present more of a problem. In many applications neither Pink nor is White the biggest problem but physical noise sources not yet under control. Frequently these sources are correlated and can be detected by looking for discrete peaks in the output spectrum. |
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Point detector |
A chemical detector which monitors for the presence and concentration of a chemical species at a fixed point rather than monitoring down a column of air. Areas can be monitored using a number of fixed points for a line of sight detector. |
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Portable Detector |
A detector that a person can carry around in the field. There are a number of detector mobility terms commonly used: fixed point or not mobile, transportable or movable on a truck, portable or moveable by a person, miniature or worn on a uniform, and embedded or included in another structure such as a uniform. |
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Process Analyzer |
A device that automatically monitors an important variable in a process stream. Process Gas Chromatographs and Process IR are commonly used in chemical processes to adjust plant parameters in real-time. An analyzer unlike a monitor can identify and quantify components in a mixture of complex materials. Analyzers generally add value to the customers product or process and monitors are safety devices. |
http://www.swagelok.com/services |
Process Control Algorithm |
A computer algorithm for controlling a plant or process. A common algorithm is a PID control algorithm standing for Proportional Integral Derivative (PID). Normally a Process Control Algorithm will be much more complex than simply a linear control algorithm and will involve logic and thresholds on variables. When a chemical detector is evaluated and installed its impact on the Control strategy and algorithm will be considered and the algorithm updated. |
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Process Control Computer |
A specialized computer used in chemical plants and oil refineries to implement control algorithms such as PID with additional logic. Many chemical detectors including area monitors and process analyzers are interfaced to operators through the Process Control Computer. |
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Process Gas Chromatograph |
One of the original chemical instruments used in process work, Process GC can be used in the control of the cracking process in an oil refinery. |
http://www.automation.siemens.com/w1 |
Process Mass spectrometer |
A mass spectrometer, often a low resolution residual Gas Analyzer, used to make process measurements in a chemical processing plant. |
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Short Definition |
References |
Q |
Quality Factor of a resonant circuit. Important for piezoelectric chemical detectors because if one loads the coating with too much mass or otherwise constrains the resonator and a read out system relying on a change in frequency will not work. |
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Quartz Crystal Microbalance |
A chemical sensor based on a quartz crystal with a coating that selectively adsorbs a target chemical changing its mass and changing the resonant frequency of the crystal. The change in crystal frequency can be read out electronically. |
Term |
Short Definition |
References |
Remeadition System |
System used to clean up a contaminated site which involves either the removal of contaminated material or treating the contaminated soil or groundwater in place. One version frequently called "Pump and Treat" pumps groundwater out of the ground, removes contaminants, and returns the water. A sensor measures the contaminant concentration and when it drops below a threshold value the process is complete. |
http://www.amazon.com/Design-Remediation-Systems-Jimmy-Hock/dp/1566702178 |
Residual Gas Analyzer |
A low resolution, ruggedized, mass spectrometer used for process control applications |
http://www.ors-labs.com/ResidualGasAnalysis.php?gclid=CJOQ3YXBxasCFaJgTAodrSoW3A |
Residual Life Indicator |
A device used to predict the life of an activated carbon bed or other filter based on measuring its exposure to background contaminants such as fuel vapor. |
http://www.dodsbir.net/sitis/archives_display_topic.asp?Bookmark=37638 |
Resolution |
Resolution refers to the sharpness of spectral peaks and the narrowest range of wavelengths, bands, or frequencies that can be separated from their neighbors. |
http://en.wikipedia.org/wiki/ |
Ring Down Spectrometer |
A highly sensitive infrared spectrometer that sends a pulse of infrared radiation back and forth between two mirrors in a gas cavity until the radiation is absorbed and no longer detectable. This technique allows for very long path lengths and thus great sensitivity in a compact instrument. |
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ROC Curve |
Receiver Operating Characteristic Curve is a graphical technique which, in the case of chemical sensors, plots false alarm rate against minimum level of detection. It is a more rigorous technique than guessing some concentrations and running a number of samples at succesively lower concentration. |
http://en.wikipedia.org/ |
Term |
Short Definition |
References |
Saturation |
The concentration above the span where the response of a chemical detector is no longer linear. |
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Sampling Frequency |
1/ rate (expressed as a frequency in Hz, MHz etc) of digitization by an analog to digital computer |
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Sampling System |
The "front end" of a chemical detector, process analyzer etc. where sample is drawn from the process stream or ambient air. |
http://www.iceweb.com.au/ |
Selectivity |
Ability of a chemical detector to differentiate between target chemicals and background chemicals which may be present in its environment. |
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Semiconductor Gas Detector |
A gas sensor based on semiconductor materials. Simple PN junctions and various FET configurations are possible. |
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Sensitivity |
Ratio between a change in output signal and change in concentration of the chemical material |
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Stack Monitor |
A chemical detector adapted to detecting target chemicals in stack gas. Stack gas is frequently hot and wet making the design of a stack sampling and monitoring system a challenge. |
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Short Term Exposure Limit |
For a toxic chemical, the concentration below which it is safer to work for a breif period of time without irritation or severe tissue damage. |
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Spectrophotometer |
A device for measuring the intensity of light as a function of wavelength over the visible spectrum and may include near IR and near UV. A spectrophotometer may be required for quantitative analysis of colorimetric detectors. |
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Switching Power Supply |
Switching power supplies convert 60 Hz current from a wall outlet to a much higher frequency (25 kHz) before going through a now much smaller step down transformer. The problem with switching power supplies is that they add a relatively high noise frequency noise signal to the input power to very sensitive amplifiers and an analog converter so the noise source which may be above the bandwidth of the detector causes an aliasing error and may appear as random noise. Smoothing out this random noise with a digital filter may greatly degrade the performance of the detector. |
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Surface Acoustic Wave (SAW) device |
Surface Acoustic Wave (SAW) devices are delay lines developed as electronic filters. Much like Quartz Crystal sensors they are modified for chemical sensing by applying a coating that adsorbs the target chemical thus changing its mass. One approach to an electronic readout is to make the SAW device an element in an oscillator and measure the oscillator frequency shift as the device is exposed to a target chemical. |
http://en.wikipedia.org/wiki/ |
Term |
Short Definition |
References |
Thermal Conductivity Detector |
The simplest of gas chromatography detectors. Detects chemical vapors eluting from a GC column by comparing their thermal conductivity to the carrier gas (normally helium) using a Wheatstone bridge. Very similar electrically to a catalytic bead detector where the heating element changes temperature and therefore its resistance which is measured in a Wheatstone bridge. |
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Thermocouple and Thermistor |
Most chemical instruments, detectors, and sensors are sensitive to temperature and finding a convenient way to measure the temperature of a cell or silicon block is an important problem. Thermocouples which consist of a junction of dissimilar metals along with a separate reference junction are a very common of measuring temperature on a surface but may not be accurate enough for most detector applications. A more accurate alternative, although it produces a nonlinear signal and therefore requires more computer processing is the thermistor. One of the references discusses the tradeoffs between thermocouple and thermister in a pharmaceutical application. One of the best ways to compensate for temperature in a chemical instrument is to use some measurable feature of the chemical signature to compensate for temperature changes. For example in Ion Mobility Spectrometry (IMS) a reference signature can be captured every few minutes by running a filtered air signature and then using the reactant ion peak which should be present in every signature as a reference. |
http://en.wikipedia.org/wiki/Thermocouple |
Threshold Limit Value (TLV) |
The upper limit of safe long term exposure to a particular chemical. These numbers are published and easily obtained. |
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Threshold of Detection |
Also called the Limit of Detection, Lower Limit of Detection, etc.. The lowest level at which the detector or sensor can reliabily detect the presence of a target chemical in a background. The method for determining the Threshold of Detection given in the referenced article may be over simplified because it assumes a laboratory environment and great control over the variables. The Receiver Operating Characteristics (ROC) curve may convey more information about the actual Limit of Detection, and environmental extremes need to be specified. |
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Time of Flight Mass Spectrometer (TOFMS) |
A mass spectrometer that ionizes a sample in a vacuum and separates out ionic species by accelerating them down a drift tube in a vacuum, detecting them at the end of the drift tube with an electrode in a Faraday Cup. TOFMS is an old technology that has found recent life in the analysis of large macromolecules. It can also be made into a ruggedized device. Ion Mobility Spectrometry (IMS) at one time was called the "poor man's cousin" of TOFMS but the technologies operate on very different principles are produce very different results. IMS forces ions down a drift tube at low or atmospheric pressure where the ions undergo collisions with neutral molecules and drift at a particular speed rather than accelerate. |
http://en.wikipedia.org/ |
Tin Oxide Gas Sensor |
A particular type of semiconductor gas sensor. The reference contains a tabular list of gases. |
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Transfer Hose |
Many commodity chemicals are distributed to customers through rail cars or tank trucks and considering the transportation system is critical to ensuring their safety. One key component is the transfer hose which is used for loading and unloading. Loading and unloading operations are fertile ground for chemical sensor application along with safety valves. |
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Transmitter |
Sometimes Transmitter refers to the entire field unit in its housing of a chemical detector including the sensor, amplifiers, encoders, etc and sometimes the term just refers to the encoder and electronic interface. Frequently detectors are placed remotely and often have very different technology but ultimately all of the data must be converted to the 4 to 20 mA system for transmission over plant cables to the process control computer. |
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Troubleshooting Instrumentation |
Chemical Detectors need regular maintenance, calibration, and repair. Traditionally this task has fallen on the plants Instrument Technicians but budget problems and the need to show a profit have forced many end users to severely cut back on plant maintenance personnel and shop equipment. Instrument maintenance and troubleshooting has been contracted out to third parties. Frequently diagnostic software is built into software in the detector or the process control computer. It is often possible to watch the slow degradation of a signal and predict when maintenance will be required. |
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Type Classification |
Defining the safety standards to which a chemical detection system must conform. Perhaps the ultimate safety system is to design the chemical instrument to be intrinsically safe. See the discussion in the references. |
http://www.chemicalplantsafety.net/hazard-control/introduction-to-hazardous-area-classification/ http://www.measure.org/geninfo/prod_cert.htm |
Term |
Short Definition |
References |
Upper Explosive Limit (UEL) |
Gases and vapors are only explosive over a range in air. The Upper Explosive Limit is the upper limit on the range. Above the UEL there is not enough oxygen to produce the explosive reaction. |
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Short Definition |
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Vacuum Gauge |
Mass spectrometers operate at a vacuum and other sensors may require a reduced pressure for proper operation. |
http://matec.org/ps/library3/ |
Voltage Controlled Oscillator (VCO) |
An electronic component that produces an alternating current at a particular frequency bases on an input voltage provided by an electronic circuit. In chemical detection may form part of a feedback circuit for measuring a chemical response from a piezoelectric chemical sensors. |
Term |
Short Definition |
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Water Curtain |
A safety device for the mitigation of toxic chemical clouds which may form when hydrogen fluoride or other toxic gases are accidentally released. |
Term |
Short Definition |
References |
X-ray Spectroscopy |
X-Ray spectroscopy is an efficient real time technique for detection metals released into the atmosphere from incinerators and processing plant effluent. The reference discusses work with adapting X-Ray spectroscopy to Continuous Emissions Monitoring (CEM). |
Term |
Short Definition |
References |
None |
Please recommend glossary terms for this section. |
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Term |
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Zero and Span |
Many chemical detectors operate over a linear range from Minimum Level of Detection to a maximum. Detectors are calibrated by allowing them to sample zero air provided in a can or tank and a Span Gas which has the maximum concentration. The linear range of the detector is then set. |
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Zero Air |
Dry hydrocarbon free air used to calibrate instruments and also as a gas supply for other instruments such as infrared instruments. Often times when designing chemical detection instruments including Continuous Emission Monitors (CEM) the plant including chemical plants and power plants can be assumed to have a zero air supply. |
http://hiq.lindegas.com/international/ |
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