Bump testing and calibrating gas detectors.

What is a bump test?

A bump test is a quick check that a gas detector still works. You expose the detector to a known gas, briefly, and confirm each sensor responds and every alarm goes off. It proves gas can reach the sensors and the alarms work. It does not prove the readings are accurate. Most crews bump test with a 4 gas calibration gas before each day’s use.

What is LEL?

LEL stands for lower explosive limit: the lowest concentration of a flammable gas in air that can ignite. Below it the mix is too lean to burn. Gas detectors read flammable gas as a percent of the LEL. Methane’s LEL is about 5% by volume in air, so 50% LEL methane is 2.5% methane by volume, the concentration used in most methane calibration gas and 4 gas mixes. OSHA treats a confined space as hazardous above 10% of the LEL.

Bump test vs calibration: what is the difference?

A bump test checks function: do the sensors respond and do the alarms sound? A calibration adjusts the detector so its reading matches a known concentration of gas. A calibration check sits in between: it compares the reading to the test gas without adjusting it. If a detector fails a bump test or calibration check, give it a full calibration before using it again. If it fails the calibration, take it out of service.

How often should a gas detector be calibrated?

Follow the detector manufacturer’s schedule. OSHA’s bulletin on calibrating portable gas monitors (SHIB 11-26-2024) recommends a bump test or calibration check before each day’s use, and a full calibration whenever a detector fails one. Test more often after a sensor is exposed to high gas levels, after a drop or other damage, after a sensor change, or when the detector changes hands.

How do you calibrate a gas detector?

1. Check the calibration label or manual for the gas, concentration and flow rate your detector needs.
2. Zero the detector in clean air, or with zero air if the area is not clean.
3. Attach a regulator that fits the cylinder valve and gives the right flow, then connect the calibration cup or tubing.
4. Start the detector’s calibration mode and apply the gas until the readings settle.
5. Let the detector adjust (span) to the cylinder values, then confirm each sensor passed.
6. Record the date, cylinder lot number and results. OSHA recommends keeping calibration records for the life of each instrument.

What is calibration gas?

Calibration gas is a mixture with a known, certified concentration of one or more gases, balanced in air or nitrogen. Detectors are tested and adjusted against it. Use gas that is certified and traceable to NIST standards, within its expiration date. McNeill ships NIST-traceable calibration gas in single gas and multi gas mixes.

What is span gas?

Span gas is the calibration gas used to set the upscale reading of a detector: the known concentration the detector is adjusted to match. Zero gas, usually zero air or nitrogen, sets the zero point. A calibration uses both.

What are OSHA’s bump test requirements?

No OSHA standard sets a bump test schedule. The confined space standard (29 CFR 1910.146) requires the atmosphere to be tested with a calibrated direct-reading instrument before entry: oxygen first, then flammable gases, then toxic gases. OSHA’s bulletin SHIB 11-26-2024 recommends a bump test or calibration check before each day’s use, following the manufacturer’s instructions. The bulletin is advisory, but it is how OSHA explains what “calibrated” means.

How do you dispose of calibration gas cylinders?

1. Empty the cylinder fully: run it through a regulator in a well-ventilated area until no gas flows.
2. Mark it “Empty”.
3. Recycle it as scrap metal where local rules allow. Some recyclers want the valve removed or the cylinder punctured with a cylinder recycling tool first.

Cylinders that held toxic or reactive gas, such as ammonia, chlorine or phosphine, may need to go through hazardous waste. Check the cylinder’s SDS and your local rules. Questions about a specific cylinder? Ask McNeill.