Temperature gauge

Discussion in 'The Projects Forum' started by dannycelluk, Jul 10, 2011.

  1. dannycelluk

    Thread Starter New Member

    Jul 10, 2011
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    Hello

    I am new to the forums and my knowledge of circuit design is poor but i am capable of following a circuit diagram.

    I would like to make a temperature gauge that is suitable for measuring vehicle intake temperatures (charge temperatures)

    Does anyone have any suggestions for the type of thermocouple i would need to use in such a high flow environment?

    Also i would like the value to be displayed on a 7 seg display, can this be acheived easily without the use of a microprocessor? as i have no means of programming one.

    Any suggestions/circuit diagrams would be appreciated.

    Thanks

    Dan
     
  2. #12

    Expert

    Nov 30, 2010
    16,284
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    The smaller the sensor, the quicker the response time, if that matters. A thermocouple is the smallest one I know of. It's just 2 wires of different metals welded together. You can buy a thermistor that adjusts to air temperature changes by 63% in 10 seconds in still air, a lot faster in a moving stream of gasses.
     
  3. dannycelluk

    Thread Starter New Member

    Jul 10, 2011
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    Thanks for the reply, any suggestions on a thermistor type/number?

    The housing of the sensor needs to conduct heat slowly, or the heat from the intake manifold could affect the readings.

    The sensor also needs to be sturdy enough to withstand a very high flow.
     
  4. #12

    Expert

    Nov 30, 2010
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  5. debe

    Well-Known Member

    Sep 21, 2010
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    Check out car wreckers theres plenty of modern vehicles that use a thermistor in the inlet manifold to sense air intake temp, they are normaly very reliable.
     
  6. debe

    Well-Known Member

    Sep 21, 2010
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    Some pics of a vehicle inlet manifold air temp sensor using a thermistor. This particular unit is also a manifold air pressure sensor. Picked up for free from wreckers.
     
  7. jeff52

    New Member

    Jul 8, 2011
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    Some multi meters use a k type T couple to scene air temp, also UNIVERSAL UEI makes a stand alone meter that scenes air, and liquid temps. :)
     
  8. Jaguarjoe

    Active Member

    Apr 7, 2010
    770
    90
    IAT thermistors are a cheap and easy solution except they are non linear. A plain jane $10 LCD voltmeter could be arranged to display the IAT output but the numbers would not be in degrees.
    Adding 2 fixed resistors to the IAT will do a remarkable job of linearizing them. Formulas to do this are on the web. If I remembered to bookmark them the last time I used them, I'll post it. Honeywell sells linear thermistor sensors based on this method.
    You could use the IAT/voltmeter combination without linearization, you'll just need a lookup table or a graph of R vs T.
     
  9. dannycelluk

    Thread Starter New Member

    Jul 10, 2011
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    Thanks for the replys.

    I dont really want to use my multimeter to read the temperature, i would like to mount an LCD display on the dashboard to show Air Charge temperature,

    This will enable me to modify the inlet to produce coldest charge temperatures. e.g diffferent intercooler different materials used etc.

    Sorry maybe i didnt make myself clear,

    what i was worried about is heat transfer from the intake manifold touching the legs of the thermistor and affecting the reading.

    Maybe some rubber insulation on the 2 legs of the thermistor would help reduce/slowdown heat transfer.

    Range needs to be from -10 - approx 80 degrees.

    Something similar to this...

    http://www.electronics123.com/s.nl/it.A/id.399/.f
    Thanks again
     
    Last edited: Jul 11, 2011
  10. debe

    Well-Known Member

    Sep 21, 2010
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    There is an IC the size of a small plastic transistor that gives a linear output voltage that can be read in deg C for every mv, cont remember the type no .
     
  11. wayneh

    Expert

    Sep 9, 2010
    12,123
    3,047
    That's the LM35, and it would be a good choice here. All you need to produce a readout is a voltmeter circuit. No calibration curve.
     
  12. #12

    Expert

    Nov 30, 2010
    16,284
    6,797
    That link to the $25 thermometer kit you posted seems like just the right thing. So...why not use the obvious? You want to do it cheaper? You want to get experience building things?

    One more idea, the leads of a thermocouple do not conduct heat anywhere near as well as a stream of gasses. I think that is not a problem.
     
  13. Pencil

    Active Member

    Dec 8, 2009
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    Edit: Wayneh beat me on the reply. I didn't see the second page.


    Are you thinking LM35 (centigrade)?

    Fahrenheit model LM34.
     
    debe likes this.
  14. dannycelluk

    Thread Starter New Member

    Jul 10, 2011
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    Two reasons,

    1. I want experience making the circuit
    2. All the ones i can find are very cheap and look cheap too! I also cant seem to find one with a backlit screen

    Lookng at the links you provided, I think the LM35 is perfect for what i want, Now i just have to design a curcuit to ensure that the correct Mv is going to it at the correct temperature.

    Im guessing i will need a Zero and span pot adjustment?
     
    Last edited: Jul 12, 2011
  15. dannycelluk

    Thread Starter New Member

    Jul 10, 2011
    5
    0
    right ive had a good read through the links you provided, but how do i convert a voltage into a numeral digit? i.e. at 60mV, provide voltage to 7 seg display at ABC?

    thanks for your help
     
  16. debe

    Well-Known Member

    Sep 21, 2010
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    184
    Danny check data sheets on theLM35 its 10Mv/ deg C just feed to a LCD Mv meter & it will read in Deg C.
     
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