All our gas safety alarms have sensors that measure specific gas levels in the air. Because some gases are lighter or heavier than air they will tend to “pool” at different heights in an enclosed area. Therefore, when mounting a gas safety alarm you should ensure to mount the sensor at a height closest to where the gas will be sensed first.
Note that the molecular weight of fresh air is approximately 28.9. Gases with lower molecular weights will tend to rise above fresh air in an enclosed area or room. Gases with higher molecular weights will tend to sink below fresh air toward the floor.
When deciding where to mount a gas safety alarm sensor, find the gas you are monitoring on the list, then note whether it is heavier or lighter than air. This will tell you where to mount the gas sensor.
For example, when measuring for nitrogen, a gas safety alarm's oxygen level sensor should be mounted higher on the wall to detect an oxygen deficient environment. When using a CO2 safety alarm, the CO2 sensor should be mounted near the floor.
It's important to note that this chart shows you where gases "pool" over time in an enclosed area. In the short-term, high gas levels will first occur at a gas leak source. Therefore it is important to mount the sensor as close to any likely sources of gas leaks (tanks, cylinders, valves, manifolds, etc.) as possible.
Of course, while this chart should be used as a general guideline, when using any gas safety monitor always check the manufacturer's instructions first before mounting the sensor. Their experience in leak detection and industry regulations will be invaluable in your safety alarm installation plan.
Gas Safety Alarm Mounting Height Chart
| Level | Gas | Gas Symbol | Molecular Weight |
| Near Ceiling | Ammonia | NH3 | 17.0 |
| Florine | F | 19 | |
| Hydrogen | H2 | 2.0 | |
| Helium | He | 4.0 | |
| Methane | CH4 | 16.0 | |
| Nitrogen | N2 | 14.0 | |
| Eye level: 48-60 inches (122-152 cm) | Fresh Air | n/a | 28.9 |
| Carbon Monoxide | CO | 28.0 | |
| Ethylene | C2H4 | 28.1 | |
| Hydrogen Fluoride | HF | 20 | |
| Oxygen | O2 | 15.9 | |
| Floor Level: 12-18 inches (30-46 cm) | Argon | Ar | 39.9 |
| Acetone | C3H60 | 58.1 | |
| Benzene | C6H6 | 78.1 | |
| Butane | C4H10 | 58.1 | |
| Carbon Dioxide | CO2 | 44.1 | |
| Chlorine | Cl | 70.9 | |
| Diesel Fuel | C12H23 | 44 | |
| Ethanol | C2H60 | 46 | |
| Gasoline | C8H18 & C7H16 | ~60 | |
| Hydrogen Chloride | HCl | 36.46 | |
| Hydrogen Sulfide | H2S | 34.1 | |
| Isobutylene | C4H8 | 56.1 | |
| Isopropyl Alcohol | C3H80 | 60.1 | |
| Methanol | CH30H | 32 | |
| Nitrous Oxide | N20 | 44 | |
| Nitrogen Dioxide | NO2 | 44 | |
| Natural Gas | NG | 44 | |
| Ozone | O3 | 48 | |
| Propane | C3H8 | 44 | |
| R-Rated Refrigerants | R-22 (C2CIF2), R-123 (C2HF3Cl2), R-124 (C2HF4Cl), R-142b (C2H3F2Cl) | 86+ | |
| Sulfur Dioxide | SO2 | 64 |
Common Fixed Gas Sensor Mounting Heights
Carbon Dioxide (CO2) Sensor Mounting Height
CO2 is heavier than air. Therefore, fixed CO2 safety monitor sensors should be mounted approximately 12 inches (30 cm) off the floor.
Note that dangerous CO2 levels above 3% are virtually unheard of indoors. What is more common are CO2 levels above 1,200 ppm. Read more about high CO2 levels in the home here.
Carbon Monoxide (CO) Sensor Mounting Height
CO gas is approximately the same weight as air. However, it also rises with warm air. Therefore, the United States Environmental Protection Agency (EPA) recommends placing a carbon monoxide detector on walls approximately 5 feet above the floor or at eye level. The EPA also recommends at least one CO monitor in every bedroom and at least one on every floor of a building including the basement.
Note, however that in a peer-reviewed study in 2012, researchers found that carbon monoxide mixes quickly with circulating fresh air, so it doesn't matter how high on the wall or ceiling your CO detector should be mounted.
Also note that several manufacturer's offer combined carbon monoxide and smoke detectors. While the EPA recommends you put them approximately 5 feet above the floor and at least 12 inches from the ceiling, it's okay to mount them on the ceiling too. Just make sure you mount them at least 4 inches from where the wall meets the ceiling (called an air "dead zone").
Propane or LP Sensor Mounting Height
Propane is heavier than air. Therefore, fixed propane safety sensors should be mounted 4-6 inches (10-15 cm) off the floor. In addition, the sensor should be placed as close to the source of a potential leak as possible.
Oxygen Sensor Mounting Height
Oxygen deficiency alarms report the lack of oxygen by volume as it is displaced by another gas. Therefore, if the gas of concern is heavier than air (for example, chlorine), the oxygen sensor should be mounted close to the floor. If several gases are being monitored, the oxygen sensor should be mounted at least 5 feet above the floor, depending on how the gas is stored to measure the oxygen level in a room before someone could become asphyxiated from lack of oxygen.
Refrigerant Sensor Mounting Height
Refrigerant gases like hydrofluorocarbons (HFCs) are much heavier than air, so the refrigerant sensor should be mounted 4-6 inches (10-15 cm) off the floor. However, it is equally important if possible to place the sensor as close to the source of a potential leak, but not near a fan or other source of moving air.
Common Questions
Should gas sensors be mounted at breathing levels (4-6 feet)?
Conventionally, the "breathing zone" is defined as the zone within a 10 inch (0.3 m) radius of a person's nose and mouth. Assuming an average height about 5-6 feet, you'd think all fixed gas detectors should be mounted in the same area.
The problem with this is that gas sensors activate alarms. You want the alarm to sound when the gas level rises, long before you breath it. Because different gases have different weights, you should mount the sensor either higher or lower depending on its weight.
Of course, the sensor is different than the wall-mounted display. If you have a display separate from the sensor, it makes sense to mount the display where it is easiest to read.
Why do I need a gas sensor safety alarm for every kind of gas? Wouldn't a low-oxygen alarm work for all gases?
You may remember the movie Apollo 13, where the astronauts were in danger of carbon dioxide poisoning. You can watch the clip here. The problem wasn't too little oxygen, but too much CO2 in the air.
The same thing happens with carbon monoxide. This poisonous gas can kill you long before a low-oxygen sensor would even alarm. The same goes for radon, a poisonous gas that can cause breathing problems with long-term exposure.
In general, low-oxygen alarms are best used in enclosed areas where high levels of inert gases in pressure containers like nitrogen could be released and quickly displace the oxygen in the room.
Where should I mount a wall-mount gas detector?
Besides taking into consideration the height of the sensor from the floor, the other thing to keep in mind is the distance from the sensor to the gas source.
The closer you mount the sensor to the source of a potential gas leak, the sooner it will "sense" the leak. Consult with your fixed mount gas detector supplier to determine how many sensors you should use in a large building or a factory.
When you select a place to mount your sensor, also notice if there are air return ducts, fans, or doors to the outside nearby. A constant flow of fresh air will lower the reading.
If you are looking for more information about mounting your fixed gas detection device, contact us or learn about our fixed gas detection monitors and safety alarms here.