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DS1820 temp sensor odd results

Discussion in 'Electronic Components' started by Eric Yancey, Oct 24, 2006.

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  1. Eric Yancey

    Eric Yancey Guest

    Hello,

    I recently installed a temperature monitoring system for my home workshop.
    The primary goal is to monitor the heat capacity and dissipation of a
    concrete slab floor with radiant heat tubing. When I installed the heating
    tubes before the concrete was poured, I embedded a fairly short (5 feet or
    so) piece of PEX tubing to house a temperature sensor.

    I built kit 145 from here: http://kitsrus.com/kits.html to interface with
    four dallas DS18S20 sensors. One sensor measures ambient air temp, two
    measure the temperature of the fluid entering and exiting the floor, and the
    fourth is inserted in the terminated PEX tube as described above.

    The problem I'm having is that the 4th sensor is reporting flaky results. I
    replaced the original sensor but the replacement is exhibiting the same
    behavior as the original. Here is some sample data:

    07:45:00 78.35
    07:44:30 74.07
    07:44:00 71.15
    07:43:30 72.50
    07:43:00 75.20
    07:42:30 67.42
    07:42:00 74.62
    07:41:30 67.32
    07:41:00 75.97
    07:40:30 71.15
    07:40:00 73.96
    07:39:30 69.80
    07:39:00 69.80
    07:38:30 78.01
    07:38:00 73.96
    07:37:30 57.87
    07:37:00 70.57

    02:01:00 61.25
    02:00:30 86.11
    02:00:00 84.76
    01:59:30 59.56

    Around midnight the sensor was reporting temperatures consistently around
    60F which I believe is the actual temperature of the slab. From around
    2:00AM this morning it started reporting inconsistent values, and now,
    around 8:00AM, it is reporting temperatures anywhere from 57F to 78F, which
    I know is not correct (the temp of a concrete slab cannot fluctuate 30
    degrees in 8 minutes).

    Does anyone have any ideas on why this may be happening?

    I apologize for the length of the message and thanks for any help!

    Eric Yancey
     
  2. Jamie

    Jamie Guest

    lost soles of those lying in the cement below that slab? :)

    Radon gas?, some element mix in the concrete that maybe reacting
    to a near by R.F. signal at times?
    who knows! :)
     
  3. Eric Yancey

    Eric Yancey Guest

    Haha, well, it was built on an ancient indian burial ground :)

    The RF signal is an interesting idea, but 3 of the other sensors are within
    3' of the flaky one, so who knows?

    I'm gonna pull the sensor this evening and see if I get accurate readings
    outside of the slab.

    Eric
     
  4. Graham W

    Graham W Guest

    Then swap it with another one you assess as working properly?
     
  5. Some thoughts.

    Did you try to change the connection so to eliminate a faulty chip or a bad
    connection? Same for the sensors.

    Radiation (electromagnetic radiation) of any source can disturb the signal
    from the sensor. Twist the wires carfully or try a piece of shielded cable
    and ground the shield.

    The sensor may not make a good thermal contact with the concrete, so your
    results may reflect the temperature of the air in the tube instead of the
    concrete around. You can try to close the tube using a cork or fill the tube
    with fine dry sand (as for birds cages so you can remove it using a vacuum
    cleaner).

    petrus bitbyter
     
  6. Uwe Bonnes

    Uwe Bonnes Guest

    ....
    Read 9 Bytes after the CONVERT_T command and the conversion complition.
    That way you get the CRC8 and you can check the data for integrity.

    Discard and reread if CRC8 fails.

    Bye
     
  7. mike

    mike Guest

    If your test of the sensor outside the slab shows as I suspect that
    it is OK, I think your issue may be due to temperature fluctuations in
    the still curing concrete. IIRC, concrete, when it is curing generates heat.
    This heat will gradually fall off towards the ambiant temperature in
    time, but as you said that the slab was recently laid, maybe that has
    not had time to complete in the center. You could double check by
    drilling a hole (remember where the pipes are?) and inserting a mercury
    thermometer to see if the same thing happens.
    As the fluid is in motion it will not exhibit the swings as not enough
    time will have occured between entry and exit for transfer. As it is a
    heating system, I guess the liquid's temperature will be higher than
    that of the slab anyway, so heat transfer will be away from the liquid.
     
  8. Eric Yancey

    Eric Yancey Guest

    Thanks Mike and everyone else for your replies and suggestions. I should
    have been a bit more clear - the concrete was installed about a year ago so
    it should be mostly cured by now. Yesterday I pulled the sensor from the
    PEX, taped it to the floor, and placed a metal clamp on top of it. Since
    then the temps have been pretty consistent, varying only by half a degree or
    so between samples over the last 24 hours.

    I'm considering wrapping some sort of insulating material around the sensor
    and then putting it back in the floor. Not sure what else to try at this
    point?

    Thanks again,

    Eric
     
  9. Can you replace the sensor with a short or a resistor and see what happens?
     
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