Temp sensor and its placement to know the temperature of the 'New York' today

Discussion in 'General Electronics Chat' started by Willen, Feb 20, 2016.

  1. Willen

    Thread Starter Member

    Nov 13, 2015
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    Lets say it is 12 noon now. if I placed a sensor in my yard or in a street, exposed to the Sun light, it shows 38°C. If I placed a sensor to inside a building then it shows 27°C. If I placed inside the front room, it shows 33°C and if I placed inside the back room it shows just 22°C. What is the temperature of the surrounding place (I said 'New York' for example) now? And where to place the sensor to get valid reading?

    I am planning to make simple analog thermometer using LM35A.
     
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The senor needs to be out of the sun and the air must be able to flow around it.
    On the belgian page you will see an example of a box with spaces to block the sunlight and allow the air.
    http://www.wirtzfeld.be/blog/2006/07/26/woensdag-26-juli-2006/
    There are two pictures of the box, one closed and one open.

    Bertus
     
  3. Alec_t

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    Sep 17, 2013
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  4. Willen

    Thread Starter Member

    Nov 13, 2015
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    Hi,
    Hm...Stevenson Screen!
    Now I am focusing on the sensor component- to get accurate temperature reading the sensor's case should be a better heat conductor (I think). LM35A case has similar black epoxy as other TO-92 transistors. Is it a nice heat conductor or won't it block the surrounding temperature to reach to the chip inside?
     
  5. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Atmospheric temperature measurements aren't ususally measured on a second-by-second basis, so the LM35 case will be fine.
     
  6. GopherT

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    Nov 23, 2012
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    A TO-92 has a simple thermal time constant. Essentially a logarithmic decay of current temp to new temperature, similar to charging / discharging a capacitor through a resistor. In the is case, thermal conductivity of the To-92 package limits the rate of heat transfer and the mass and materials determine the heat capacity of the part. The conductor leads also contribute to the thermal conductivity into the sensor point in the part.

    You can try to remove the plastic package to reduce the time constant. And, increase thermal conductivity of the leads into the package by attaching small "radiators" to each but that would also add to the heat capacity of the part so everything is a trade-off.

    As you search, you may find that thermocouples have the shortest time constant but will certainly be more difficult to interface with the controller than an LM35.
     
  7. MrChips

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  8. GopherT

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    Nov 23, 2012
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    Are you paying attention, chips?

    He understands what a Stevenson screen is (or anything else that makes shade). He asked about the LM35 and seems concerned with the thermal response rate.

    Not keeping up with the conversation is understandable but accusing someone of not paying attention is unforgivable if you are the one not paying attention.
     
  9. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Solid state temperature sensors like the LM35 get a significant amount of thermal energy through the leads, whereas something like platinum thermistor gets most of its energy through the case. When measuring air temperature, it is important to remember that air is a reasonably good insulator. The sensor is reporting the temperature of the air within 1 inch. In still air this can be a significant source of error.

    ak
     
  10. Sensacell

    Well-Known Member

    Jun 19, 2012
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    Please God don't let this decay into a discussion about global warming.
     
  11. Nykolas

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    Aug 27, 2013
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    Well, you just started it! E
     
  12. bertus

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