FLAMMABLE / COMBUSTIBLE GAS INDICATORS (CGI)


FIGURE 4.9 Combustible Gas Indicators (CGI).


Direct reading instruments are the most effective in determining flammable hazard zones, and one of the best is the Combustible Gas Indicators (CGI). This text is not a definitive technical guide to direct reading instruments, but a guide for understanding what the instruments are telling responders and what tactics might be employed. A multi-gas meter is an important tool in an emergency response to a natural gas emergency. A Combustible Gas Indicator can indicate if atmospheres are coming close to flammable and dangerous.


CGI Types and Operation


CGIs are designed to measure combustible gases and give the user a reading of flammability based on the specific concentration of gas in the air. As we saw in Chapter 3, the reading is usually a percentage of the LEL of the product (0%-100% of the LEL). There are several operating methods for meters. CGIs can use catalytic bead sensors, metallic oxide sensors (MOS) or Wheatstone bridges. Each technology has its advantages and disadvantages. A good source for additional information on Combustible Gas Indicators is Chris Hawley’s Hazardous Materials Incidents published by Delmar (2001).


Advantages


The best method for safety and relative accuracy is to calibrate before each use and never exceed the meter’s warning alarm (depending on the meter, usually 10% of the LEL). Staying below 10% of the LEL keeps the responder below potential explosive atmospheres. This procedure also allows an organ- ization to develop an effective “hot zone” or “exclusion zone” using 10% of the LEL as the zone delineation.


Disadvantages


Meters are only accurate for the gas to which they are calibrated. Using them on other gases does not make them useless or inoperable, but does mean that


59 STREET SMART TIP


Responders should not run into a building sounding like a pack of bloodhounds—sniff… sniff…sniffing as they go. Use the correct meter!!


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