Every permit-space entry depends on a direct-reading gas monitor, and every monitor drifts. Sensors age, get poisoned, get wet or get dropped. Two checks catch problems before they hurt someone.
Bump test vs. calibration
| Bump test (functional check) | Calibration | |
|---|---|---|
| What it does | Exposes sensors to a known gas long enough to confirm they respond and the alarms activate | Adjusts the instrument's readings to match a known concentration of test gas |
| Changes the settings? | No | Yes |
| Time | Under a minute, often in a docking station | A few minutes per instrument |
| When | Before each day's use (recommended) | Per the manufacturer's schedule, and whenever a bump test fails |
A calibration check is similar to a bump test but checks the reading against the known gas concentration within a tolerance, without adjusting.
What OSHA recommends
OSHA's Safety and Health Information Bulletin on verifying calibration of direct-reading portable gas monitors recommends a bump test or calibration check before each day's use, in accordance with the manufacturer's instructions. If the instrument fails, it should be fully calibrated, and if it fails calibration, it should be removed from service and the manufacturer contacted. The confined space standard itself requires testing with a calibrated direct-reading instrument (1910.146(c)(5)(ii)(C)).
Step by step: a daily bump test
- Turn the monitor on in fresh air and let it warm up and zero.
- Check the calibration gas cylinder: correct gases and concentrations, and not expired.
- Apply the gas through the cap, hose or docking station at the specified flow rate.
- Confirm each sensor responds and the audible, visual and vibrating alarms activate.
- Let the readings clear in fresh air.
- Record the result (date, instrument, pass or fail, person). Docking stations keep this automatically.
- If anything fails, calibrate. If calibration fails, tag it out.
Sensor types and what fools them
- Catalytic bead (LEL) sensors need oxygen to work and can be poisoned by silicones, lead compounds and some sulfur compounds, or inhibited by H2S. Very high concentrations of flammable gas above the upper explosive limit can also produce false low readings. Some monitors use infrared (IR) LEL sensors, which don't need oxygen and resist poisoning but don't detect hydrogen.
- Electrochemical sensors (oxygen, H2S, CO) age over time, can dry out in heat or flood in humidity, and may show cross-sensitivity to other gases. For example, hydrogen can affect some CO sensors.
- Photoionization detectors (PIDs) measure many volatile organic compounds but need correction factors for the specific chemical.
Sampling tips that matter as much as calibration
- Use the sampling pump and hose for pre-entry testing; diffusion monitors alone can't reach into the space.
- Allow time for the sample to travel the hose length, per the manufacturer.
- Keep the hose clean and dry, and check the pump for a blocked inlet.
- Test at multiple levels for stratified gases.
Sources and official references
Last reviewed: October 25, 2026 · Free general education based on OSHA 29 CFR 1910.146 and 29 CFR 1926 Subpart AA. It does not replace the site-specific training, written program and hands-on evaluation your employer must provide, and it does not issue a certificate. Nothing you enter on this page is saved or sent.