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Temperature monitoring

Discussion in 'Electronic Repair' started by [email protected], Apr 11, 2007.

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  1. Guest

    I need a unit that can detect a downward change in temperature which
    can be adjustable in the area of 31 degrees F. The trip point must be
    accurate to one degree or better. The sensing element needs to be a
    probe which can be remoted from the control unit by means of a cable.
    The control unit will be mounted in a weather proof box ouside. The
    unit must provide a contact closure at the preset temperature and hold
    it until the temperature rises. It has to be battery powered,
    preferably from 12.0VDC. The units that I've found thus far do not
    have the accuracy required. Does any one know of any such equipment
    available? Thanks, Lenny Stein, Barlen Electronics.
     

  2. Omega sells industrial process control stuff that will do it, but its
    usually overpriced.
    http://www.omega.com/

    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
  3. DaveM

    DaveM Guest

    How long does the sensor cable have to be?
    Does the adjustment have to track a scale or can it be set by trial and error
    until it's at the desired set point?
    How much current do the contacts have to handle?
    Do you need a display of set point and/or actual temperature?
    How much hysteresis between on/off conditions?

    --
    Dave M
    MasonDG44 at comcast dot net (Just substitute the appropriate characters in the
    address)

    Life is like a roll of toilet paper; the closer to the end, the faster it goes.
     
  4. lmcclaf

    lmcclaf Guest

    The first thing that popped into my mind was a programmable thermostat
    like I have in my house. Mine is a Hunter model that cost me about
    $17. It is run on a single AA battery (or maybe it's AAA, I don't
    remember). The temp can be set in either F or C and when the temp gets
    1 degree below what is set then the unit switches on the furnace until
    the temp is 1 degree above the set temp and then it turns off again.
    Wouldn't be too difficult to remove the sensor to make it remote I
    don't think.

    Cheers,
    Lawrence
     
  5. That's tough.
     
  6. James Sweet

    James Sweet Guest


    What are you trying to use it for? You could try a Dallas DS1822 or
    related part and a microcontroller such as a Pic or AVR, that hardware
    will be cheap and should meet your specs but it will require some
    firmware to be written.
     
  7. There are very few mechanical thermostats which can work that close in
    differential. The only one I can recall used a mercury thermometer with
    leads embedded in the glass tube at the desired temperature.

    Even a design with electronics needs careful design to get that close of a
    differential. However I wonder if the spec. needs to be that accurate since
    this appears to be an anti freeze system.
     
  8. James Sweet

    James Sweet Guest

    The Dallas sensors I refer to have a straight digital output so there's
    no fiddly analog design to deal with. Still, that is quite a tight range.

    If he provides some more detail about the intended application it will
    be easier to give suggestions.
     
  9. Guest

    This is the application:
    A nearby apple orchid apparently is in danger of having the trees
    damaged due to an early Spring frost. Every year around May they have
    gotten this box out. Inside is a temperature monitoring control unit
    and what appears to be a low power radio transmitter. There is also a
    probe, a thermistor I suspect mounted inside a small piece of PVC pipe
    which is connected to about 50 feet of shielded cable. This is placed
    out in the field. The transmitter pages out via an antenna which is
    mounted in the orchard. The whole business is powered by a 12.0 volt
    wet cell. This antenna was described to me as "being about ten feet
    tall". So I'm assuming its a CB unit of sorts but until I run an SWR
    test I won't know for sure. The temperature control unit has a switch
    with positions from 26 to 35 degrees F. You select the temperature you
    want to alarm at and a contact closure occurs. This powers the
    transmitter and sends a signal to the pagers. The critical temperature
    is 31 degrees F. This control unit was damaged by water and is
    useless. I tried to trouble shoot it but there are house numbers on
    the semiconductors and the manufacturer is out of business. I took a
    chance and connected the transmitter to a CB antenna and I have
    determined that the RF portion of this Rube Golgberg works. However,
    since I didn't like the idea of hours of transmitter on time when I
    rebuild it I intend to use a one shot timer after the new temperature
    control unit so that I'll get a 10 second closure and then power will
    be removed from the transmitter. When the temperature warms the timer
    will reset and be ready for the next shot. Apparently during this two
    to three week period a dip to 31 degrees requires they get some huge
    fans out to warm the trees. This is the reason for the page. As much
    as I'd like to spend the time designing a system to do this the
    customer is a commercial orchard and probably will be willing to spend
    a few bucks to just have the situation resolved. I hope this clarifys
    things a little. Thanks, Lenny.
     
  10. James Sweet

    James Sweet Guest


    I doubt you need accuracy of 1 degree, but you should be able to achieve
    something close. Have a look at this part
    http://datasheets.maxim-ic.com/en/ds/DS1821.pdf. It's a complete
    programmable thermostat on a chip.

    If you want to avoid programming, have a look at this one
    http://www.national.com/pf/LM/LM34.html 1 degree accuracy, linear output
    in degrees F, all you need then is a comparator, some precision
    resistors and a precision voltage reference.
     
  11. http://hpsweb.honeywell.com/Cultures/en-US/Products/wireless/default.htm

    may be a starting point.
     
  12. Bill Jeffrey

    Bill Jeffrey Guest

    Lenny -

    If I understand the application, you don't need accuracy, but you do
    need repeatability. In other words, if you want the unit trip when the
    true temperature is 31 degrees, but the unit tells you that the
    temperature is 27 degrees - who cares? Just set the trip point indicator
    at 27 degrees. Or even (dare I say it?) put a new label on the trip
    point switch. Essentially what you are doing is calibrating the unit.

    Repeatability is a different animal. To follow the above example, if the
    true temp is 31 degrees, and the unit says 27 degrees, the next question
    is - what does it say tomorrow? And the day after? Does it still say 27
    degrees? If it consistently says 27 degrees, then you have what you
    need. But if the reading drifts from day to day, you are in trouble.

    Did that make sense? Accuracy, repeatability, and resolution are not the
    same thing.

    Bill Jeffrey
     
  13. John

    John Guest

    Probably can be done as a one-off design for under $150.

    PICAXE microprocessor (under $10)
    DS18B20 temperature sensing chip ($5)
    one 2n2222 transistor (under $1)
    78L05 regulator (under $3)
    one relay (depends on the load)
    about 60 lines of code (priced by the hour or by the project)

    The PICAXE would read the DS18B20 chip every minute (or whatever) and
    compare the temperature to the set point. If the temp is low, it
    would operate the relay to power the transmitter for 10 seconds (or
    whatever). At this point, the PICAXE can set a flag that the alert
    has been sent, and continue to check the temp every minute. If the
    temp is still low after 15 minutes (30 minutes, 1 hour, whatever), it
    can turn the transmitter on again (same 10 seconds or longer).

    If you want spares, you could duplicate the circuit as many times as
    you wish.

    John

    Email for details: picaxe at jecarter dot com
     
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