Combustible gas indicator

A concentration of 35,000 ppm in air produces unconsciousness in 30—40 minutes. This concentration also constitutes a serious fire and explosion hazard, and should not be permitted to exist under any circumstance. Any person exposed to ethyl ether vapor of any appreciable concentration should be prompdy removed from the area. Recovery from exposure to sublethal concentrations is rapid and generally complete. Except in emergencies, and then only with appropriate protective equipment, no one should enter an area containing ether vapor until the concentration has been found safe by measurement with a combustible-gas indicator. 

The combustible gas indicator must be cahbrated using the appropriate cahbratinggas, such as methane or pentane. 

Direct reading samplers include simple devices such as colorimetric indicating tubes in which a color change indicates the presence of the contaminant in air passed through the tube, or instruments which are more or less specific for a particular substance. In the latter category are carbon monoxide indicators, combustible gas indicators (explosimeters) and mercury vapor meters, as well as a number of other instruments. 

Several manufacturers make explosimeters or combustible gas indicators. Although they differ somewhat in design and operating features, their operation is based on the fact that a measurable amount of heat is released when a combustible gas or vapor is burned. Most meters contain a battery-operated electrical circuit known as a Wheatstone bridge, which is balanced by means of controls on the outside of the instrument. 

On the simplest type of instrument (an explosimeter) only one scale is provided, usually with readings from 0 to 100% LEL. However, the detectable changes produced by combustion are too small to be measured accurately in the presence of the low concentrations of contaminants usually encountered in evaluating potential health hazards. For example, the LEL of even the most explosive gas is of the order of 1 %, or 10,000 ppm, which is well in excess of the toxic limit for any gas. Therefore, explosimeters or combustible gas indicators which have only a 0-to-l(X)% LEL explosive scale are not suitable for environmental health testing in the ppm range. More sensitive instruments, including the type used in sampling for environmental health purposes, have a dual scale, in which the second, more... 

The manufacturer s instructions for operating a combustible gas indicator should be carefully reviewed before the device is used. In general, all explosimeters require a brief initial warm-up period so that the batteries can heat the filaments. Then battery strength should be checked, and the zero scale adjusted. When zero is established and the indicating needle is stable, the instrument is ready for use. 

A.C.G.I.H., "Manual of Recommended Practices for Combustible Gas Indicators and Portable Direct Reading Hydrocarbon Detectors, A.C.G.I.H Cincinnati, Ohio, 1980. 

Detection of Arsenical Blood Agents with PIDs or FIDs may be possible. In addition, because of the high flammability of the agents, combustible gas indicators can be used to screen for agents. However, the detection limits will be far in excess of lethal limits. Detection and identification with FT-IR is possible provided that the appropriate reference spectra are available. 

Methane Anaerobic bacterial action Combustible-gas indicator... 

Direct Reading Instruments — Combustible Gas Explosimcters one of the most uselul instruments of the direct reading type, also known as a combustible gas indicator. As the names suggest, instruments of this type were designed to detect the presence of explosive or combustible gases in the air. Safety checking is still their principal application, and many of them are suitable only for this purpose. 

The user of any instrument should be thoroughly familiar with precautions to be taken in its operation users of combustible gas indicators must also be aware of interfering gases and vapors which can create major aberrations in instrument response. One such precaution is that the 0-to-100% scale should be used first, to determine whether an explosive atmosphere exists and to prevent overloading to O-to-10% LEL scale. For example, note the typical meter responses to methane gas shown in Figure 17, particularly those at the LEL, in the explosive range, and above the UEL. If the pointer of the meter travels into the red portion of the scale and remains there, an explosive concentration is present. However, if the pointer climbs rapidly to the red area and then falls back to zero, there is either a concentration above the UEL or a gas mixture which lacks sufficient oxygen to support combustion. Whatever the reason, if the pointer touches the red portion of the scale, the gas tester should leave the area immediately. 

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