Using a gas detector correctly comes down to a consistent routine: zero it in clean air before each use, understand what the readings mean, position it where gases actually collect, and keep it calibrated. Whether you’re entering a confined space, working near fuel lines, or monitoring an industrial facility, the steps below will help you get reliable readings and stay safe.
Startup and Fresh Air Zeroing
Every session with a portable gas detector starts the same way. Turn the unit on in a known clean-air environment, well away from vehicles, chemical storage, or any potential gas source. The instrument needs this baseline to measure deviations accurately. Most units will run through a self-test, checking the sensors, battery, and alarm functions automatically.
Once the self-test completes, the detector performs a “fresh air zero.” This tells the sensors that the current atmosphere is normal: about 20.9% oxygen and zero combustible or toxic gases. If you zero the device in contaminated air, every reading afterward will be wrong because the detector treats that contaminated baseline as normal. Some models zero automatically; others require you to press a button to confirm. Check your manual for the specific sequence, but the principle is universal: clean air first, then zero.
What the Readings Mean
Most multi-gas detectors display four channels at once: oxygen concentration, lower explosive limit (LEL), and one or two toxic gas readings, commonly carbon monoxide (CO) and hydrogen sulfide (H₂S). Each uses a different unit of measurement, and knowing the safe ranges is essential.
- Oxygen (O₂): Normal air is 20.9%. A low alarm typically triggers at 19.5%, and a high alarm at 23.5%. Below 19.5% you risk impaired judgment and, at much lower levels, unconsciousness. Above 23.5%, materials ignite more easily.
- LEL (combustible gases): Displayed as a percentage of the lower explosive limit, not a percentage of the atmosphere itself. A reading of 10% LEL means the air contains 10% of the concentration needed to ignite. Standard low alarm is 10% LEL; high alarm is 20% LEL. At 100% LEL, the air can explode with a spark.
- Carbon monoxide (CO): Measured in parts per million (ppm). The low alarm is typically set at 35 ppm, and the high alarm at 70 ppm. Prolonged exposure above 35 ppm causes headaches and dizziness.
- Hydrogen sulfide (H₂S): Also measured in ppm. Low alarm at 10 ppm, high alarm at 20 ppm. H₂S is particularly dangerous because at high concentrations it paralyzes your sense of smell, so you can’t rely on the familiar rotten-egg odor to warn you.
Understanding TWA and STEL Alarms
Beyond the instant “low” and “high” alarms, most detectors calculate two time-based exposure values. The TWA (time-weighted average) tracks your cumulative exposure over an 8-hour shift. For carbon monoxide, the TWA alarm is set at 35 ppm, meaning if your average exposure across the workday hits that level, the detector alerts you even if no single reading spiked high enough to trigger an instant alarm.
The STEL (short-term exposure limit) works over a rolling 15-minute window. It catches brief but intense exposures that the TWA might smooth over. For H₂S, the STEL alarm is 15 ppm. For CO, it’s 200 ppm. If you see a STEL alarm, it means you’ve been exposed to a concentrated burst that could cause harm even if it didn’t last all day. Leave the area immediately when either alarm activates.
Where to Position the Detector
Gas behavior depends on density relative to air, and positioning your detector incorrectly can mean it never picks up a hazard sitting two feet above or below it.
Heavier-than-air gases, like propane and hydrogen sulfide, sink and pool near the floor. When monitoring for these, hold or mount the sensor 6 to 12 inches (15 to 30 cm) from the ground. Lighter-than-air gases, like methane and hydrogen, rise toward the ceiling, so the detector should be positioned high. Carbon monoxide has nearly the same density as air, so it disperses evenly. Place the sensor in the breathing zone, roughly 4 to 6 feet (1.2 to 1.8 meters) from the floor, where you’re actually inhaling.
For confined space entry, always test the atmosphere before you go in. If your detector has a remote sampling pump or you have an external pump with draw tubing, lower the probe to the bottom of the space first, then pull it upward in stages. This catches both heavy gases that settled at the bottom and lighter gases collecting at the top. Never assume a single reading at the opening represents conditions at the bottom of a tank or pit.
Using a Sampling Pump
Many portable detectors can switch between diffusion mode (the air naturally flows across the sensor) and pump mode, where an internal or external pump draws air through tubing. Pump mode is what you need when you can’t physically reach the area you’re testing, like the interior of a vessel before entry.
When using detector tubes with a hand-operated sampling pump, snap both tips off the glass tube, then insert it into the pump inlet with the arrow on the tube pointing toward the pump body. Pull the pump handle out in one smooth motion until it locks, then release it. Wait about 30 seconds for the sample to draw through. After the sampling time, grasp the handle and rotate it 90 degrees. If the handle snaps back to its starting position on its own, the system is airtight and your reading is valid. If the handle stays extended by 5 mm or more, there’s a leak, and the reading won’t be accurate. Check the tube seal and try again.
With electronic sampling pumps built into the detector, you simply attach tubing to the inlet port and turn the pump on. Keep in mind that longer tubing means a longer delay before the reading stabilizes, since the air takes more time to travel from the sample point to the sensor. A common rule of thumb is about one second of delay per foot of tubing.
Bump Tests and Calibration
A bump test is a quick functional check. You expose the detector to a known concentration of test gas and confirm the alarms trigger. According to New York State Department of Labor guidelines, you should bump test before each day’s use. The test takes about a minute and tells you the sensors are actually responding. It does not adjust the readings.
Full calibration is more thorough. You expose each sensor to a certified concentration of gas and the instrument adjusts its readings to match that known value. Calibration frequency depends on the manufacturer’s recommendations, but monthly intervals are common in many workplaces. If a detector fails a bump test, you must fully calibrate it before using it again. If it can’t pass calibration, take it out of service.
OSHA recommends keeping calibration records for the life of each instrument. This documentation protects you during inspections and helps track sensor degradation over time. Most modern detectors store calibration logs internally, but backing them up on paper or in a database is good practice.
Common Mistakes That Lead to Bad Readings
Zeroing in contaminated air is the most frequent error. If you calibrate your baseline next to a running diesel engine, the detector will treat that CO level as zero, and every subsequent reading will be falsely low. Always move to a genuinely clean area, ideally outdoors and upwind from any equipment.
Ignoring sensor lifespan is another common problem. Electrochemical sensors for toxic gases typically last two to three years. Combustible gas sensors may last three to five years. As sensors age, they respond more slowly and less accurately. A detector that passed calibration six months ago might not pass today.
Relying on a single reading at the entrance of a confined space rather than sampling at multiple depths can miss stratified gas layers. And turning the detector off between readings to save battery means you lose continuous TWA and STEL tracking, which defeats half the purpose of wearing one.
Regulatory Requirements
Several OSHA standards specifically require gas monitoring: the permit-required confined spaces standard (29 CFR 1910.146), the hazardous waste operations standard (29 CFR 1910.120), and the grain handling facilities standard (29 CFR 1910.272). Beyond these, the General Duty Clause requires employers to protect workers from recognized hazards, which in practice means gas detection is expected wherever airborne chemical or oxygen-deficiency risks exist.
OSHA recommends that employers develop written standard procedures for calibrating and using gas monitors, including documentation verifying proper maintenance. While the recommendation itself isn’t a citation-worthy regulation, the absence of a monitoring program in a recognized hazard environment can still result in a General Duty Clause violation. Keeping a simple log of bump tests, calibrations, and any failed readings gives you a clear record that the instruments were functioning properly each day they were used.