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SHT1x Calibration — Parallax Forums

SHT1x Calibration

DmageeDmagee Posts: 20
edited 2012-01-12 19:03 in BASIC Stamp
I have two SHT1x connected to two different BS2 boards running the sample code and both sensors are reading as much as ten to eleven degrees higher than actual room temp. One SHT is connected directly to a Home Work Board and the other is eighteen inches away from all other electronics.
Right now the SHT is reading 78.1F and 25%H.
Actual room temp and humidity are 75F and 31%H.
What can I do to get a true reading?

Thanks, Dave.

Comments

  • WBA ConsultingWBA Consulting Posts: 2,934
    edited 2012-01-02 19:06
    Sounds like they might need a bit of reconditioning. What that means is that the "polymer glob" on the front of the sensor needs to be rehydrated. You can look at the notes in the Sensirion Datasheet or you can do it like I have done with 4 sensors that I had with odd readings.

    The datasheet states:
    IMPORTANT: After soldering the devices should be stored
    at >75%RH for at least 12h to allow the polymer to rehydrate.
    Otherwise the sensor may read an offset that
    slowly disappears if exposed to ambient conditions.
    Alternatively the re-hydration process may be performed at
    ambient conditions (>40%RH) during more than 5 days.

    What I have done is place the sensors on the window sill of my bathroom for a day or two. The steam from a few showers rehydrates the polymer glob and then I let them sit in a warm/dry environment for a day or two (I chose the top of my Wii console that is inside my entertainment center and is always warmer then the room). After that, all 4 sensors were within 2 degrees of each other and within 5% humidity of each other. (and my SHT15 reference sensor)

    Keep in mind also, that the SHT11 is very sensitive, so keep the two very close to each other while testing them. Maybe place a small cup over them to prevent ambient airflow from disturbing the readings.
  • DmageeDmagee Posts: 20
    edited 2012-01-03 10:37
    Thanks, I'll try rehydrating them.
  • Tracy AllenTracy Allen Posts: 6,664
    edited 2012-01-04 12:10
    Your question was about temperature, not humidity, is that right? The temperature measurement should not be affected much by the state of hydration. 10 degrees off would be unacceptable, but your example was 78.1°F/25% for the SHT vs 75°F/31% for the room. I have to ask, how did you determine the room "actual"?
  • DmageeDmagee Posts: 20
    edited 2012-01-05 19:19
    I have three stand alone units that read both temp and humidity, all three are +-1°. They are store bought not analytical laboratory grade units.
    One of the sht sensors that has been in a 75% humidity environment for a week and the temp is +1° in comparison to the other instruments.
    I don't know if this is because of the high humidity or that it has been running for a week. I still think that the rh reading is still 5% higher than actual rh but that might get better over time. The other sht has been siting un powered for a day in the same 75% humidity environment and I will test it tomorrow.
  • DmageeDmagee Posts: 20
    edited 2012-01-05 20:37
    Ok. I got bored and and plugged in the sht. The sht reading is 80.5°tF and the room is 71° to 72°.
    I also got a new sht in the mail today so I tested it too. The new sht reading is 81.9°.

    I don't need any instruments to tell me that this room is not in the 80s.

    So what is going on?


    Thanks.
  • DmageeDmagee Posts: 20
    edited 2012-01-12 19:03
    OK one last try, I'm still trying to get this SHT and STAMP to read temp and relative humidity. So far it's readings are not what I thought that I would get.
    I borrowed an instrument from the lab today to see if I was wrong or the SHT was wrong. It's a Hygrometer, Thermometer, and Dew Point measuring instrument that has been calibrated and is due for calibration on 8/22/13. So I'm pretty sure that I'm starting out on solid ground with this instrument. I have my Parallax SHT and the other probe that also uses a SHT as a sensor about .5" from each other.

    The readings from the STAMP and SHT are 71.4tF and 79.4rh

    The readings from the certified instrument are 68.39tF and 76.78rh

    That makes a 3.1 degree F difference and a 2.7 percent rh difference.

    That seams like a big difference to me, is that an acceptable difference for an instrument that is supposed to measure to the tenth of a degree?
    Am I expecting more than its capable of? I'll just stop asking questions if I am expecting to much.

    Thanks for any help. I'm not trying to make anyone angry, just trying not to get shot down by my coworkers when I propose an idea.
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