COMBUSTIBLE GAS METERS FOR SAFETY ENGINEERING
What Are Combustible Gas Meters?
These meters use elements which are made of various materials such as platinum or palladium as an oxidizing catalyst. The element is one leg of a Wheatstone bridge circuit. These meters measure gas concentration as a percentage of the lower explosive limit of the calibrated gas.
The oxygen meter displays the concentration of oxygen in percent by volume measured with a galvanic cell. Other electrochemical sensors are available to measure carbon monoxide, hydrogen sulfide, and other toxic gases. Some units have an audible alarm that warns of low oxygen levels or malfunction.
Calibration of Combustible Gas Meters
Before using the monitor each day, calibrate the instrument to a known concentration of combustible gas (usually methane) equivalent to 25%-50% LEL full-scale concentration.
The monitor must be calibrated to the altitude at which it will be used. Changes in total atmospheric pressure from changes in altitude will influence the instrument's measurement of the air's oxygen content. The unit's instruction manual provides additional details on calibration of sensors.
Special Considerations.
• Silicone compound vapors, leaded gasoline, and sulfur compounds will cause desensitization of the combustible sensor and produce erroneous (low) readings.
• High relative humidity (90%-100%) causes hydroxylation, which reduces sensitivity and causes erratic behavior including inability to calibrate.
• Oxygen deficiency or enrichment such as in steam or inert atmospheres will cause erroneous readings for combustible gases.
• In drying ovens or unusually hot locations, solvent vapors with high boiling points may condense in the sampling lines and produce erroneous (low) readings.
• High concentrations of chlorinated hydrocarbons such as trichloroethylene or acid gases such as sulfur dioxide will depress the meter reading in the presence of a high concentration of combustible gas.
• High-molecular-weight alcohols can burn out the meters filaments.
• If the flash point is greater than the ambient temperature, an erroneous (low) concentration will be indicated.
If the closed vessel is then heated by welding or cutting, the vapors will increase and the atmosphere
may become explosive.
• For gases and vapors other than those for which a device was calibrated, users should consult the manufacturer's instructions and correction curves.
Maintenance of Combustible Gas Meters
The instrument requires no short-term maintenance other than regular calibration and recharging of batteries. Use a soft cloth to wipe dirt, oil, moisture, or foreign material from the instrument. Check the bridge sensors periodically, at least every six months, for proper functioning.
A thermal combustion-oxygen sensor uses electrochemical cells to measure combustible gases and
oxygen. It is not widely used in the area offices.
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