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 Module is an epoxy encapsulated device which controls the output of the FuelGuard system.   The Alarm Module provides connectors for four Leak Detection Modules.  (If more than four sensors are attached an Expansion Module is also required.)  The Alarm Module provides power to the electronics in the enclosure.  The power is fused by a standard automotive 3 amp "minifuse" to protect the components.  The Alarm Module interprets the output of the Leak Detection Modules and activates the relays and the annunciator indicator lights.   The Alarm Module provides four independent (single pole/double throw) two amp relays, one for fail, and three for gas alarms.  These relays don't have to be wired for the Alarm Module to function but they are intended to operate external signaling devices such as horns and lamps or to affect controls such as ignition interlocks.   Connection to these relays is done through four push-on connectors that go on the left side of the Alarm Module.  The supplied wiring is color coded.  The three gas alarm relays (trace, moderate & significant) are normally energized or normally de-energized.  Thus when an alarm condition occurs, the relay coil is energized, the relay contacts are pulled in and the common contact is connected to the NO contact.   (See FuelGuard Drawing)

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.

This is an optional panel that installs on the dashboard of the AFV.  It is about the size of a credit-card.  It incorporates an audible alarm, indicator lights for three levels of gas alarms, the sensor/system OK and FAIL lights and a silence button.  The silence button momentarily silences the audible alarm and also tests the operation of the indicator lights and buzzer.  It connects to the Alarm Module.  See FuelGuard Drawing.

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).   See FuelGuard Calibration.

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

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.

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

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.

The FuelGuard cables are equipped with automotive connectors.   These locking connectors prevent accidental disconnects.  They are keyed so they cannot be installed backwards.

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

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.

The FuelGuard Leak Detection Modules, Alarm Module are usually housed in a Nema 4X enclosure.  It can be sized for one to eight sensors.  The enclosure is made of lightweight plastic and protects the electronics from accidental damage as well as from dirt and water.   The enclosure has a windowed front which makes it easy to read the LED displays on the Leak Detection Modules.  There is no need to open the enclosure except for initial installation or component replacement.  Sensor calibration can be completed and latched alarms can be reset without ever opening the FuelGuard enclosure.  See FuelGuard Drawing.

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

Expansion Module
The FuelGuard is designed to accommodate up to eight sensor points.  If a user wishes to connect more than four sensors, the Connector on the Alarm Module labeled "EXPANSION" connects to an Expansion Module which interfaces to Leak Detection Modules for up to four additional sensor points.  The Expansion Module monitors the Leak Detection Modules that are connected to it, and passes alarm information back to the Alarm Module which signals alarms for all eight sensors.

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.

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.

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.

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).

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.

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.

FuelGuard System
The FuelGuard System is composed of three major components:  The Alarm Module, Leak Detection Modules with display and Sensors.  Sometimes a system is attached to an Annunciator (not pictured).  The Alarm Module and the Leak Detection Modules (one for each sensor) are housed in a NEMA 4X enclosure which protects them from dirt and water.  The sensors and the Annunciator are connected to the electronics in the enclosure by automotive connectors.   See FuelGuard Drawing.
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Gas is a phase of matter which expands indefinitely to fill a containment vessel. Characterized by a low density.

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.

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.

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.

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.

Leak Detection Module is an epoxy encapsulated module which controls the operation of an attached sensor, displays its reading and passes information to the Alarm Module.  It is usually housed in the FuelGuard NEMA 4X Enclosure.  The Leak Detection Module's LED display provides a linear reading of the concentration of combustible gas the the sensor is seeing, displayed as a % of the Lower Explosion Limit (LEL).  See FuelGuard Drawing.

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.

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.

Mobile refers to a continuous-monitoring instrument mounted on a vehicle such as, but not 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.

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

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.

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

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.

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.

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

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.

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)

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

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. 

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.

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.

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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