Best 4 Gas Monitor for Confined Space (2025 update)

What Does 4 Gas Monitor for Confined Space Measure?

A 4 gas meter measures carbon monoxide (CO), oxygen (O2), hydrogen sulfide (H2S), and combustibles (EX). The combustibles sensor is non-specific, meaning that it can detect a variety of gases. Acetone, industrial solvents, alcohol, ammonia, lacquer, thinners, benzene, methane, butane, naphtha, ethylene oxide, natural gas, gasoline, propane, halon, refrigerants, hydrogen sulfide, and toluene are just some of the combustibles that a 4 gas monitor can detect.

What Does a 4 Gas Meter Do?

The primary purpose of a 4 gas monitor is to alert users to any hazardous air quality environments. These instruments protect workers from unseen workplace gas hazards. 4 gas monitors warn the users of any unsafe air quality environments through audible, visual, and vibration alarms.

What Are the Different Types of 4 Gas Monitors?

4 Gas Monitor for Personal Protection The majority of 4 gas monitors fall into the portable category. These are enclosed units with 4 sensors that can be clipped to your body to provide continuous personal protection.

4 Gas Monitor with a Pump for Point Sampling This type of 4 gas monitor contains a built-in or external pump. A probe is used to enable point sampling. Before entering a confined space, a sample can be drawn for analysis while the user remains in a safe location. These devices are also useful to track down toxic sources and access hard-to-reach locations.

4 Gas Monitor for Stationary Fixed Wall The fixed wall mounted 4 gas monitor units provide continuous protection at a stationary location. These units are more expensive and offer protection in a localized indoor space, such as an engine room or industrial factory setting. Fixed wall 4 gas monitors are also useful in confined spaces with heavy occupational traffic, like a mining shaft.

How Do 4 Gas Sensors Work?

A 4 gas monitor is composed of electronics and four sensors. The gas sensors convert the detected gas concentration to an electronic signal for analysis by the on-board microprocessor. From there, the processor outputs the reading to the display screen. If the readings exceed the pre-set alarm values, the alarms will be triggered to warn the user.

As the name suggests, there are four sensors used in a 4 gas monitor. The O2, H2S, and CO sensors are electrochemical cells. The combustibles sensor reading %LEL is most often recorded by a catalytic bead pellistor sensor. 

Gas	Sensor Technology	Pros	Cons
Combustibles Explosive (EX %LEL)	Catalytic Sensor The combustible gas oxidizes on the bead, changing the conductivity of the element. From there, the resistance change is proportional to the combustible concentration. Calibrated to methane, the typical detection range is 0-100% LEL.	Low cost Very reliable and simple to use Best for %LEL range	Not selective Consumes power Not desirable for high % absolute volume  Requires at least 10% oxygen concentration to operate
Carbon Monoxide (CO) Hydrogen Sulfide (H2S) Oxygen (O2)	Electrochemical Sensors The gas reacts with a working electrode which triggers an electrochemical redox reaction. The current generated is proportional to the concentration of gas detected.	Small and cheap Easy to integrate with electronics Immediate ON	Limited life from 24 to 36 months Influenced by temperature and humidity

Is Using a 4 Gas Monitor Mandatory?

In certain occupational situations, a 4 gas monitor is mandatory. Several OSHA standards require the use of multi-gas monitors.

• Permit-required confined spaces: paragraph (c)(5)(ii)(C) of 29 CFR 1910.146, see here.
• Hazardous waste operations and emergency response: paragraph (c)(6) of 29 CFR 1910.120 (Hazardous Waste Operations and Emergency Response), see here.
• Grain Handling Facilities: Section 5 (Entry Into Bins, Silos, and Tanks) in Appendix A of 29 CFR 1910.272, see here.

What Are Safe 4 Gas Monitor Levels?

Various government agencies and associations recommend specific gas exposure limits. Below are some examples for each gas included in a typical 4 gas monitor.

Oxygen (O2) Deficiency

Carbon Monoxide (CO) 

Agency	Recommendation / Requirement
World Health Organization (WHO)	9 ppm average over 8 hours
Environmental Protection Agency (EPA)	9 ppm average over 8 hours
The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE)	9 ppm average over 8 hours
National Institute for Occupational Safety and Health (NIOSH)	35 ppm average over 10 hours 200 ppm ceiling value
Occupational Safety and Health (OSHA)	50 ppm average over 8 hours
American Conference of Governmental Industrial Hygienists (ACGIH)	25 ppm average over 8 hours

 

Hydrogen Sulfide (H2S) 

Agency	Recommendation / Requirement
National Institute for Occupational Safety and Health (NIOSH)	NIOSH Recommended Exposure Limit (REL): 10 ppm, 10-minute ceiling Concentration considered immediately dangerous to life and health (IDLH): 100 ppm
Occupational Safety and Health (OSHA)	10 ppm average over 8 hours
American Conference of Governmental Industrial Hygienists (ACGIH)	1 ppm average over 8 hours 5 ppm average over 1 hour

 

Combustibles, Explosives (EX) %LEL (Methane)

	Recommendation / Requirement
National Institute for Occupational Safety and Health (NIOSH)	1,000 ppm 8 hour TWA [methane] = 2%LEL 50,000 ppm (5%vol) IDHL Immediately Dangerous To Life or Health [methane] = 100%LEL
	Factory default alarms for LEL are set 20% for low. At Forensics Detectors, alarms are set at 50% for high.

 

Do 4 Gas Detectors Indicate Which Gas Is Being Detected?

Yes. they do. Since there is an array of 4 sensors, they present the corresponding gas levels being detected. However, for the combustibles sensor, it cannot discriminate from the hundreds of different combustible gases that may be present. The majority of combustibles gas detectors are calibrated to methane. Therefore, the values presented during detection will be with respect to methane.

What Is a 4 Gas Monitor Used For?

4 gas monitors are used for personal protection in hazardous gas environments where air quality may be compromised. Confined spaces can pose a safety risk, as highlighted below:

• Industrial Tank: A gas leak detector with a gooseneck is placed in a tank before the worker enters to sniff for any residual combustibles that may have accumulated. When the readings are within safe limits, the worker may enter the tank. They will also carry 4 gas detectors while in the tank to protect from oxygen deficiency and combustible residue. 
• Plumber/Electrician: Trade workers such as plumbers and electricians sometimes enter confined spaces. Closed spaces in residential and building spaces often do not have proper ventilation, leading to a lack of fresh air. In this case, oxygen deficiency is a major concern and a 4 gas detector is an excellent choice for the worker.
• Wastewater/Sewerage: The wastewater industry keeps our sewers and sewerage systems working. When cleaning these areas, workers are often in confined places where biological breakdown may emit combustibles. Along with methane and hydrogen sulfide exposure, wastewater and sewerage spaces may be oxygen deficient. Personal protection using a 4 gas monitor is a wise decision.
• Mining: Similar to wastewater, mining also poses the risk of methane and hydrogen sulfide exposure. Carbon monoxide, a toxic gas, may also be present and oxygen levels may be depleted. In these situations, using a 4 gas monitor is a safety necessity.

Can a 4 Gas Monitor Be Used to Detect a Gas Leak? 

No, we do not recommend using a 4 gas monitor for pinpointing suspected gas leaks in a pipe, joint, or appliance. A gas leak detector that employs a gooseneck, semiconducting metal oxide sensor, and ppm reading is the most desirable detector for this specific application. 

How Do I Select a 4 Gas Monitor?

To select the best 4 gas monitor that serves your purpose, follow this guide. It is important to narrow down your selection by thinking about these questions:

• Do I need a built-in pump or probe with the unit?
• How often will I use it? What battery capacity do I need?
• What is my budget and operating cost budget?
• Do I need advanced functions such as data-logging, bluetooth, or printing?
• Do I need specific accreditations with the monitor, such as ATEX or a NIST traceability calibration certificate?
• Do I require a belt clip for attaching onto my clothes?
• Is after sales support, USA headquarters, or fast calibration service required?
• Does a standard 4 gas monitor meet my application requirements? Or do I need to custom order a specific 4 gas monitor with other detected gases?

What Are 5 Gas and 6 Gas Monitors?

Similar to a 4 gas monitor, 5 gas and 6 gas monitors includes extra sensors in the hardware to detect additional gases. Below is a picture of a 6 gas monitor with built-in pump. This unit detects CO, H2S, EX, O2, CO2, and NO2.

How Long Do 4 Gas Monitor Sensors Last For?

4 gas monitors using catalytic bead sensors to detect combustibles will usually work for over 5 years. Electrochemical cell technology sensors, used to detect O2, H2S, and CO, typically last between 24 to 36 months.

How Do I Test My 4 Gas Monitor?

The best way to test your 4 gas monitor is to expose it to a known gas source, typically multi-gas bump gas.  Generally referred to as bump testing, this is good practice to perform daily, especially in personal protection applications. 

What Is a 4 Gas Monitor Bump Test?

• Bump testing is a procedure when one exposes the gas detector to a small amount of “blast” target gas to ensure the detector operates and alarms as programmed.
• The function of this test is to verify proper operation and build user confidence, particularly in hazardous applications.
• It is recommended to bump test when first purchased and follow with weekly checks. If using in LIFE THREATENING and/or DANGEROUS applications, bump test daily. See video explanation here. 

Can You Detect Hazardous Gas Accumulation By Smell In Confined Spaces?

Never rely on smell alone to detect gases in confined spaces. Some hazardous gases are odorless, while others can quickly overwhelm your sense of smell.

What Is 4 Gas Monitor Calibration?

4 Gas monitor calibration is the technical task of adjusting the detector to provide a more accurate gas reading. Because the sensors may degrade or drift over time, we typically suggest to calibrate your 4 gas monitor between 6 to 12 months. This time period is typical for gas detectors. 

Gas calibration is a technical task and requires certain key pieces of equipment. This includes a multi-gas calibration gas bottle, gas regulator, tubing, calibration cap fitting, and your 4 gas detector. You need to confirm with the manufacturer to determine the recommended calibration gas concentration levels and mixtures. For the Forensics Detectors 4 gas monitors, we recommend gas calibration with a mix of CO at 200 ppm, H2S at 25 ppm, O2 at 18%, and Methane at 50%LEL.

Be attentive and calibrate daily if:

• The user is employing the device as an analytical tool where accuracy is paramount.
• The user is using the device in an extreme environment (temp and humidity extremes create sensor drift).
• The user is undertaking an imminently dangerous or extreme application.
• Bump testing fails. In this case you must calibrate to make sure the sensors and monitor are able to alarm.
• The 4 gas detector is alarming in fresh air. This means that a sensor has potentially drifted past an alarm set-point (the oxygen sensor tends to drift low).

Calibration and bump testing is mandatory because injury or death will occur if the device fails. Take it very seriously. 

Also, it is prudent to have a calibration schedule that the owner or employees strictly follow.

How Do I Take Care of My 4 Gas Monitor?

• Store your 4 gas detector at a normal room temperature - about 70F with 50%RH (well within operating specifications).
• Store it away from electromagnetic or magnetic sources, such as phones.
• Store it in a clean environment where no dust or particles exist.
• Store it away from any exhaust gas, concentrated vapors, or harsh chemicals.
• Clean the casing of your detector with a damp cloth.
• Store it in a stable place where there are no vibrations or continuous shaking.

How Do I Properly Use a 4 Gas Monitor?

When operating a 4 gas monitor, there are some important tips to consider. Obviously, be smart and read your product manual. In addition, keep these tips in mind:

• Turn ON the 4 gas monitor in fresh air. 
• Ensure the 4 gas monitor is within its calibration period.
• Ensure the 4 gas monitor has been bump tested and validated as operational. Ensure levels have come to baseline. CO, H2S and EX should read 0ppm and O2 should be around 20.9%.
• Check alarm set-points. Set your alarms as desired.
• If undertaking analytical measurements, keep the unit stationary. Ensure humidity and temperature are also tracked and are as constant as possible.
• If using a pump, turning the pump ON/OFF will change the pressure and may affect readings. Take data point readings when either the pump is off or on after 60 seconds so the "pressure change" does not create inaccurate readings.

What Is the Difference Between ppm and %vol Detector Readings?

A typical 4 gas meter presents oxygen readings as a percentage. The H2S and CO readings will be displayed in ppm. The combustibles will be shown as %LEL. It is important to know how these measurements are related.

The typical scale of concentration for any gas is %vol. The means what % of the air volume is taken up by the gas volume. There are other measures based on weight and mols, but gas detection primarily involves readings on a volumetric basis.

The part per million (ppm) scale has a direct conversion from %vol. 10,000ppm = 1%. Both of these values are interchangeable. Once ppm values enter the thousands, it is customary to start using %vol. Fo example, it is easier to say 1.1 %vol instead of 11,000 ppm.

Conversion Example

Oxygen % value = (oxygen ppm / 1,000,000) x 100%

For example, if we have 5,000 ppm of oxygen, then we obtain:

Oxygen % value = (5,000 / 1,000,000) x 100%

Oxygen % value = 0.5%

Quick Conversion From ppm to %vol

100 ppm = 0.01%

1,000 ppm = 0.1%

10,000 ppm = 1%

100,000 ppm = 10%

1,000,000 ppm = 100% 

What Is the Difference Between ppm and %LEL Detector Readings?

When your 4 gas monitor reads the combustible levels, it uses %LEL. This is different than ppm and %vol units. %LEL represents a percentage of the lower explosive limit of a particular combustible. This can be a bit tricky, but follow along slowly to understand the difference:

Each combustible has a different explosive limit in air, and therefore a different lower explosive level (LEL).

For example, methane will explode in air at 5% volume (which is 50,000 ppm). This is called the 100% Lower Explosive Limit. In other words, 100% LEL = 5% volume. When methane concentration levels reach 100% LEL, the gas will explode if an ignition source is present. For propane, 100% LEL = 2.1% volume, and for hydrogen, 100% LEL = 4.0% volume.

So the %LEL scale is based entirely on explosive limits. As you can imagine, each gas has different explosive limits.

Example: If our 4 gas detector is reading 5% LEL and it has been calibrated to methane, then 5% of [5%vol] = 0.25 %vol or 2500 ppm.

Final Words

4 gas monitors are essential gas detection devices that simultaneously measure the levels of carbon monoxide (CO), oxygen (O2), hydrogen sulfide (H2S), and combustibles (EX) in the air. These monitors are crucial for ensuring personal safety when working in confined spaces such as tanks, vessels, silos, pits, manholes, and tunnels, where hazardous gases can accumulate. When unsafe conditions are detected, 4 gas monitors alert users through a combination of audible, visual, and vibration alarms, allowing workers to take immediate action to protect themselves. These versatile devices are widely used across various industries, including maritime, oil and gas, construction, mining, water, and wastewater, where the presence of dangerous gases poses a significant risk to worker safety. To maintain the highest level of safety and ensure accurate multi-gas detector operation, it is essential to perform regular bump tests and calibrations on the 4 gas monitors. By adhering to these best practices, workers can rely on these life-saving devices to provide real-time, reliable gas detection in even the most challenging environments.

About The Author Dr. Kos Galatsis ("Dr.Koz") is the President of FORENSICS DETECTORS where the company operates from the scenic Palos Verdes Peninsula in Los Angeles, California. He is a subject matter expert on gas sensor technology, gas detectors, gas meters and gas analyzers. He has been designing, building, manufacturing and testing toxic gas detection systems for over 20 years. Everyday is a blessing for Dr. Koz. He loves to help customers solve their unique problems. Dr. Koz also loves spending time with his wife and his three children going to the beach, grilling burgers, and enjoying the outdoors. Read more about Forensics Detectors here. Email:  drkoz@forensicsdetectors.com Phone: +1 424-341-3886