What are the four types of gas detector sensors, how does each one work, and how do you read a 4-gas meter?
Portable gas monitors use four sensor technologies. Catalytic bead sensors read combustible gas as percent of the lower explosive limit by burning a trace of it on a heated bead; they need oxygen and can be poisoned by silicone and sulfur compounds. Electrochemical sensors read oxygen and the toxic gases (H2S, CO, SO2, NO2, NH3, HCN, Cl2, PH3) in parts per million or percent by volume through a chemical reaction that produces a current; they are compact and accurate but can cross-read other gases, hydrogen on a CO sensor being the common case RKI Instruments describes. Infrared sensors read gases that absorb infrared light, methane and carbon dioxide among them, without oxygen and without poisoning, but cannot see hydrogen, acetylene, ammonia or carbon monoxide. Photoionization detectors read volatile organic compounds by ionizing them with an ultraviolet lamp of 10.0, 10.6 or 11.7 eV. A 4-gas meter shows four numbers at once: combustible gas in percent LEL (clean air reads 0), oxygen in percent by volume (clean air reads 20.9), hydrogen sulfide and carbon monoxide in ppm (clean air reads 0), each with its own alarm set points; everything else on the screen is status, such as battery, pump, time, peak and the calibration-due reminder.
RKI Instruments 72-PAX-D GX-3R Pro 4-Gas Monitor, LEL/O2/H2S/CO, Li-ionFour-gas monitor: catalytic LEL, electrochemical O2, and a dual electrochemical H2S/CO sensor, as RKI lists it (RKI data)Why the sensor type is worth knowing. The display hides the chemistry, and the chemistry sets the limits of what the number means. A catalytic LEL reading in a nitrogen-purged tank is wrong, because the bead needs oxygen to burn the gas; RKI’s FAQ on inert atmospheres says so and offers a dilution fitting or an infrared sensor instead. A CO reading of 40 ppm beside a battery charger may be hydrogen, which is why RKI builds a hydrogen-compensated CO sensor with an extra electrode. A PID reading is a total of every ionizable vapor the lamp can see, scaled to isobutylene, not a reading of any one chemical. And the oxygen number is the only one that should not read zero: 20.9 percent is clean air, and a monitor showing 20.9 with all other channels at 0 is the picture of a space with nothing in it, which is what the pre-entry test under 1910.146 is looking for.
The four sensor types, as the manufacturer describes them
From RKI Instruments’ FAQ and product pages; the gases are the ones RKI lists for each technology.
How to read the 4-gas display, line by line
The numbers below are RKI's presets, user adjustable; the 19.5, 23.5 and 10 percent lines are OSHA's.
- Combustible gas: a number followed by % LEL, 0 in clean air. 10 percent LEL is OSHA's hazardous-atmosphere line and RKI's first alarm; 25 and 50 are the second and third. RKI's GX-Force suppresses readings below 2 percent LEL to zero so the display does not flicker in clean air.
- Oxygen: a number followed by % or % vol, 20.9 in clean air. Below 19.5 is deficient and above 23.5 enriched under 1910.146(b); RKI alarms at 19.5 (low), 18.0 (low-low) and 23.5 (high). A falling oxygen number with nothing else moving means something is displacing air: nitrogen, argon, CO2, or oxygen being consumed.
- Hydrogen sulfide: ppm to one decimal, 0.0 in clean air (RKI suppresses below 0.3 ppm). RKI alarms at 5, 30 and 100 ppm with TWA 1.0 and STEL 5.0.
- Carbon monoxide: ppm, 0 in clean air (RKI suppresses below 2 ppm). RKI alarms at 25, 50 and 1,200 ppm with TWA 25 and STEL 200.
- Status: the GX-Force manual lists the display items as operational status, clock, battery level, peak reading, pump status and the calibration notification; the GX-3R Pro has a full-dot display that lights and rotates automatically. Three LEDs flashing every 30 seconds on an RKI instrument is its reminder that a bump test or calibration is due or that a gas alarm has occurred.
- Alarms: a gas alarm on RKI instruments is flashing LEDs, a continuous buzzer, a flashing reading and vibration; a trouble alarm (sensor disconnection, low battery, circuit error, calibration range error, and on the GX-Force pump or flow) is an intermittent buzzer with an error message. The alarm guide covers low, high, TWA and STEL in detail.
- What a gas monitor is called: the trade says monitor, detector or meter for the same instrument; a single-gas unit worn on the collar is also called a clip or a badge; a pumped unit a sample-draw monitor; a multi-sensor one a 4-gas, 5-gas or multi-gas. RKI calls its portables gas monitors and gas detectors interchangeably.
Gases a 4-gas and 5-gas monitor can carry, the sensor type, and the NIOSH IDLH
| Gas | Sensor type (RKI) | Range RKI prints | NIOSH IDLH | OSHA PEL |
|---|---|---|---|---|
| Combustible gas (LEL) | Catalytic bead | 0 to 100% LEL | — (10% LFL is the OSHA confined-space line) | — |
| Oxygen | Electrochemical | 0 to 40.0% vol | — (below 19.5% is deficient) | — |
| Hydrogen sulfide | Electrochemical | 0 to 200.0 ppm | 100 ppm | C 20 ppm 50 ppm [10-minute maximum peak] |
| Carbon monoxide | Electrochemical | 0 to 2,000 ppm | 1200 ppm | TWA 50 ppm (55 mg/m³) |
| Carbon dioxide | Infrared | 0 to 10,000 ppm or 0 to 10.0% vol | 40,000 ppm | TWA 5000 ppm (9000 mg/m³) |
| Sulfur dioxide | Electrochemical | 0 to 100.0 ppm | 100 ppm | TWA 5 ppm (13 mg/m³) |
| Nitrogen dioxide | Electrochemical | 0 to 20.00 ppm | 13 ppm | C 5 ppm (9 mg/m³) |
| Ammonia | Electrochemical | 0 to 400.0 ppm | 300 ppm | TWA 50 ppm (35 mg/m³) |
| Hydrogen cyanide | Electrochemical | 0 to 30.0 ppm | 50 ppm | TWA 10 ppm (11 mg/m³) [skin] |
| Chlorine | Electrochemical | 0 to 20.00 ppm (SC-04) | 10 ppm | C 1 ppm (3 mg/m³) |
| Phosphine | Electrochemical | 0 to 20.00 ppm (SC-04) | 50 ppm | TWA 0.3 ppm (0.4 mg/m³) |
| Volatile organic compounds | PID, 10.6 eV | 0 to 4,000 ppm (PID-002LA) | By compound | By compound |
Source: NIOSH Pocket Guide to Chemical Hazards (current online edition) (IDLH and PEL); RKI Instruments GX-3R Pro product page and specification table (rkiinstruments.com/product/gx-3r-pro-gas-detector, rkiinstruments.com, fetched Nov 8 2026); RKI Instruments 04 Series product page and specification table (rkiinstruments.com/product/04-series-single-gas-detectors, rkiinstruments.com, fetched Nov 8 2026). The IDLH column is why the toxic gases are called highly toxic: hydrogen cyanide and phosphine at 50 ppm, chlorine at 10 ppm, nitrogen dioxide at 13 ppm.
Questions people ask
What are the four types of gas detectors?
By sensor: catalytic bead for combustible gas (percent LEL), electrochemical for oxygen and the toxic gases (ppm), infrared for methane, hydrocarbons and CO2 without oxygen, and photoionization (PID) for volatile organic compounds. RKI Instruments’ FAQ sets catalytic against infrared and describes hydrogen interference on the electrochemical CO sensor.
How does a gas detector sensor work?
Catalytic: a trace of gas burns on a heated bead and the heat changes its resistance. Electrochemical: the gas reacts at an electrode in a cell and the current is proportional to the ppm. Infrared: the gas absorbs infrared light at its wavelength and the loss is measured. PID: an ultraviolet lamp knocks electrons off molecules whose ionization energy is below the lamp’s and the current is counted.
How do you read a 4 gas meter?
Four numbers: combustible gas as % LEL (0 in clean air, 10 percent is OSHA’s line and RKI’s first alarm); oxygen as % (20.9 in clean air, alarms at 19.5 and 23.5); H2S in ppm to one decimal (RKI alarms at 5, 30 and 100); CO in ppm (RKI alarms at 25, 50 and 1,200). Status items on RKI’s GX-Force include battery, peak reading, pump status and calibration notification; a gas alarm is flashing LEDs, a continuous buzzer, a flashing reading and vibration.
What does the 4 gas meter detect?
Oxygen deficiency and enrichment, flammable gas, hydrogen sulfide and carbon monoxide, the atmosphere 29 CFR 1910.146 tests for. It does not detect ammonia, chlorine, SO2, NO2, HCN, CO2 or solvent vapors without an added sensor.
What is an electrochemical sensor?
A small cell with electrodes and an electrolyte in which the target gas reacts and produces a current proportional to its concentration; it reads oxygen and the toxic gases, needs a little oxygen to work (RKI’s FAQ on inert atmospheres), can cross-read other gases, and lasts a few years (RKI warrants its standard sensors 3 years).
Which sensor is used to detect LPG gas?
A catalytic bead combustible sensor, calibrated to methane or isobutane, reads propane and butane as percent LEL; an infrared hydrocarbon sensor reads them too and works without oxygen. The LEL sensor guide on this site covers both.
What sensor detects gas?
It depends on the gas: combustibles on a catalytic bead or infrared sensor, oxygen and toxics on electrochemical cells, solvent vapors on a PID, and carbon dioxide on an infrared sensor. The table on this page lists the gases a 4-gas and 5-gas monitor can carry with the sensor type and the NIOSH IDLH.
What are the different types of gas sensors?
The four in portable monitors (catalytic, electrochemical, infrared, PID) plus the thermal conductivity sensor RKI uses on the GX-6100 for percent-volume readings in oxygen-free gas, and the metal-oxide semiconductor sensors found in fixed systems and consumer alarms.
What are some highly toxic gases that can be fatal?
The table on this page lists the NIOSH IDLH values of the gases portable monitors read: hydrogen sulfide 100 ppm, carbon monoxide 1,200 ppm, hydrogen cyanide 50 ppm, chlorine 10 ppm, nitrogen dioxide 13 ppm, sulfur dioxide 100 ppm, ammonia 300 ppm, phosphine 50 ppm, carbon dioxide 40,000 ppm. The toxic and flammable gases guide on this site covers more of them.
How much do electrochemical sensors cost?
The replacement sensors in the band on this page show their prices; RKI’s single-gas electrochemical sensors (CO, H2S, O2) are at the low end, the H2S/CO combination, infrared CO2 and PID sensors above them.
Which sensor is used for gas detection in a confined space?
All four kinds in one instrument: the four-gas monitor’s catalytic LEL sensor, electrochemical oxygen, H2S and CO sensors, with an infrared or PID sensor added when the space calls for it.
The ratings, test results and standards shown here are the manufacturers' published values, reproduced as published. Nothing on this page is a recommendation by Bayshore Safety that any product is suitable for a particular task, hazard or workplace. Selecting safety products and protective equipment is the responsibility of the end user and the employer, based on their own hazard assessment and the regulations that apply to their work. Confirm every value on the manufacturer's own documentation before use.