A bump test is a quick check that exposes a gas detector to a known gas to prove its sensors respond and its alarms go off. A calibration adjusts the detector so its readings match a known concentration of gas. Both are needed: the bump test tells you the detector works today, and calibration keeps its readings accurate over time.
Why detectors need checking
Gas sensors are chemical and electronic devices that change with age and use. Over time they drift, lose sensitivity or fail outright, and the display can still look normal. Common causes include:
- Normal ageing of electrochemical sensors used for oxygen and toxic gases
- Exposure to high concentrations of the target gas
- Poisoning or inhibition of catalytic flammable gas sensors by silicones, lead compounds, some sulphur compounds and other substances
- Extreme heat, cold, humidity and moisture
- Physical damage from drops, dust and dirt blocking the sensor openings
A detector that fails silently gives a worker false confidence in an atmosphere that may be lethal. That is why routine testing is a core part of any H2S or confined space program.
Bump testing
A bump test, sometimes called a functional test, exposes the detector to a test gas at a concentration high enough to trigger its alarms. It confirms that gas can reach the sensors, that each sensor responds, and that the audible, visual and vibrating alarms work. It does not adjust the readings.
How often
Many manufacturers recommend a bump test before each day's use, and many employers and industry practices require it. Follow the manufacturer's instructions for your model, your program and any provincial requirements, and choose the more frequent where they differ. Also bump test after a drop, after exposure to a high concentration or a poison, and whenever you doubt the readings.
How it is done
- Turn the detector on in fresh air and let it complete its start-up and self-test.
- Check the display, battery, sensor filters and housing for damage.
- Apply the test gas using the regulator and cap or docking station specified by the manufacturer.
- Confirm each sensor responds within the expected time and that all alarms activate.
- Let the readings return to normal in fresh air before use.
- Record the result, or let the docking station log it.
If the detector fails, take it out of service and calibrate it. If it fails again after calibration, have it serviced or replaced.
Calibration
Calibration compares the detector's readings to a known concentration of gas and adjusts it to match. A full calibration usually includes a zero, or fresh-air setting, followed by a span adjustment using certified calibration gas.
How often
The manufacturer specifies a maximum calibration interval for each model. Calibrate whenever a bump test fails, after a sensor is replaced and whenever the readings are in doubt. Some employers calibrate more often than the minimum, particularly in harsh conditions.
Doing it properly
- Use calibration gas that matches the manufacturer's specification for concentration and gas mix.
- Check the expiry date on the gas cylinder. Reactive gases such as hydrogen sulphide have a limited shelf life.
- Use the correct regulator, flow rate and tubing. Some reactive gases need specific tubing materials.
- Zero the detector only in air known to be clean.
- Have calibration done by a trained person or an authorized service centre.
Bump test and calibration side by side
| Bump test | Calibration | |
|---|---|---|
| Purpose | Proves sensors respond and alarms work | Adjusts readings to match a known concentration |
| Changes the detector? | No | Yes, sets zero and span |
| Typical frequency | Commonly before each day's use; follow the manufacturer and your program | At the manufacturer's maximum interval, and after a failed bump test |
| Who does it | Each trained user, or a docking station | A trained person, docking station or service centre |
| What to record | Date, detector, pass or fail, person | Date, detector, gas lot and expiry, readings before and after, person |
A typical case
A contractor's crew uses a docking station in the site trailer. The station reports every detector as passed, but during an audit someone notices that the test gas cylinder on the dock expired some months earlier. Reactive gases such as hydrogen sulphide can lose concentration over time, so the station may have been applying less gas than it assumed. The fix is to replace the cylinder, calibrate the fleet with in-date gas, and add the cylinder expiry date to the weekly site inspection checklist. The lesson is that automated records are only as reliable as the gas and the setup behind them.
Common mistakes
- Bump testing at the end of the shift to fill in the log, instead of before use
- Holding a test gas cap loosely so gas escapes before reaching the sensors
- Zeroing a detector in a truck or building where the air is not known to be clean
- Sharing one detector between several workers in different locations
- Leaving a failed detector in the charging rack where someone else picks it up
Docking stations help, but only if used. Automated docking stations can bump test, calibrate and keep records with little effort, and many lock out a detector that has missed a test. They work only if every detector goes into the dock as required and the gas cylinders on the station are kept in date.
Records and program practices
Keep a record for each detector showing its serial number, bump test and calibration dates and results, sensor replacements, and who did the work. Records show that your detectors were working on the day of an entry, help you spot sensors that are wearing out, and support audits and investigations. Good practices include:
- A clear rule in your program for bump test and calibration frequency
- A tag or display indicator showing the next calibration due date
- A place to quarantine detectors that fail
- Training for every user on bump testing their own detector
- Spare detectors so a failed unit does not tempt someone to work without one
- Regular checks of fixed gas detection systems by qualified people, as the manufacturer requires
Bump test results are also worth recording on confined space permits. See Atmospheric testing and gas detection and Hydrogen sulphide (H2S) hazards for how detectors are used in practice.
How ehZee can help
As part of H2S and confined space work, we review your detector fleet, testing practice and records, and write clear bump test and calibration rules into your program. See our H2S Alive training service.
Common questions
Can we skip the bump test if the detector was calibrated recently?
No. A sensor can fail between calibrations. The bump test is the only way to confirm the detector responds on the day you use it.
Is a self-test at start-up the same as a bump test?
No. The start-up self-test checks the electronics, battery and alarms, but it does not expose the sensors to gas, so it cannot confirm they respond.