Best Practices for Gas Sensor Mounting Height

Posted: 5/18/23

science of gas density

Gravity is Undefeated 

When a small stone is dropped into a lake, why does it sink? Alternatively, why does a kayak float when the stone doesn’t? It’s obviously not the object’s weight–a loaded kayak weighs more than the stone. Spoiler alert–it’s the object’s density. Learn more about how this applies to gas sensor mounting below.

Gas sensor applications have been growing in recent years and are expected to continue to grow for the foreseeable future. Parking garages, vehicle repair and storage facilities, fire stations, and bus barns are common places that call for carbon monoxide (CO) and nitrogen dioxide (NO2) gas sensing.

    Why Do Manufacturers Have Differing Recommendations for Mounting Height?

    Various sensor manufacturers have different recommendations for sensor mounting height, especially for NO2 gas. Some recommend mounting the sensor as high as possible (theorizing that since vehicle exhaust is hot, the gas will rise and the sensor can detect the gas and initiate the exhaust sequence before it reaches breathing height), some at breathing height (that’s where people are, so that’s where the gas should be detected), and some as low as possible (based on the chemical density of NO2 gas being higher than ambient air).  Let’s look at each of these NO2 sensor mounting options individually:

    CO2 NO2 sensors

    High Mounting Option

    It is true that vehicle exhaust is hot and that temperature affects the density of gases. NO2 is less dense the hotter it gets, but it cools very quickly when mixed with ambient air (typically in less than 5 minutes). As such, NO2 gas may cool before it gets to the ceiling, and any NO2 sensor mounted high may never get the opportunity to detect the gas.

    Note: There are some NO2 applications, specifically for vehicles with stack exhaust that pushes the exhaust fumes immediately upward to the ceiling, where mounting NO2 sensors as high as possible can be effective.

    CO2 NO2 sensors

    Breathing Height Option

    At first glance this option makes sense. It’s reasonable to think: “Most people are between 5-6 feet tall, breathing air that’s 5-6 feet from the floor, so that’s where we should mount the NO2 sensor.” But upon further analysis, mounting NO2 sensors at breathing height can be challenging because children and people in wheelchairs are breathing at < 5 feet from the floor, and vehicle repair/maintenance technicians are frequently on the floor or under vehicles. Because NO2 gas rises when hot and then falls when cooled, measuring NO2 gas at 5-6 feet from the floor is equivalent to catching a falling knife.

    CO2 NO2 sensors

    Low Mounting Option

    The density of ambient air is approx. 1.275 kilograms per square meter (kg/m3) at 0 C. The density of NO2 gas is approx. 3.663 kg/m3 at 0 C–nearly three times as dense as ambient air. In every scenario and every application, NO2 gas will sink to the floor of the building. Temperature changes, vehicle movement, wind, and ventilation will cause the air (and all the gases therein) to move, but like the stone in the lake mentioned above, NO2 gas will always find its way to the floor. As such, the best practice is to mount NO2 sensors as close to the floor as possible.

    CO2 NO2 sensors

    Gravity Wins–Every Time

    You can’t beat gravity. I was reminded of this fact the hard way a while back when I fell from a ladder. What is true for ill-fated home improvement projects is also true for gas sensor applications. ACI gas sensors are designed to provide the most accurate and reliable readings of toxic or combustible gases and to be installed where the respective gas will most likely be found. As such, the mounting height recommendations are based on the density of the gas being measured. The most accurate, lowest cost, the best-looking sensor is worthless if installed in the wrong spot.

    We understand the utility of mounting all gas sensors at one height, or multiple sensors in one enclosure. But when dealing with toxic gases that are hazardous to occupants, do you really want to fight gravity? What’s best for the installer isn’t always best for the occupant. Because NO2 gas is highly toxic at low concentrations, it’s critical to exhaust as much of it as possible. 

    We also recommend that local building codes are considered for gas sensor placement. For example, some codes call for NO2 sensors to be mounted as high as possible. And while that differs from our recommendation, we don’t suggest picking a fight with your building inspector. Having separate sensors for CO & NO2 allows you to meet all building codes–having multiple sensors in one housing does not. 

    Our Sales and Technical Support team has years of experience with these applications. We’re happy to consult on your next job to ensure full coverage of the space with the fewest devices. Please contact us at any time. 

    See the full list of our mounting height recommendations.

    Written by: Brant Kasbohm,, 608-831-9876

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