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HDC1080: Temperature offset compensation

Prodigy 130 points

Replies: 11

Views: 176

Part Number: HDC1080

Hello,

you wrote in the original question that a temperature offset because of pcb-heat will result in a relative humidity value lower than actual.

This results in my question:

If I calculate the absolute humidity (out of temperature with offset and humidity with error) - does it result in the same value as if I calculated the absolute humidity out of correct temperature without offset and correct humidity without error?

This would be the same effect when I have a fixed volume of gas with known temperature and absolute humidity. If I heat this gas, the absolute amount of humidity in this volume of gas will stay the same, while the temperature will increase and the relative humidity will be lower than before.

Am I right and can I use this dependency or are the dependencies and math operations more complex? Or is there a problem, that the gas in my case is not heated but only the sensor?

BR

Matthias

  • Dear Matthias - 

    The HDC10x0 humidity sensors have very low power consumption in sleep mode so their self-heating is very limited. The power consumption during the measurement increases and it might cause self-heating. In order to mitigate this effect it is suggested to not exceed more than 2 measurements per second at high resolution (1 measurement of temperature and 1 measurement of relative humidity).The length of the active state depends on the acquisition time: higher the resolution, longer is the active state.

    Also, in order to correctly sense the ambient temperature and humidity, the HDC1080 should be positioned away from heat sources on the PCB. Generally, it should not be close to any heat sources (Like LCD or regulator). Again, to minimize any self-heating of the HDC1080 it is recommended to acquire at a maximum sample rate of 1sps (RH + Temp). In home systems, humidity and the temperature monitoring rates of less than 1sps (even 0.5sps or 0.2sps) can be still effective.

    BR-

    Josh

  • In reply to Josh Wyatt:

    Hi Josh,

    did you read the original question that is related to this actual question?

    My sensor is as far away from heat sources as possible but the pcb is heated by voltage regulator, LEDs, µCs and gas sensors.

    My measurement rate is at 1 sps.

    Please again refer to my above written questions.

    Matze

    Hello,

    you wrote in the original question that a temperature offset because of pcb-heat will result in a relative humidity value lower than actual.

    This results in my question:

    If I calculate the absolute humidity (out of temperature with offset and humidity with error) - does it result in the same value as if I calculated the absolute humidity out of correct temperature without offset and correct humidity without error?

    This would be the same effect when I have a fixed volume of gas with known temperature and absolute humidity. If I heat this gas, the absolute amount of humidity in this volume of gas will stay the same, while the temperature will increase and the relative humidity will be lower than before.

    Am I right and can I use this dependency or are the dependencies and math operations more complex? Or is there a problem, that the gas in my case is not heated but only the sensor?

    BR

    Matthias

    BR

    Matthias

  • In reply to Matze:

    Dear Matthias - 

    In the formula below, temperature (T) is expressed in degrees Celsius, relative humidity (rh) is expressed in %, and e is the base of natural logarithms 2.71828 [raised to the power of the contents of the square brackets]:

    this shows that any temp offset would be inversely proportional to the AH. If you can measure the heating due to the other components, then you can apply the offset as you see fit, i think.

    BR-

    Josh

  • In reply to Josh Wyatt:

    Dear Josh,

    I already know and use this formula.

    So if I only use the absolute humidity for my software, does the temperature error and the resulting relative humidity error affect the calculated absolute humidity value or do the temperature error and the resulting relative humidity error compensate each other in the calculation formula for the absolute humidity?

    BR

    Matze

  • In reply to Matze:

    Dear Matze -

    I am finding it hard to follow you here. 

    any error in a measurement would of course carry through when used in a calculation.

    BR-

    Josh

  • In reply to Josh Wyatt:

    Dear Josh,

    let me try to explain my point to you.

    I take the above mentioned absolute humidity formula and solve it for the relative humidity like "Relative Humidity [%] = ... ". I increase the ambient air temperature (not the pcb temperature) and keep the absolute humidity fixed. The fomula will result in a lower relative humidity.

    In my case: The actual absolute humidity is fixed because it can not be changed by the pcb heat. My pcb heat increases the temperature measurement of the HDC (temperature error). It results in a lower relative humidity value (relative humidity error). Is this the same dependency as explained above in the absolute/relative humidity formula?

    BR

    Matze

  • In reply to Matze:

    Got it - so - IF you get any temp offset from from PCB heating, it will impact the %RH measurement returned from the sensor, as it uses temperature measurement as part of the the %RH calculation.

    BR-

    Josh

  • In reply to Josh Wyatt:

    Yes, but does this pcb temp offset value impact the %RH measurement with the same %RH error value as if there was no pcb temp offset but the ambient air temperature was increased by the same temperature value?

    e.g. 1: ambient air temperature increases by 1°C. Relative humidity decreases by 1%. The calculated absolute humidity stays on the same value.

    e.g. 2: pcb temperature increases by 1°C. Does the relative humidity decrease by the same value 1% as in e.g.1?

    BR - Matze

  • In reply to Matze:

    Matze - 

    the AH and RH are proportional (if one goes up the other does as well) - the temp is inversely proportional to the AH...so i don't think #1 can ever be true.

    what is your end application or goal here?

    BR-

    Josh

  • In reply to Josh Wyatt:

    AF can stay fixed if temperature increases and relative humidity decreases.

    An increase of temperature can not change the amount of water in the air so it can not change the absolute humidity.

    So the temp is inversely proportional to relative humidity if absolute humidity is unchanged and temperature is proportional to absolute humidity if relative humidity is unchanged and absoluite humidity is proportional to relative humidity if the tempertature is unchanged.

    am I right?