1. Gas sensors can fail without warning. Electrochemical sensors are similar to a battery in design and can fail without warning.  Catalytic combustible sensors can be *poisoned by gases they see in the environment. Although gas meters have on board diagnostics, the gas meter will not warn you, if the sensor has lost sensitivity to gas.  The only way to determine a failed or damaged sensor is to perform a **bump test or calibration.

2. It’s the law. OSHA’s Safety and Health Information Bulletins SHIB 05-04-2004 states “Employers should keep calibration records for the life of each instrument. This record enables operators to quickly identify a ***DRPGM that has a history of excessive maintenance/repair, or is prone to erratic readings, and to track drift of the sensors to determine when they need replacement.”   OSHA 1910.146(d)(4) states that an employer must provide equipment that is properly maintained. 

3. Pump not working. Some gas meters monitor the pumps current draw and indicate low air flow alarm if the current draw is high.  If a leak exists in the pump, hose or filter then a blockage will not cause a high pump current condition or a low flow alarm.

4. Defective alarms. When bump testing and calibrating you gas meter, make sure that both the audible, visual and vibration (if so equipped) alarms work.

 

5. Slow response. Reaction or response time slows as a sensor ages. These meters are designed to provide alarms quickly (usually under 15 seconds).  A slow responding sensor may not warn personnel of a dangerous condition quickly.

 

6. Non-repeatable results. If there is air leakage, the gas sensors will not be exposed to consistent airflow. After every calibration you should bump test the gas meter to insure the gas readings are repeatable.

7. Dirty filters.Dirty or blocked filters will prevent gas from reaching a sensor.

8. Contamination. Contamination of the hose and filters can remove (scrub) the gas you are trying to detect. For example: Hydrogen Sulfide (H2S) gas reacts with water.  If there is water in the hose or filter, H2S gas may not reach the sensor.

 

9. Low response. Air leakage will dilute the air sample, resulting in little or no response.  Many sensors measure gas in the very sensitive PPM gas range.  Any dilution of the air sample will greatly diminish a PPM gas reading.

 

10. Out of Calibration. As sensors age, they lose their sensitivity to gas. A calibrated meter compensates for the sensors deterioration by amplifying the sensors signal output.

 

  *Poisoning: Some gases can permanently damage a catalytic sensor, causing it to lose sensitivity to gas.

 

**Bump Test (or Function Check) – OSHA defines a bump test as “a qualitative function check in which a challenge gas is passed over the sensor(s) at a concentration and exposure time sufficient to activate all alarm settings.

   The purpose of this check is to confirm that gas can get to the sensor(s) and that all the instrument’s alarms are functional. The bump test or function check does not provide a measure of the instrument’s accuracy. When performing a bump test, the challenge gas concentration should trigger the DRPGM’s alarm(s).”

***DRPGM: Direct Reading Personal Gas Meter

Responsibility for the use of any and all information contained in this article is strictly and solely that of the user.