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Gas Detection


Aerosol is a suspension in air (or gas) of minute particles of a liquid or a solid.

Alarm is an audible, visual, or physical presentation designed to warn the instrument user that a specific level of a dangerous gas/vapor concentration has been reached or exceeded.

Alarm Only Instrument is an instrument providing an alarm(s) which does not have an integral meter or other readout device indicating current concentration levels.

Alarm Set Point is the selected gas concentration level where an alarm is activated.

Ambient Air is air to which the sensing element is normally exposed.

Analyzer is an instrument which can determine qualitatively and quantitatively the components in a mixture.

Approved is acceptance by the authority having jurisdiction. In Delphian literature, the term is considered synonymous with "listed" and "certified."

Area Monitor is a term that is often misleadingly applied to gas monitoring sensors that are installed in a regular grid pattern throughout an area requiring monitoring. A true area monitor must be able to measure the concentration of a substance at any point in three dimensional space in a defined value or it must be able to indicate the total quantity of a substance that has penetrated a defined volume.

BASEEFA.  British approval agency.  The Electrical Equipment Certification Service (EECS) is based at the Health and Safety Laboratory's Buxton research center in Derbyshire, England. It provides a range of testing and certification services primarily related to equipment and systems intended for use in potentially explosive atmospheres.

Certain conditions can cause a sensor not to function. when this happens, normal gas sensing is blocked until the conditions are removed. The most common block is lack of oxygen. Oxygen deprivation can be caused by sensor flooding and a clogged flame arrestor.

CALIBRATION is the procedure used to adjust the instrument for proper response (e.g., zero level, span, alarm and range). Also see Remote Calibration

Calibration Gas is the known concentration(s) of gas used to set the instrument span or alarm level(s).

Common Alarm Module provides a relay contact transfer whenever any one gas detection alarm occurs in a zone.

Catalysis is a phenomena in which a relatively small amount of material augments the rate of reaction without itself being consumed.

Catalytic Bead Sensors


Clean Air [Zero air] is air that is free of any substance that will adversely affect the operation of or cause a response of the instrument. Clean air is also called "zero air" or "zero gas".

A form of sensor poisoning where a chemical reaction takes place which coats the surface of a sensor, to the extent where it is unable to sense a gas. For instance exposure to a small concentration of a volatile silicon compound can kill a catalytic sensor in less than five minutes. Special poison resistant CHC sensors are available from Delphian.

Combustible Hydrocarbon (CHC) is any organic gas or vapor which when mixed with air or oxygen is capable of the propagation of flame away from the source of ignition when ignited. See CHC Sensors.

Combustion is the rapid oxidation of a material evolving heat and generally light.

This sensor allows the user to switch sensor elements and flame arrestors instead of the whole sensor. A sensor is thereby replaced with a minimum of cost in time and money.

Constant Temperature Operation

Consumables are those materials or components which are depleted or require periodic replacement through normal use of the instrument.

Control Unit/Module [Controller] is that portion of a multi-part gas detection instrument which is not directly responsive to the gas but which responds to the electrical signal obtained from one or more detector heads to produce an indication, alarm, or other output function. The control unit contains the operating controls such as zero, span and alarm setpoint adjustments along with readouts, status indicators, recorder outputs and relay contacts.

CSA  An approval agency based in Canada.
Canadian Standards Association

DC Controller.  Digital Controller

Detector Head is the gas-responsive portion of a gas detection instrument located in the area where sensing the presence of gas is desired. Its location may be integral with or remote from any circuitry.

Diffusion is a process by which the atmosphere being monitored is transported to the gas-sensing element by natural random molecular movement. This movement is accelerated by thermal energy.

Electrical Noise
Electrical noise has been creating problems ever since the first antennas were installed and lightning storms interfered. Almost every form of electrical or electronic device has had to live with the problem at one time or another. Today, we recognize that almost any device which operates on the principle of moving an electron from one point to another can be a receiver of electrical noise. Since the problem first became obvious to early radio engineers, the term ‘radio frequency interference’ (RFI) was most often applied. As engineers spread the use of electronics beyond its beginnings in radio communication, it was found that electrical interference didn’t confine itself to the radio spectrum. The newer term electromagnetic interference (EMI) acknowledges the fact that electrical interference encompasses the entire electromagnetic spectrum from the lowest magnetic frequencies through the highest microwave frequencies.  Delphian's RFI Filter

Electrochemical Sensors

Volatilization - a form of sensor poisoning which removes the sensor catalyst. Special poison resistant CHC sensors are available from Delphian.

Explosion is an uncontrolled chemical reaction which generates a large amount of heat and gas in a short period of time.

Fail Safe. Any system that cannot fail in any mode without providing a directly observable indication of failure. Consider an electrical relay with a set of contacts that are open when it is un-powered. If a power source and a light bulb are connected in series with the contacts, the lamp will glow when the relay is energized. If the goal of this system is to insure that the relay has power, then this system is said to be fail safe. If the lamp, relay contacts, lamp power source relay coil, or the relay coil power supply fail, then the lamp extinguishes itself providing a directly observable foolproof indication of failure.

Fault Tolerance Ability of a functional unit to continue to perform a required function in the presence of faults or errors (IEC 61508-4, 3.6.3)

Fault Tree Used in safety engineering to quantitatively determine the probability of a safety hazard.

FIT Failure in Time (1x10-9 failures per hour)

Fixed Installation. The terminology commonly used to indicate that a gas monitor is permanently installed, such as in the control panel of a control room. Occasionally gas monitors are mounted in vehicles, such as fire trucks or tankers. These are also generally referred to as fixed installation monitors.

Flame Arrestor
Its purpose is to prevent flame propagation from the heated sensor side of the sensor. It reduces the differential cooling effect of wind on the sensing element and protects the sensor from flooding by a high velocity of gas. It also protects the sensing element from damage during handling. It should be checked for proper attachment and fit, and for signs of corrosion, dirt or moisture. An area must be declassified (free from Combustible Gas) before a flame arrestor can be removed from a catalytic system. See CommonSensor.

Flameproof or ExplosionProof
Flame paths through housing flanges or flame arrestors are dimensioned such that the hot, gaseous products of internal explosion, which may leak out of the enclosure, will be sufficiently cooled to prevent igniting the specific flammable surroundings. The enclosures are strong enough to withstand the internal explosions without damaging these controlled flame paths. Surfaces that may contact the flammable atmospheres have maximum temperatures below the ignition point of the specific gas or vapor to be encountered.

Flammable (Explosive) Limits. For gases or vapors which form flammable mixtures with air or oxygen, there is a minimum concentration of vapor in air or oxygen below which propagation of flame does not occur on contact with a source of ignition. There is also a maximum proportion of vapor or gas in air above which propagation of flame does not occur. These boundary-line mixtures of vapor or gas with air, which if ignited will just propagate flame, are known as the "lower and upper flammable limits" (LFL and UFL) or the "lower and upper explosive limits" (LEL and UEL), and are usually expressed in terms of percentage by volume of gas or vapor in air. LEL and LFL are different terms for the same concept and can be used interchangeably. In popular terms, a mixture below the lower flammable limit is too "lean" to burn or explode and a mixture above the upper flammable limit too "rich" to burn or explode.

Flammable (Explosive) Range. The range of flammable vapor or gas-air mixture between the upper and lower flammable limits is known as the "flammable range", also often referred to as the "explosive range". For example, the lower limit of flammability of acrylonitrite at ordinary ambient temperatures is approximately 3 percent vapor in air by volume, while the upper limit of flammability is about 17 percent. All concentrations by volume of acrylonitrite vapor in air falling between 3 percent and 17 percent are in the flammable or explosive range.

Flashpoint is the minimum temperature at which a liquid gives off a sufficient vapor to reach 100% LEL (sufficient vapor to form an ignitable mixture with the air near the surface of the liquid).

Flexirack. Delphian's flexible custom-tailored rack mounting system built for the exact number of gas detection controllers required.

Sensor flooding occurs when a gas concentration at the sensor exceeds its stoichiometric mixture. The signal from the sensor reverts to zero because the mixture in the air is too gas-rich to burn.

FMECA Failure Modes, Effects and Criticality Analysis

Factory Mutual Research Corporation

Gas monitor for Alternate Fueled Vehicles.

Gas is a phase of matter which expands indefinitely to fill a containment vessel. Characterized by a low density.

Gas Detection Instrument is an assembly of electrical, mechanical and chemical components (either a single integrated unit or a system comprised of two or more physically separate but interconnected component parts) which senses and responds to the presence of gas in air mixtures.

Gas Sensing Element (Sensor) is the particular subassembly or element in the gas detection instrument which, in the presence of a gas, produces a change in its electrical, chemical, or physical characteristics.

HFT Hardware Fault Tolerance

Hydrogen Sulfide is interchangeable for: hydrogen sulfide, H2S, dihydrogen sulfide, and hydrogen sulfide gas.

IDLH (Immediately Dangerous to Life and Health) represents the maximum concentration level of a substance from which one could escape within 30 minutes without escape-impairing symptoms or any irreversible effects (For instance 300 ppm for Hydrogen Sulfide).

Ignitable Mixture A mixture within the flammable range (between the lower and upper flammable/explosive limits) that, when ignited, is capable of the propagation of flame away from the source of ignition.

Ignition Temperature is the minimum temperature necessary to initiate combustion (oxidation) and have self-sustained combustion of the solid, liquid, gas, or vapor of interest.

Infrared Detectors

An interferent is any gas other than the target gas that will cause a gas detecting sensor to give a signal. In the case of a combustible sensor, any combustible gas or vapor will cause a signal.

Intrinsic Safety Electrical apparatus and its associated wiring in the hazardous location is intrinsically safe when it is designed to operate with insufficient electrical or thermal energy, under either normal or abnormal conditions, to ignite the specific hazardous atmospheric mixture.

ISA  Instrument Society of America

LDM  Low Demand Mode, where the frequency of demands for operation made on a safety related system is no greater than twice the proof test frequency.

LDM  Leak Detection Module

Liquid is a phase of matter which is free to conform to a shape of a vessel but has a fixed volume and has a greater density than a gas.

Locating Sensors

LOPA Layer of Protection Analysis (safety)

Lower Explosive Limit (LEL)
Lower Flammable Limit (LFL)

The lower explosive limit (LEL) or lower flammable limit (LFL) of a combustible gas is defined as the smallest amount of the gas that will support a self-propagating flame when mixed with air (or oxygen) and ignited. In gas-detection systems, the amount of gas present is specified in terms of % LEL: 0% LEL being a combustible gas-free atmosphere and 100% LEL being an atmosphere in which the gas is at its lower flammable limit. The relationship between % LEL and % by volume differs from gas to gas. For data on other gases, refer to the most recent edition of the Handbook of Chemistry and Physics published by the C.R.C. Press. Typical settings for the alarm circuit are 20% for the low alarm, 40% for the high alarm and 60% for the high-high alarm.
The LEL of a gas is affected by the temperature and pressure: as the temperature increases, the LEL decreases and hence the explosion hazard increases; the relationship between LEL and pressure is fairly complex, but at approximately one atmosphere a pressure increase usually lowers the LEL. The LEL of a gas is not significantly affected by the humidity fluctuations normally encountered in the operation of a gas-detecting system.
Flammable Limits of Various Gases & Vapors

Markov Analysis Analyses the reliability and availability of systems whose components exhibit strong dependencies.

Mobile refers to a continuous-monitoring instrument mounted on a vehicle such as, but no limited to, a mining machine or industrial truck.

Monitor is an instrument used for continuous measurement of a condition which must be kept within prescribed limits.
Monitors are not the same as analyzers. An analyzer is capable of determining the quality, quantity and/or type of specific substance or substances in a mixture. A monitor continuously measures a condition which must be kept within prescribed limits.

MTBF Mean Time Between Failures is sometimes used instead of The Failure Rate, when the failure rate is constant.

National Electrical Code

National Electrical Manufacturers Association.  Technical standards for electrical equipment.  [NEMA Enclosures]

Nonsparking Nonsparking circuits are those which contain no contacts or in which contacts are isolated from the surrounding atmosphere such as by hermetic sealing.

Nominal Voltage is the voltage given by the manufacturer as the recommended operating voltage of their gas detection equipment. If a range (versus a specific voltage) is given, the nominal voltage shall be considered as the midpoint of the range, unless otherwise specified.

Nonincendive circuits are those which may spark under normal operating conditions, but which may not release enough energy to cause ignition. Circuits may contain enough energy for potential ignition should both the equipment fail and the process fail (two concurrent failures).

In its most basic form Oxidation is a chemical reaction with oxygen. Example: the oxidation of Methane (formula). In this example, molecular Oxygen is the oxidizing agent and the substance reacting with oxygen (methane) is called the reducing agent. Confusingly the reducing agent (Methane) can also be called an oxidizable gas. Compounds containing oxygen can yield oxygen in a reaction and are also called oxidizing agents. Oxidation is a reciprocal process in which one agent is reduced and one oxidized. A more complete way of describing oxidation is through the transfer of electrons. The substance oxidized loses electrons. The substance reduced gains electrons. Under suitable conditions, the oxidation-reduction reaction produces a flow of current.
See electrochemical sensor

Paper Tape Detectors

PFDAVG Average Probability of Failure on Demand (safety measurement)

PFH Probability of Dangerous Failure per Hour (safety measurement)

Point Monitor. All popular gas monitors are point monitors. This means that their sensors are only capable of indicating the concentration of a substance at a single point in three dimensional space. Selection of the proper point in three dimensional space at which to place a sensor or sample probe is the most important element in the art of gas monitoring.  (Sensor Location)

Sensors can be quickly destroyed (or poisoned) by certain materials. Even low concentrations of poisoning substances can cause serious problems. The two most common phenomena are coating and etching. [Available from Delphian: Special Poison Resistant CHC Sensors and Poison Protection Caps]

Portable refers to a self-contained, battery-operated or transportable gas monitor worn or carried by the person using it. A gas detector that can be carried.

Ported Sensor provides a 1/4" NPT connection on the side of a sensor body or sensor adaptor for delivery of calibration gas.  Typically used when sensor is located in a hard-to-reach or hazardous location.

Parts Per Million

Range is the series of outputs corresponding to values of concentrations of the gas of interest over which accuracy is ensured by calibration.

Remote Calibration
Conventional gas detectors require two people with walkie talkies, one at the controller to adjust zero and span and one at the sensor to apply calibration gas. With Delphian's one-person calibration, zero and span adjustments are made at the sensor by the same person applying calibration gas. Also see Calibration

Sequential Acknowledgment Module provides a relay contact transfer whenever any one gas detection alarm occurs in a zone, but each alarm can be sequentially acknowledged.

Sample Draw refers to a method to cause deliberate flow of the atmosphere being monitored to a gas-sensing element.

Delphian's SAGE CE Computerized Gas Monitoring System utilizes Sensor and Relay Satellites. These support eight sensor-transmitter/relay inputs converting signals to transmit over an inexpensive twisted-pair network to the central computer.

Semiconductor Sensors

Sensing Element
A sensing element is unique to each gas to be monitored. The element is constructed to plug into a circuit board in the sensor housing. Sensor elements can, with the use of a special tool, be replaced in the field. See CommonSensor.

A gas detecting sensor converts the presence of a gas or vapor into an electrically measurable signal. The sensor is the heart of a gas monitor. The system is as good as the sensor. See Sensor Technology.

SFF Safe Failure Fraction summarizes the fraction of failures which lead to a safe state and the fraction of failures which will be detected by diagnostic measures and lead to a defined safety action. (safety measurement)

SIF Safety Instrumented Function

SIL Safety Integrity Level. SIL is a measure of safety system performance. It is one means of reducing risk. It can be expressed either as a failure rate or as the amount of risk reduction. The higher the SIL, the lower the PFD (Probability of a failure on demand) for the system. A product’s SIL rating means it is suitable for use within a system with a specified SIL rating.

SIS Safety Instrumented System - Implementation of one or more Safety Instrumented Functions. A SIS is composed of any combination of sensor(s), logic solver(s) and final element(s).

Special Local Alarm Module for use with the SafeCAL and infrared detectors.  See SLAM

Solid is a phase of matter characterized by a definite volume and definite shape. A solid resists external forces to change shape.

Span is the algebraic difference between the upper and lower values of a range.

Stationary refers to a gas detection instrument intended for permanent installation in a fixed location.

Stoichiometric. The exact percentage of two or more substances which will react completely with each other leaving no unreacted residue. For example, a 7% mixture of methane by volume in air will react completely with the oxygen present leaving only CO2 and H2O as residue. If the methane concentration here is less than 7%, there would be oxygen left over. If the methane concentration were greater than 7%, there would be methane left over.

Test Gas is a known concentration of the gas to be detected diluted with clean air.

Threshold Limit Value Time-Weighted Average (TLV-TWA) is the time-weighted average concentration of a substance for a normal 8-hour work day and a 40-hour work week, to which nearly all workers may be repeatedly exposed, day after day. (OSHA)

Toxic Gas or Vapor. Any substance which causes illness or death when inhaled or absorbed by the body in relatively small quantities. H2S is a highly toxic gas.

Transfer Factors
For each CHC there is a specific minimum concentration above which an ignition source will cause an explosion or flame front propagation. This is called the Lower Explosive Limit (LEL) of each gas. The LEL is different for every gas. Through the use of conversion tables (transfer factors) it is possible to calibrate with one CHC gas and use the system to measure the % LEL of another CHC.
It is always best to calibrate a gas monitor with the gas it will be monitoring.   Each combustible gas has its own unique characteristics, as well as different LEL values.  Transfer factors, even when experimentally determined, are only approximations and will vary from sensor to sensor.  In addition, as sensors age, their response to gas changes.  The transfer factor for new sensors may not be the same for older sensors.  As of our latest FMRC Approval, we are the only manufacturer of gas monitoring systems who has permission to use transfer factors for other known gases while calibrating using methane gas.  An approved transfer factor list is available from Delphian (ask for part number 361-987-01)

A transmitter in a gas detection system amplifies the signal from the sensor and converts it into a more convenient form for transmission. Some monitors collect sensor signals without the use of transmitters.

Trouble Signal is a signal (contact, transfer, and/or visible or audible signal) advising an instrument user of conditions such as input power failure, an open circuit breaker, a blown fuse, loss of continuity to the detector head, defective gas-sensing element, or significant downscale indication

Voting Alarm Module provides a relay contact transfer when a specified minimum number of individual alarms occur in a zone.

Vapor is the gaseous state of a material below its boiling point.

Vapor Density relates the molecular weight of a gas to the molecular weight of air.

Vapor density is the weight of a volume of pure vapor or gas (with no air present) compared to an equal volume of dry air, at the same temperature and pressure. This information assists in determining the location of a sensor. A vapor density figure of less than 1 indicates that the vapor is lighter than air and will tend to rise in a relatively calm atmosphere. A figure greater than 1 indicates that the vapor is heavier than air and may travel at low levels for a considerable distance to a source of ignition and flash back (if the vapor is in the flammable range). Note that some gases such as ethane have a vapor density of 1 and may be present at low levels or may rise significantly, dependent upon ambient conditions.

Zone Alarm Modules are logic cards which provide special function contact outputs based upon the alarm status of one or more monitoring points in a gas detection zone. A zone is a user-designated group of points. See CAM, VAM and SAM.

Zero Gas
Zero gas is clean air, and is an excellent way of insuring that a small release of gas is not near the sensor while zeroing the sensor signal during calibration.


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