Bump testing confined space gas detectors

The International Safety Equipment Association recommends, at a minimum, verification of sensor accuracy before each day's use. Additional testing of instrumentation should be carried out before each new confined space entry – after the crew's lunch hour, for example.

The only way to guarantee an instrument will detect gas accurately and reliably is to test it with a known concentration of gas. Exposing the instrument to a known concentration of test gas will show whether the sensors respond accurately and the instrument alarms properly.

There are two methods of verifying instrument accuracy: a functional or bump test and a full calibration. Each is appropriate under certain conditions. A bump test verifies calibration by exposing the instrument to a known concentration of test gas. The instrument reading is compared to the actual quantity of gas present, as indicated on the cylinder. If the instrument's response is within an acceptable tolerance range of the actual concentration, then its calibration is verified. It is recommended that users check with the gas detection equipment manufacturer for the acceptable tolerance ranges.

Instruments should be "zeroed" before the bump test to give a more accurate picture of the bump test results. Also, the test should be conducted in a clean, fresh air environment. When performing a bump test, the test gas concentration should be high enough to trigger the instrument alarm. If the instrument fails a bump test, it must be adjusted through a full calibration before it is used.

During a period of initial use of at least 10 days in the intended atmosphere, calibration should be verified daily to ensure nothing is in the atmosphere to poison the sensors. The period of initial use must be of sufficient duration to ensure the sensors are exposed to all conditions that might adversely affect the sensors. If the tests demonstrate that no adjustments are necessary, the interval between checks may be lengthened, but it should not exceed 30 days. If the instrument fails a bump test, it must be adjusted through a full calibration before it is used. When calibrating an instrument, always follow the manufacturer's recommended calibration frequency and procedure.