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.
Calibration Gas is the known concentration(s) of gas used to set the instrument span or alarm level(s).
Catalysis is a phenomena in which a relatively small amount of material augments the rate of reaction without itself being consumed.
Combustion is the rapid oxidation of a material evolving heat and generally light.
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.
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.
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.
Flameproof or ExplosionProof
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.
FMECA Failure Modes, Effects and Criticality Analysis
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.
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.
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.
LOPA Layer of Protection Analysis (safety)
Lower Explosive Limit (LEL)
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.
MTBF Mean Time Between Failures is sometimes used instead of The Failure Rate, when the failure rate is constant.
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.
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)
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.
Range is the series of outputs corresponding to values of concentrations of the gas of interest over which accuracy is ensured by calibration.
Sample Draw refers to a method to cause deliberate flow of the atmosphere being monitored to a gas-sensing element.
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).
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.
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
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.
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