They work for one gas at a time, not for sewer gas as a whole. Sewer gas is a mix. A hydrogen sulfide meter reads the rotten-egg gas in ppm, a combustible gas sniffer reads methane, and an oxygen sensor reads low air. A detector tells you a gas is there. It does not find the leak.
Sewer gas is a mix, so detectors pick one part
Wisconsin's health department describes sewer gas as a mix of toxic and nontoxic gases from decaying waste. The main parts are hydrogen sulfide and ammonia (the highly toxic ones), plus carbon dioxide, methane, nitrous oxides and sulfur dioxide. Each of those behaves differently, and a sensor that reads one usually ignores the rest.
Here is what the common detector types actually measure:
| Detector type | What it reads | Units | What it tells you |
|---|---|---|---|
| Hydrogen sulfide (H2S) meter | The rotten-egg gas | Parts per million (ppm) | Whether the toxic part of sewer gas is in the air, and how much |
| Combustible gas sniffer or meter | Methane and other burnable gases | Percent of the lower explosive limit (%LEL) | Whether a burnable gas is building up. Useful for fire risk, not for a faint smell |
| Oxygen sensor | Oxygen level | Percent | Whether a gas like methane is pushing out breathable air in a closed space |
| Multi-gas meter | Usually H2S, combustible gas, oxygen and carbon monoxide | Mixed | The tool professionals use before entering a pit, vault or manhole |
| Detector tubes | One chemical per tube | ppm | A one-time spot reading |
OSHA lists detector tubes, direct-reading gas monitors, alarm-only gas monitors and explosion meters as examples of equipment for testing air where hydrogen sulfide may be present.
### What most "sewer gas detectors" for sale really are
Read the spec sheet before you trust the label. If it lists "%LEL", "combustible gas" or "natural gas, propane, methane", it is a combustible gas sniffer. It reacts to methane, which NOAA describes as a colorless, odorless gas that is lighter than air. It is built to flag burnable gas in amounts measured as a share of the explosive limit, and NIOSH puts hydrogen sulfide's lower explosive limit at 4 percent, or 40,000 ppm. Some people smell the rotten-egg gas at 0.01 to 1.5 ppm, per OSHA. So a sniffer can stay quiet while a room still smells.
If you want to know whether the smell itself is at a harmful level, you need a sensor that reads hydrogen sulfide in ppm. Your nose often notices it below the level where OSHA lists the first health effects, 2 to 5 ppm with prolonged exposure. That's useful: the smell is an early hint, and the meter tells you how serious it is.
Why your nose is not the detector
OSHA is blunt about this: do not rely on your sense of smell to warn you of harmful levels. After a while you stop smelling hydrogen sulfide even though it is still there, and at high levels the sense of smell can shut off almost at once. A room that smelled strong and now smells like nothing is not proof the gas is gone. That is the best argument for a real H2S meter in places where the gas can build up, like a sewage ejector room or a lift station wet well.
### A carbon monoxide alarm is not a sewer gas alarm
A plug-in or ceiling CO alarm watches for carbon monoxide. Carbon monoxide is not on Wisconsin's list of sewer gas parts. Wisconsin notes that carbon monoxide and methane are odorless and can cause headaches, dizziness and nausea just like hydrogen sulfide, which is why symptoms alone can't tell you which gas you're dealing with. Keep your CO alarms, and don't count on them to catch sewer gas.
How to trust a meter's reading
Trust a meter only if it has been checked recently against a known gas. Gas sensors drift. OSHA's bulletin on direct-reading monitors says every sensor's reference point shifts over time, so a drifted meter still reacts to gas but shows the wrong number. OSHA passes along the safety equipment industry's advice to run a bump test (a quick check with a known gas that confirms the alarms work) or a calibration check before each day's use, and to fully calibrate a meter that fails. A meter that sat in a truck for a year without either is a guess, not a reading.
### Where to put a meter
Placement matters because the gases in sewer gas don't mix evenly. NIOSH puts hydrogen sulfide's density at 1.19 times that of air, and OSHA says it collects in low, enclosed, poorly ventilated areas. NOAA puts methane at 0.55, so it rises. A few practical rules follow:
- For hydrogen sulfide, read low. Hold the meter near the floor drain, the base of the toilet or the lid of an ejector pit. A reading taken at eye level in the middle of a room can miss a layer near the floor.
- For methane, read high. A combustible gas sensor belongs near the ceiling or the top of a closed space.
- Read where people breathe, too. A pocket reading at the drain tells you the source. A reading at desk or bed height tells you what people are actually breathing.
### Portable meter or fixed alarm?
A portable meter is for finding and checking: you carry it to the drain, the pump room or the pit lid. A fixed alarm stays mounted in one spot and sounds when a set level is reached. OSHA's list of equipment covers both, from direct-reading monitors to alarm-only monitors. A fixed alarm only makes sense in a room where gas can build up day after day, like a sewage ejector room. In a bathroom, the right fix is closing the leak. Our page on whether you need a hydrogen sulfide detector walks through that call room by room.
A detector finds the gas, a plumber finds the path
Even a perfect reading only tells you that gas is in the room. The fix is finding how it got in. Wisconsin's health department says to follow the odor to its entry point, refill floor drains with water, check the roof vent for debris, and hire a plumber when the leak is behind a wall. The usual paths are a dried-out trap, a blocked or broken vent, a failed toilet seal, a loose cleanout cap or a cracked drain line. When none of those is obvious, a smoke test is the tool that shows the path: smoke forced into the drain and vent system comes out wherever sewer gas can.
- For a smell near a bathroom drain, start with why a bathroom drain smells like sewer.
- For a dry trap, see what a P-trap does.
- For a whole building, see sewer gas smell in a commercial building.
When to skip the detector and leave
If the smell is strong, if anyone feels dizzy, sick or has stinging eyes, or if you can't tell whether it's sewer gas or natural gas, don't stand there taking readings. Get out and call 911. Our page on the first signs of sewer gas poisoning lists the symptoms by level, and how to tell a gas leak from sewer gas covers the rotten-egg question.
HQ Plumbing is licensed and insured and offers 24/7 emergency service. We smoke test drain and vent systems with liquid-smoke equipment that forces smoke through under air pressure, then fix the trap, vent, seal or line it points to.