In its technical amendment to the (confined space) rule on May 19, 1994, Federal OSHA modified the Appendix E (Sewer Entry) language to read as follows:

" The oxygen sensor/broad range sensor is best suited for initial use in situations where the actual or potential contaminants have not been identified, because broad range sensors, unlike substance specific sensors, enable employers to obtain an overall reading of the hydrocarbons (flammables) present in the space."

The OSHA technical amendment refers to the solid state Metal Oxide Semiconductor (MOS) element as the technology for broad range monitoring. The MOS sensor technology is NOT the same as a pellistor, hot-wire or catalytic element. Although usually non-linear and consuming higher power than other sensors, the broad range sensor (MOS style) has the unique ability to dramatically increase conductivity (reduce its resistance) in the presence of only a few hundred parts per million of scores of CFC's, HCFC's, and VOC's. Since chlorinated solvents, cleaning agents, hydrocarbon fuels and organic compounds can frequently be found in sewers at or above TLV levels, the MOS or "broad range" sensor serves a unique role in screening sewer atmospheres for these possible contaminants as OSHA discussed in their technical amendment.

Catalytic sensors fail to respond to some of these materials or only respond at much higher levels that approach a significant % of the flammable level (2500 to 5000 ppm or more). Electrochemical sensors are designed to operate as substance specific cells and NOT broad range sensors.

If you wish to confirm your gas detectors operation technology in reference to the use of a metallic oxide semiconductor, the manufacturer must state, in writing, that:

"The Model 'XYZ' portable gas detector as manufactured by 'ABC Corporation' uses a solid state (metallic oxide semiconductor) broad range sensor in the manufacture of this instrument."

The Model CGS 90R portable gas detector as manufactured by Enmet Corporation uses a solid state (metallic oxide sensor) broad range sensor in the manufacture of this instrument.

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