BME - The inner temperature of sensor is higher than air temperature. How sensor calculate RH?

[steinmetz_eng]

Hello,

I have some units of BME280, and they all measure +1.0°c till +1.5°c temperature above the air temperature. For air temperature, I'm using an +-0.01°c calibrated termometer as reference.

How could the sensor calculate the air humidity, if the temperature is OFF? Bear in mind that relative humidity is relative to the air temperature. BME280 could theoretically measure absolute humidity with the die temperature, however in the relative humidity calculation IT'S NECESSARY the real air temperature and, given certain humidity levels, 1°c OFF could means an innacuracy of 5% RH.

Comparing an SHT45 (a much more accurate hygrometer (+-1%), from Sensirion) with BME280, I noticed that BME tends to read too low humidity when self-heating occurs, leading to confirm that the sensor is using the innacurate air temperature to calculate the air humidity.

I'm thinking in incorporate the BME280 in my designs because I could get three measurements in a single chip, which is really optimal and save costs.

Questions:

1) is the above right, and the inaccuracies of temperature (and self-heating) influenciating the humidity readings?

2) In case of BME688, does the 300°c of the gas hot-place influenciates in temperature and humidity readings? If so, for about how much time should I leave the sensor rest after a gas analysis reading?

Thank you for the support.

[BSTRobin]

Hi steinmetz_eng,

Thank for your inquiry.
Temperature, humidity, and pressure values can all be measured independently, with humidity and pressure requiring temperature values to compensate. As described in the BME280 data sheet, we strongly recommend using the compensation calculation in the sensor API of BME280 to obtain more accurate sensor data.

The gas sensor inside BME688 is physically separated from other sensors(temperature, humidity, pressure), and heating the gas sensor does not affect the temperature, humidity, and pressure sensors.