11.2 Four-Gas Meters and Combustible Gas Indicators
Key Takeaways
- A typical fire-service four-gas meter reads oxygen, combustible gas as percent of LEL on a catalytic CGI, carbon monoxide, and hydrogen sulfide — four different sensors, not one “toxic meter.”
- OSHA 1910.120(a)(3) defines oxygen deficiency as less than 19.5 percent oxygen by volume; fresh air is about 20.9 percent. A drop toward 19.5 percent means tens of thousands of ppm of something else may already be in the air.
- A catalytic CGI reports percent of LEL for the calibration gas (often methane or pentane), needs oxygen to work, and can read low or false in an oxygen-deficient atmosphere. Correction factors apply when the fuel is known.
- Zero percent LEL does not mean the atmosphere is non-toxic. Never use a CGI as a toxic meter; CO and H2S electrochemical sensors have their own cross-sensitivities and still do not see every poison.
- Oxygen-enriched air increases fire intensity. Many meters alarm near 23.5 percent oxygen (the 1910.146 confined-space enrichment figure); 1910.120 itself defines deficiency at 19.5 percent and does not publish a matching enrichment percentage.
11.2 Four-Gas Meters and Combustible Gas Indicators
Quick Answer: A typical technician four-gas instrument reads oxygen (O2), combustible gas as percent of lower explosive limit (LEL) on a catalytic combustible gas indicator (CGI), carbon monoxide (CO), and hydrogen sulfide (H2S). Fresh air is about 20.9% oxygen. OSHA 1910.120(a)(3) calls less than 19.5% oxygen oxygen deficiency. The CGI reports % of LEL for the calibration gas (often methane or pentane), needs oxygen to work, and does not measure toxics. Never treat a CGI as a toxic meter.
NFPA 470 11.2.2(A) lists electrochemical cells (toxic-gas sensors), flammable gas/LEL, and oxygen concentration among the technologies you must be able to select, use, read, and document. OSHA 1910.120(q)(6)(iii)(B) is still the competency: field survey instruments to classify, identify, and verify. The four-gas is how most fire-based teams meet the oxygen / fire / two common toxics slice of that job. Photoionization is the next section. Colorimetric tubes and radiation instruments are Chapter 12. This section is the meter on every technician’s chest and why it lies if you only watch the LEL digit.
Four sensors, four stories
| Channel | Typical sensor | Units on the display | What a technician can honestly claim |
|---|---|---|---|
| O2 | Electrochemical oxygen | % by volume | Whether the air is near 20.9%, deficient (<19.5%, OSHA 1910.120), or enriched enough to change fire behavior |
| LEL / CGI | Catalytic bead (pellistor) on most four-gas kits; some meters also have infrared (IR) LEL | % of LEL (0–100), relative to the calibration gas | Whether a flammable atmosphere is developing as that fuel would look to this bead — not identity, not toxicity |
| CO | Electrochemical | parts per million (ppm) | CO and some cross-sensitive interferents |
| H2S | Electrochemical | ppm | H2S and some cross-sensitive interferents |
Those four numbers arrive together. They are not one verdict. A student who says “the four-gas was zero so the product is not toxic” has already failed 11.2.2.
Oxygen: 20.9%, 19.5%, displacement, and enrichment
Normal dry air is about 20.9% oxygen. OSHA 1910.120(a)(3) defines oxygen deficiency as atmospheres where oxygen by volume is less than 19.5% — the concentration below which atmosphere-supplying respiratory protection must be provided. That is the number to memorize for this exam. It is also the common low-oxygen alarm on a four-gas.
A drop from 20.9% to 19.5% is only 1.4 percentage points, but it is not a small contamination. If another gas is displacing air, that 1.4-point drop means on the order of 6–7% of the atmosphere is something else — tens of thousands of ppm. Many toxics are IDLH long before you lose 1.4 points of oxygen. Simple asphyxiants (nitrogen, carbon dioxide, some product vapors) can push you toward 19.5% while a CGI still reads 0% LEL if the displacer is not flammable, or while a catalytic CGI reads false-low if oxygen has already fallen too far.
Enriched oxygen is a fire problem: clothing, hair, and hydrocarbons burn hotter and easier. OSHA 1910.120 defines deficiency at 19.5% and does not print a matching enrichment percentage. 29 CFR 1910.146 (permit-required confined spaces) treats greater than 23.5% oxygen as a hazardous atmosphere, and many four-gas meters alarm near 23.5% on the high side. Teach 23.5% as a common instrument alarm and confined-space figure, not as “the 1910.120 enrichment number.” Oxidizer leaks, leaking medical-oxygen cylinders, and some chemical oxygen generators are how enrichment shows up on a hazmat scene.
Always read oxygen first among the four-gas channels. It tells you whether the CGI is in its operating envelope, whether SCBA is already required for asphyxiation, and whether you are about to add air to a space that may be fuel-rich.
LEL, UEL, and what a CGI actually displays
The lower explosive limit (LEL) — also lower flammable limit (LFL) — is the leanest mixture with air that will propagate flame. The upper explosive limit (UEL) / upper flammable limit (UFL) is the richest. Between them is the flammable range. Outside that range the mixture is too lean or too rich to burn until air or fuel changes.
A catalytic CGI does not print “percent gasoline.” It burns a sample on a heated bead and reports % of LEL as if the gas were the calibration gas. Methane and pentane are the usual span gases. Correction factors (manufacturer tables) convert that reading toward another known fuel. Two exam traps live here:
- Unknown fuel, no factor. The number is a relative indication, not a true %LEL of the mystery vapor.
- Methane calibration versus heavier solvents. A bead calibrated to methane often under-reads many solvent vapors — false security, the unsafe direction. A pentane or propane calibration is often more conservative for those solvents and can over-read methane. Know the cal gas written on the cylinder and in the IAP.
Catalytic beads need oxygen. Manufacturer data commonly put the floor around 10% oxygen by volume (check your manual; it is not an OSHA number). In an oxygen-deficient tank, nitrogen-purged rail car, or CO2-inerted space, LEL can read low or zero while the space is full of fuel. Introduce air during ventilation or entry and you may drive a too-rich atmosphere down into the flammable range. That is why oxygen-first is not a classroom nicety.
Above the UEL, a CGI that never saw the mixture pass through the flammable range can also look quiet. A gasoline cargo-tank vapor space can be too rich to read as “100% LEL” the way a student expects. Do not interpret 0% LEL as “no fuel” unless oxygen is normal, the meter bump-tested, and you understand the container.
Infrared LEL sensors (if the AHJ issues them) do not use the same oxygen-burning chemistry and will not see every fuel a bead sees (hydrogen is a common IR miss). Most technician four-gas kits in service are still catalytic. If the stem says catalytic CGI, apply the oxygen-dependent rules.
CO, H2S, and cross-sensitivity — still not a universal toxic meter
CO and H2S electrochemical sensors are why the kit is called four-gas rather than O2/LEL. They are target sensors:
- CO sensors can also respond to hydrogen, some volatile organics, and acetylene. A “CO” alarm in a battery room or at a cutting job may be an interferent.
- H2S sensors can be affected by sulfur dioxide, nitrogen dioxide, and other species (sometimes a negative interference that hides H2S).
They still do not see chlorine, ammonia, hydrogen cyanide, many pesticides, or the solvent that your PID is for. Never use a CGI as a toxic meter. A CGI at 0% LEL with 100 ppm H2S on the other channel is a coherent four-gas story: the sulfide is already in IDLH territory (NIOSH IDLH for hydrogen sulfide is 100 ppm) while the flammable fraction is still tiny. H2S LEL is on the order of 4% by volume — tens of thousands of ppm — so toxicity wins the race by a wide margin. CO (PEL 50 ppm as an OSHA 8-hour limit in 1910.1000 Table Z-1; NIOSH IDLH 1,200 ppm) likewise injures far below its LEL (~12.5% by volume).
Scenario: “LEL is zero, send the team”
A confined process pit smells like rotten eggs. The four-gas on the grate reads O2 20.8%, LEL 0%, CO 0, H2S 42 ppm and climbing as the wand goes lower. The wrong sentence is “LEL is zero, it is not dangerous.” The right sentence is: oxygen supports the CGI; the CGI is not the toxic meter; H2S is already a toxic atmosphere at grade; heavier-than-air sampling in a confined volume needs more than one elevation; SCBA and the IAP action levels govern entry, not the LEL digit. If the same pit had read O2 16% and LEL 0%, you would also distrust the CGI and treat the space as oxygen-deficient until proven otherwise.
If you remember one sentence: read oxygen first, treat catalytic LEL as percent of the calibration gas that needs air to work, and never let a 0% LEL CGI talk you out of CO, H2S, or any other toxic.
Fresh air on a four-gas meter shows about 20.9 percent oxygen. Which statement matches OSHA 1910.120 and catalytic CGI behavior?
A process pit four-gas reading is oxygen 20.8 percent, LEL 0 percent, carbon monoxide 0 ppm, and hydrogen sulfide 42 ppm. What is the correct interpretation?
What does a catalytic CGI percent-of-LEL reading represent, and how do calibration gas and correction factors affect it?