Skip to content
AkerminMeasure. Monitor. Improve. Compare Products
Learning Center

What a Four Gas Detector Is For, and What It Is Not

The four gas detector is the most misunderstood instrument in safety equipment. It gets bought for the wrong jobs, trusted for the wrong readings, and skipped on the maintenance that makes it work. This page explains what the instrument actually measures, what it cannot tell you, and where the money is better spent on something else.

By the Akermin Editorial Team · 10 min read

As an Amazon Associate, Akermin earns from qualifying purchases. Product links on this page are affiliate links.

A four gas detector is a worn personal alarm that continuously measures four things: oxygen concentration, combustible gas as a percentage of the lower explosive limit, carbon monoxide, and hydrogen sulfide. It warns you when any of those crosses a threshold. It is not a leak detector, not a confined space pre-entry kit, not a replacement for a calibrated fixed system, and not a device that tells you where the gas is coming from. If you need to find a leak, you need a searching instrument like a gas leak detector or a combustible gas detector. If you need to enter a confined space, you need a calibrated four gas monitor plus a pre-entry test with a pumped instrument, and often a second person. The four gas monitor is the alarm you wear, not the survey you perform.

The four channels, and what each one is actually telling you

A four gas detector carries four sensors. They are not four versions of the same thing. Each measures a different physical property, and each fails in its own way.

Oxygen is measured as a concentration in air, normally 20.9 percent by volume. The sensor is usually a galvanic cell that produces a small current proportional to oxygen partial pressure. It tells you if the air is oxygen deficient, which happens in pits, tanks and any space where displacement or consumption has occurred, or oxygen enriched, which makes everything burn faster. It does not tell you why the oxygen is low. It does not tell you what replaced it.

Combustibles are measured as a percentage of the lower explosive limit, or LEL. The sensor is usually a catalytic bead or an infrared cell. It burns or absorbs the gas and reports how close the atmosphere is to the concentration that will ignite. A reading of 10 percent LEL means you are a tenth of the way to the lower flammable limit. It does not tell you which combustible gas is present unless the instrument is calibrated for that gas. Most are calibrated on methane or propane. If the gas is something else, the number is approximate. The LEL scale is not a concentration scale, and treating it as one is a common mistake.

Carbon monoxide is measured in parts per million. The sensor is an electrochemical cell. CO is a chemical asphyxiant: it binds to haemoglobin and stops oxygen being carried. The four gas monitor gives you a number and an alarm. It does not give you a carboxyhaemoglobin level, and it does not replace a medical assessment.

Hydrogen sulfide is also measured in parts per million, usually by an electrochemical cell. H2S is the sewer gas that deadens your sense of smell at low concentrations and kills at higher ones. The monitor gives you a number. It does not tell you whether the concentration is rising or falling unless you watch it over time.

What the alarm does, and what it does not do

The four gas detector is an alarm. That is its job. It sits on your chest or belt and makes noise, light and vibration when a channel crosses a threshold. The thresholds are set by the manufacturer or by your site rules, and they are not the same as exposure limits. A typical alarm might be set at 19.5 percent oxygen, 10 percent LEL, 35 ppm CO and 10 ppm H2S. Those are alarm points, not legal limits. The legal limits are the workplace exposure limits, and they are different numbers.

The alarm does not tell you what to do. It does not tell you where the gas is. It does not tell you whether the atmosphere is safe to enter, because it only measures at the point where the instrument is. If you are standing at the entrance to a tank and the monitor reads clean, the monitor has told you that the air at the entrance is clean. It has told you nothing about the air at the bottom of the tank.

This is the single most important limitation. A four gas monitor worn on the body is a personal monitor. It measures the air you are in. It is not a survey instrument. It is not a pre-entry test. For pre-entry testing you need a pumped instrument with a hose, and you need to test at multiple depths and in multiple locations before you enter. A monitor with a pump can do that, but only if you use it that way. The Forsensal pumped four gas monitor is an example of an instrument that can be switched between pump and diffusion mode, but the pump is only useful if the operator understands sampling.

The difference between a four gas monitor and a leak detector

A leak detector is a searching tool. It is designed to find the source of a gas release. It usually has a higher sensitivity, a faster response, and a probe that you move around until the reading rises. It is not worn on the body. It is held in the hand and pointed at suspect joints, valves and fittings. The Testo gas leak detector is that kind of instrument. It is not a four gas monitor, and a four gas monitor is not a leak detector.

The confusion arises because both can detect combustible gas. But they are optimised for different tasks. A four gas monitor is optimised to alarm at a safe threshold. A leak detector is optimised to find a small concentration and trace it to its source. If you use a four gas monitor to look for a leak, you will walk around until the alarm goes off, and by then you are standing in a flammable atmosphere. That is not a leak survey. That is an incident.

The same distinction applies to the Inficon combustible gas detector. It is a leak detector, not a personal monitor. It measures combustible gas, but it does not measure oxygen, carbon monoxide or hydrogen sulfide. It is not a four gas detector. If you need to know whether the atmosphere is safe to breathe, you need the four channels. If you need to find where the gas is coming from, you need the leak detector. Some instruments can do both, but they are not the same tool.

There is also a category error in the other direction. A four gas monitor is not a confined space entry kit. It is one part of one. Confined space entry requires a pre-entry test, a permit, a standby person, and often a respirator. The monitor is the alarm that tells you to leave. It is not the thing that makes the space safe.

The maintenance that makes the difference between an instrument and a decoration

A four gas detector is only as good as its last calibration. The sensors drift. The oxygen cell depletes. The catalytic bead can be poisoned by silicones or lead. The electrochemical cells have a finite life. If you do not calibrate, you do not know what the instrument is measuring. You only know what it is displaying.

Calibration is not a check that the instrument turns on. It is a check that the instrument responds correctly to a known concentration of gas. It requires a calibration gas cylinder, a regulator, and a procedure. Some instruments have a calibration certificate in the box. The Forsensal four gas monitor is supplied with a calibration certificate, but that certificate is a record of the factory calibration. It is not a substitute for field calibration. The certificate tells you the instrument was calibrated before it left the factory. It does not tell you that it is calibrated now.

The maintenance nobody mentions is the bump test. A bump test is a short exposure to gas to confirm that the sensors respond and the alarm sounds. It is not a full calibration. It is a functional check. Many sites require a bump test before each use. If you do not bump test, you may discover that the alarm does not work when the gas is real. The listing for the NOYAFA four gas detector states that it responds within 0.5 seconds and alerts continuously until gas levels return to normal. That is a claim about response time. It is not a claim about calibration status. The listing does not publish a calibration interval. Not stated in the listing bullets.

Sensor life is another hidden cost. Electrochemical cells typically last two to three years. Catalytic beads can last longer but are fragile. Oxygen cells have a shelf life. If you buy a four gas detector and put it in a drawer, the sensors will still age. The instrument will be out of calibration before you ever use it. This is why a four gas detector is a poor choice for occasional use. If you only need it once a year, hire one or borrow one from a calibrated source. Buying one and leaving it in a cupboard is a waste of money and a false sense of security.

Where the four gas detector stops being the right answer

The four gas detector is the right answer when you need a personal alarm for the four most common hazards in general industry: oxygen deficiency, combustible gas, carbon monoxide and hydrogen sulfide. It is the wrong answer in several specific situations.

When you need to find a leak. A four gas monitor is not a leak detector. It will alarm when you are already in the gas. It will not tell you where the gas is coming from. Use a leak detector. Placing a propane detector is a different problem again, because propane is heavier than air and behaves differently from natural gas.

When you need to enter a confined space. A four gas monitor is not a confined space entry kit. You need a pre-entry test with a pumped instrument, a permit, and a standby person. The monitor is the alarm you wear, not the survey you perform. If the space is potentially explosive, you also need intrinsically safe equipment and a gas-free certificate.

When the hazard is not one of the four gases. A four gas detector does not detect chlorine, ammonia, sulfur dioxide, hydrogen cyanide, or any of the hundreds of other toxic gases. It does not detect carbon dioxide, which is an asphyxiant but not one of the four channels. It does not detect volatile organic compounds. If you need those, you need a different instrument, often a single gas detector or a photoionisation detector.

When you need a fixed system. A four gas detector is portable. If you need continuous monitoring of a plant room or a process area, you need a fixed gas detection system with remote sensors and a control panel. A portable monitor is for people, not for plant.

When you need a medical diagnosis. A four gas detector measures the atmosphere, not the person. It does not tell you whether someone has been exposed, and it does not measure carboxyhaemoglobin. If someone is unwell, you need medical help, not a gas reading.

Finally, a four gas detector is not a substitute for training. The instrument is a tool. The person wearing it needs to know what the alarms mean, what to do when they sound, and how to check that the instrument is working. Without that, the instrument is a decoration. A single gas detector for hydrogen sulfide is a different tool with a different purpose, and it is worth understanding the difference before you buy.

The instrument you wear, not the instrument you point

A four gas detector is a personal alarm. It measures four channels at the point where you are standing. It is not a leak finder, not a confined space entry kit, and not a replacement for calibration and bump testing. If you need to find a leak, buy a leak detector. If you need to enter a confined space, buy a pumped monitor and follow the procedure. If you need a personal alarm for the four common gases, a four gas detector is the right tool, provided you maintain it and understand what it is telling you. The rest is training and discipline.

Frequently Asked Questions

Can a four gas detector detect a natural gas leak in a home?

It can detect combustible gas, but it is not a leak detector. A four gas monitor is designed to alarm when the atmosphere reaches a threshold, not to trace a leak to its source. For finding a leak, use a leak detector. For protecting people in a home, a domestic gas alarm is a better fit than an industrial four gas monitor. The four gas monitor is an industrial personal alarm, not a domestic appliance.

Does a four gas detector measure carbon dioxide?

No. The four channels are oxygen, combustibles, carbon monoxide and hydrogen sulfide. Carbon dioxide is not one of them. If you need carbon dioxide, you need a different instrument. The same applies to chlorine, ammonia and most other toxic gases. A four gas detector is not a general purpose gas detector.

How often does a four gas detector need calibration?

The listing does not publish a calibration interval. The general practice in industry is a bump test before each use and a full calibration at intervals set by the manufacturer or by site rules, often every six months. The instrument is only as good as its last calibration. A calibration certificate in the box is a record of factory calibration, not a guarantee of current accuracy.

Can I use a four gas detector for confined space entry?

Not on its own. A four gas monitor is one part of a confined space entry kit. You need a pre-entry test with a pumped instrument, a permit, a standby person, and often a respirator. The monitor is the alarm you wear, not the survey you perform. If you use a diffusion monitor to test a space before entry, you are testing the air at the entrance, not the air at the bottom.

What is the difference between a four gas detector and a leak detector?

A four gas detector is a worn personal alarm that measures four channels continuously. A leak detector is a hand-held searching tool that finds the source of a gas release. They are optimised for different tasks. A four gas monitor will alarm when you are already in the gas. A leak detector will help you find where the gas is coming from before you are in it.

Is a four gas detector worth buying for occasional use?

Usually not. The sensors age even when the instrument is not used. An instrument that sits in a drawer will be out of calibration before you need it. If you only need a four gas monitor occasionally, hire one or borrow one from a calibrated source. Buying one and leaving it unused is a waste of money and a false sense of security.