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Effects of Weather and Altitude on CO2 Sensors

Posted on January 10, 2020

If you are a user of CO2 sensors in industries such as HVAC, BMS, Horticulture, Livestock, or if you are measuring CO2 for any reason, you may not be aware just how much ambient pressure can affect your readings. In fact, seemingly small pressure variations caused by altitude and changing weather conditions can have a surprisingly large influence.

Most CO2 sensors use the NDIR (Nondispersive Infrared) principle to measure CO2 concentrations. All non-elemental gasses (Including CO2) will absorb infrared energy at a specific wavelength and, as such, can be measured by an NDIR sensor. Put simply, the sensors will measure the amount of infrared energy (at a wavelength specific to the target gas) that reaches a detector from an IR source within the sensor unit.

When the ambient pressure rises or decreases, the number of gas molecules within the measurement chamber will also rise or fall. Because CO2 sensors are typically calibrated at sea level, any change in pressure can fool the sensor into believing that the concentration of gas has changed, and your readings will become inaccurate.

The new EE894 NDIR sensor module from E+E offers four measurement parameters – CO2, relative humidity, temperature and ambient pressure. This module is now also used in a wide range of E+E CO2 transmitters and probes. 

The EE894 module uses active onboard pressure compensation to ensure that you have the best accuracy regardless of your weather conditions or altitude.

Why should I care?

If we take a typical HVAC application, CO2 sensors are installed into a building to ensure air quality standards are met. They do this by informing the HVAC system when fresh air is required. This ensures that people inside the building are not exposed to high levels of CO2, which can significantly affect the wellbeing and productivity of occupants.  Also, by monitoring levels of CO2 and introducing fresh air only when necessary, significant energy savings can be achieved. By boosting your ventilation system “On-demand” you will reduce energy consumption as your HVAC system is not heating or cooling a constant flow of outside air. On top of this, you are not running fans unnecessarily.

If your CO2 sensors are reading high, then your HVAC system will be running constantly to try and bring the perceived CO2 levels down. If the CO2 sensor is reading low, then your system will not be introducing adequate fresh air for the occupants of the building. 

So how much can Weather conditions and altitude affect my CO2 readings?

Fig. 1: Weather impact at sea level without pressure compensation

Fig. 2: Weather impact at sea level with pressure compensation

As the above graph shows, a sensor in 1500 ppm of CO2 can show a difference from low-pressure weather (around 980 mbar) to high-pressure weather (Around 125 mbar) of over 100 ppm.

With active pressure compensation, this potential difference is reduced to less than 10 ppm.

The effects of altitude are even more pronounced!

Fig. 3: Measurement deviation at 400 ppm

Fig. 4: Measurement deviation at 1500 ppm

(For altitude 600 metres or higher, the graph shows the minimum measurement error of a device without pressure compensation.)

Figures 1 to 4 show that the residual pressure dependency of a device with active pressure compensation is less than 1/10 of the pressure dependency without compensation. For instance, the measurement error caused by the weather changes at 1500 ppm, which is 105 ppm for a device without pressure compensation, becomes less than 10 ppm with active pressure compensation.

In a similar way, the measurement error caused by 1500 m altitude at 1500 ppm, which is 250 ppm for a device without pressure compensation, becomes less than 25 ppm with active pressure compensation.

What does this mean in the real world?

The potential difference in readings of a CO2 sensor (at 1500 ppm), when installed in Paris, would be close to 600 ppm less when installed in a high-altitude city such as Bogota. Even sensors installed in Madrid would show a difference in reading of around -160 ppm compared to Paris

The EE894 NDIR modules and the range of E+E transmitters using this sensor will show a difference of under 16 ppm from Paris to Madrid and less than 60 ppm in installations from Paris to Bogota.

By using the E+E Elektronik CO2 sensors and transmitters with onboard active pressure compensation, you can be sure that your readings are as accurate as you need them to be, regardless of your application or process.

For further information on our range of pressure compensated E+E CO2 sensors and transmitters, please contact us today.

Alternatively, please contact our sales team on the below telephone number.

Tel: 01926 614263

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