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Temperature Offset w/ SHT-11 — Parallax Forums

Temperature Offset w/ SHT-11

Greg NortonGreg Norton Posts: 70
edited 2008-01-25 12:23 in General Discussion
I've built a simple indoor/outdoor temperature and humidity setup using two SHT-11's from Parallax. While testing it out on my desktop, I notice that the indoor sensor is starts out with approximately the same reading as the outdoor one (they are now sitting about 2 feet apart on my desk, both should see the same conditions). However, over the course of running for a few minutes, the indoor sensor starts to read increased temps relative to the outdoor one. It rises to about 5 degrees F above what the outdoor one reads and stays there.

I swapped the SHT-11 modules between the indoor and outdoor, and the same behavior was observed. I don't think it's my particular sensor. It seems like it must be something about my circuit.

Since the outdoor setup will run on batteries, I am controlling power to the outdoor SHT-11 using a Stamp pin (only supplying power when I wanted to take a reading). I implemented the same thing on the indoor setup but that changed nothing.

I changed the program on the indoor setup to ensure that the self heating element is off just to be sure. I never turned it on anyway, but this also didn't change the behavior.

Anyone else got ideas on why this is happening?

Thanks.
Greg

Comments

  • Bruce BatesBruce Bates Posts: 3,045
    edited 2008-01-23 06:07
    Greg -

    I believe the SHT-11 sensor will need a bit of time to settle. Here are some application notes on using that sensor from Dr. Tracy Allen's web site:
    http://www.emesystems.com/OL2sht1x.htm

    I trust there are no sources of heat where you've placed the indoor sensor, such as a desk lamp, an overhead reflector lamp in a ceiling fixture, soldering station, TV or where there is a radiator underneath it.

    Regards,

    Bruce Bates

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  • Greg NortonGreg Norton Posts: 70
    edited 2008-01-23 18:48
    Bruce,

    Thanks for the info, there is a lot of useful information on that site.· This project is still in the breadboard stage so there is no other circuit element within a 1.5" radius.· Both the indoor and outdoor sensors are still this way.· I conducted the same experiment with all of the lights off, with the same results.

    Greg
  • mosquito56mosquito56 Posts: 387
    edited 2008-01-24 19:16
    Not sure if it helps but here we go. I have a temperature sensor that came with the education kit of the propellor. It never reads the same 2 days staight. I am also using it on a breadboard. There seems to be a problem with consistancy on a breadboard. I.E. Capacitance, Induction etc from the board itself. Also, line lengths from sensor to inputs will·cause problems.

    ·I think once you solder everything into a board you might see more·consistant results.

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  • sam_sam_samsam_sam_sam Posts: 2,286
    edited 2008-01-25 12:23
    Greg

    I have a Data Logger that uses a SHT-11 sensor when you first turn the power on the SHT-11 sensor take a few minutes to settle down
    ·
    mosquito56

    There seems to be a problem with consistancy on a breadboard. I.E. Capacitance, Induction etc from the board itself. Also, line lengths from sensor to inputs will·cause problems.

    ·I think once you solder everything into a board you might see more·consistant results.

    I saw the same thing when i was play with this sensor so there is something to it




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    Sam
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