Gas Detectors & Bump Testing
Many deadly gases have no color or smell, and your nose can’t be trusted even with the ones that do. A portable gas detector is the only way to know whether the air in a confined space, trench, tank or plant is safe to breathe. But a detector only protects you if it’s working, tested and used the right way.
- Construction
- General Industry
- Utilities
- Oil & Gas
What a 4-gas monitor measures
The most common portable monitor checks four things:
| Reading | Why it matters | Typical alarm settings |
|---|---|---|
| Oxygen (O2) | Normal air is 20.9% oxygen. Low oxygen causes confusion, collapse and death. High oxygen makes things burn explosively. | Low alarm 19.5%; high alarm 23.5% |
| Combustible gas (LEL) | Shows how close the air is to the lower explosive limit of flammable gases like methane, propane and gasoline vapor | Low alarm usually 10% of the LEL |
| Carbon monoxide (CO) | From engines, generators, heaters and fires. Odorless and toxic. | Often 35 ppm (some set 25 ppm) |
| Hydrogen sulfide (H2S) | From sewers, manure, oil and gas, and pulp mills. Kills your sense of smell at high levels. | Often 10 ppm (some set 1 to 5 ppm) |
Alarm settings are set by your employer, based on regulations, exposure limits and the manufacturer. Know what your monitor’s alarms mean before you use it.
Other types of detectors
- Single-gas monitors: often worn all shift for CO or H2S.
- Photoionization detectors (PIDs): measure solvents, fuels and other volatile organic compounds at low levels.
- Specialty sensors: ammonia, chlorine, sulfur dioxide, hydrogen cyanide, nitrogen dioxide and others for specific plants and processes.
- Area monitors: larger units set up to watch a whole work area and sound an alarm for everyone.
- Fixed detectors: permanently mounted in plants, garages and refrigeration rooms.
Bump testing vs. calibration
| Bump test | Calibration | |
|---|---|---|
| What it is | A quick check that exposes the sensors to test gas to make sure they respond and the alarms go off | Adjusting the sensors so their readings match a known concentration of test gas |
| How often | Before each day’s use (industry recommendation) | As the manufacturer requires, and whenever a bump test fails |
| Takes | About a minute | A few minutes |
Sensors can fail without warning. They can be damaged by drops, water, extreme temperatures, or poisoned by silicones, lead and other chemicals. A monitor that hasn’t been bump tested may show “all clear” in deadly air. The International Safety Equipment Association recommends a bump test before each day’s use.
Before you use it
- Check that the battery is charged and the monitor turns on and passes its self-test.
- Make sure calibration and bump test dates are current.
- Zero it in fresh, clean air, away from exhaust and the work area.
- Bump test it with the test gas, and confirm every sensor and alarm (sound, light and vibration) works.
- Check the filter, inlet and sampling tubing for dirt, water or cracks.
- Record the test, if your program requires it.
Testing a confined space or trench
- Test from outside first, using a sampling pump and tube, before anyone enters.
- Test in order: oxygen first, then combustible gases, then toxic gases. Most combustible sensors need oxygen to read correctly.
- Test at the top, middle and bottom. Some gases are heavier than air (H2S, propane, gasoline vapor) and collect low. Others are lighter (methane) and collect high. CO is about the same weight as air.
- Give the pump time: allow enough time for the sample to travel through the tube, usually a couple of seconds per foot plus the sensor response time.
- Keep monitoring while people are inside. Conditions can change as work goes on.
See Confined Spaces and Access, Egress, Spoils & Water.
Wearing a personal monitor
- Wear it in your breathing zone, within about 10 inches of your nose and mouth, such as on your collar or chest.
- Keep it on the outside of your clothing, not in a pocket or under a jacket.
- Don’t cover the sensor openings with tape, stickers or dirt.
- Keep it on all shift if you work around possible gas hazards.
When it alarms
- Leave the area immediately and go to fresh air. Don’t try to finish what you were doing.
- Warn others and keep them out.
- Tell your supervisor. Don’t go back in until the source is found, ventilated or controlled, and the air tests safe.
- Never assume it’s a false alarm. Investigate before re-entry.
Danger: Never enter a space to rescue someone who collapsed unless you are a trained rescuer with the right equipment, including supplied air. In many confined space deaths, more than half of the victims were would-be rescuers.
Things that fool gas detectors
- Low oxygen: catalytic combustible sensors read low or zero when oxygen is low, even if flammable gas is present.
- Cross-sensitivity: some sensors react to other gases. Hydrogen can make a CO sensor read high.
- Very high gas levels: some combustible sensors can go over range and then read low. Leave immediately on any over-range alarm.
- Sensor poisoning: silicone sprays, lead and some cleaners damage combustible sensors.
- Temperature, humidity and water: extreme conditions affect readings. Keep the inlet out of liquids.
Daily checklist
- Battery charged and self-test passed
- Zeroed in fresh air
- Bump test passed today, with every alarm working
- Calibration date is current
- Filters, inlet and tubing are clean and undamaged
- I know the alarm settings and what to do when it alarms
- Worn in my breathing zone, not covered
Standards & references
- 29 CFR 1910.146: Permit-required confined spaces (general industry)
- 29 CFR 1926.1203: Confined spaces in construction, general requirements
- 29 CFR 1926.651(g): Hazardous atmospheres in excavations
Related: Confined Spaces · Hydrogen Sulfide · Carbon Monoxide · Hot Work Permits
Last updated: