Posted: 12.10.2021 08:36
Does the Aranet4 compensate the CO2 reading for pressure and temperature (via gas law)? Or does one need to manually calibrate if at a higher altitude?
Posted: 12.10.2021 09:40
As stated in the Aranet4 datasheet, CO2 sensor of the device is calibrated at standard atmospheric pressure. If the device has to be used at high altitude, manual calibration of the unit should be performed. It is not intended to use the device higher than 4000 m (13 000 ft) above the sea level.
Manual calibration instruction can be found here: https://forum.aranet.com/all-about-aranet4/how-can-i-calibrate-my-aranet4/
Posted: 31.10.2021 09:15
I don't believe manual calibration properly corrects for altitude. If done properly, the calibration function (manual or automatic) simply adds or subtracts a fixed offset and is only intended to adjust for sensor drift. Calibration is done outdoors and adjusts the reading to an assumed baseline CO2 concentration (420 ppm according to the Aranet4 manual). For example, if the monitor reads 340 ppm outside, manual calibration will adjust the reading to 420 ppm and will thereafter add 80 ppm to readings until the unit is re-calibrated.
In addition to baseline calibration, NDIR sensors require altitude compensation, which involves a percentage adjustment based on temperature and atmospheric pressure. The formula is simply the ratio between standard sea level temperature & pressure and actual temperature & pressure (see link: The Effects of Temperature and Altitude on CO2 Measurement).
Since the Aranet4 already measures temperature and atmospheric pressure, it would be relatively easy to compensate for altitude in firmware. I have not yet tested the Aranet4 but the above response from aranet-techsupport suggests that Aranet4 doesn't properly compensate for altitude pressure. The calibration offset will only be correct at the baseline concentration - in this case, 420 ppm.
As it turns out, the effects of temperature and altitude can be substantial and the error grows larger at higher elevations and higher CO2 concentrations. For example, let's say the sensor is factory calibrated at standard sea level pressure & temperature and then placed in service in Denver (5,430 ft elevation). If outside temperature is, say, 30F and pressure is nominal for Denver, the monitor should read 360 ppm prior to calibration (because there are fewer CO2 molecules at 5,430 ft). After calibration, the monitor reads 360 + 60 offset = 420 ppm. Let's say actual indoor CO2 concentration is 1,800 ppm at 75F. The NDIR sensor will only detect 1,438 ppm. The calibration adjustment will add 60 ppm so the reading will be 1,498 ppm. That means the altitude error in this case is -302 ppm!
My elevation is 4,340 ft and I've tested four CO2 monitors thus far and only one correctly compensates for altitude. It doesn't have an atmospheric pressure sensor but instead has a user defined altitude setting, which is almost as good. I just ordered an Aranet4 and will post my findings here as soon as I can run some tests.
Posted: 01.11.2021 09:22
Thank you for posting.
The atmospheric pressure change at higher altitudes is do compensated when measuring CO2 with Aranet4. No "offset" but calculation accordingly to sensor element manufacturer's recommended formula. Compensation calculation is programmed in Aranet4 firmware and is performed at every measurement.
This compensation is not used when Automatic calibration is selected, thus Manual calibration is needed when you transfer Aranet4 to another location at a different height over sea level.
Posted: 01.11.2021 18:56
Thank you for clarifying! I assumed otherwise becuase your previous response didn't directly answer the question re: pressure & temeprature compensation. Also, I found no mention of this capability / feature in the docs so I sent an email to Aranet support a couple of weeks ago to confirm P/T calculations and never received a reply.
AFAIK, no other consumer marketed CO2 monitor does P/T calculations, so it seems like something worth pointing out in the docs, don't you think?
Posted: 02.11.2021 08:16
Thank you for your suggestion about Aranet4 documentation improvement.
And by the way, your e-mail to Aranet support was received and answered on October 20. Please check your spam or junk e-mail boxes.
Posted: 06.11.2021 18:44
I purchased an Aranet4 monitor and can confirm, based on testing, that it does indeed compensate for atmospheric pressure.
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