Pysense temperature and humidity not correct



  • Hi,

    I'm using the pysense for a project where i need ambient temperature and humidity.
    But the temperature that I get is not quite right, 30 degrees +.

    Is this normal?

    Thanks,
    Marvin


  • administrators

    hi guys,

    Temperature compensation is probably a polynomial function:

    • a first degree, T_ambient = a * T_sensor + b, and for this 2 points should be measured (ambient and sensor temperature) in order to find 'a' and 'b' coefficients
    • maybe for some designs a 2nd degree is a better model, and in this case 3 points are required to find the 3 coefficients

    This is points can be easily found reading the sensor temperature. The rest can be done in with a piece of paper, or Excel(Trendline) or Matlab/Octave.
    To understand more, here is a general description of the compensation methods, for the SI sensor: https://www.silabs.com/documents/public/application-notes/AN1026-Si70xx-Temp-Sensor-Designers-Guide.pdf



  • @cristobal There is no simple solution for the calibration, since it depends on many factors of the assembly, like open board or device in a box, orientation, airflow, .... Genrally I would assume, that multiplicative correction works better, but to know it well you have to make a test series.



  • Hi @catalin
    I'm not sure if a linear adjustment (e.g. -7 degrees) will work because in the other forum somebody mentioned that reducing to 75% of measured temperature worked better for him. Can't you do a calibration and simply add that to the pycom github libraries for the sensor, providing additionally adjusted_temperature values or something like that? (I assume that such calibration would not be very different from device to device, so that a general calibration solution could be found and implemented.)


  • administrators

    Hi @cristobal and @AptusMarvin,
    You could use a simple temperature offset to correct SI7006A20 readout temperature, taking into consideration your application sleep regime.
    So, start your app (transmit data, do go_to_sleep), and measure temperature, let's say you have 30 deg C, and your ambient is 23, so you have a 7 deg offset. Let the board enough for the ambient to transfer (at least 30 min). Use this offset to compensate temperature for the future readings.
    Try this setup, in a different ambient (outside, or in the fridge), but also for at least 30 mins. The offset method should offer +/-1 deg of precision.
    In my experiments, I've obtained 2 deg offset, for a simple python script which wakes up LoPy every 5 mins, and stays active 30 secs(sending some data over Wifi).



  • My pysense is also measuring 29°C (MPL3115A2) or 32°C (SI7006A20) when the real ambient temperature is quite a few degrees lower (probably around 23-24°).
    The sensor shield is is not in a case. I am using a GPy. This makes the temperature sensors totally unusable unfortunately.


  • administrators

    Ok, Marvin, this happens on my board, too. The sensor measures basically the thermal paddle which is connected with the PCB ground plane. I'll give more details on the topic: https://forum.pycom.io/topic/2001/pysense-accuracy



  • No the ambient temperature measured is around 30 degrees all the time. It's not in a closed case. The pcb is quite hot due to the *py.


  • administrators

    Hi Marvin,

    You're saying that the temperature obtained from the Pysense is 30 degrees more than the Ambient Temperature? Like, you're obtaining 55 degrees Celsius, instead of the ~25, that you probably have on your desk?

    Could you give more details, do you have the Pysense in a closed case, battery is also inside? Together with LoPy(or any *py board) these are heat sources that are increasing temperature on the PCB.

    Or maybe the sensor is broken, could you check humidity, also?


 

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