High resolution skin temperature monitoring

Discussion in 'The Projects Forum' started by AlGeorge, Nov 16, 2009.

  1. AlGeorge

    Thread Starter New Member

    Nov 16, 2009
    5
    0
    Hi all. I am designing a PIC or AVR with temperature sensor.
    Two I have found are the Linear Active Thermistor LM35 and Microchip mcp9700. Unfortunately, they do not have the resolution I need:
    Range 33c-40c (less than 7degC range)
    Accuracy better than .01C
    Response seconds
    Distance skin contact, or intimate to skin

    Both chips above only do 10mv/1c. Sure they can be boosted by another analog device, but specs demand minimal parts.
    The 8-bit PIC comparator is likely not as suitable as the 10-bit AVR device.

    I have never designed temperature sensors, and all designs seem to cater for wider temperature ranges.
    Which leads me to the main question - given the above parameters, is there an IR receiver that does not need a sender, with better resolution than aforementioned?
    There is a ic manufacturer Vishay, which seems to be one of the market leaders, but I really need to ascertain if IR is better than the previous mentioned packages?

    Hope I've provided sufficient information here. There are 2 other threads on this forum, but do not address the needs mentioned here.
    Any suggestions gratefully received.
    Kind regards,
    Alistair
     
    Last edited: Nov 16, 2009
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    You may get the resolution you need with a different A to D converter (not comparator). The hard part is the level of voltage you are working with. You can try to resolve fractions of millivolts with something like a 22 bit audio A to D, or you can offset and amplify the LM35 outout to the 7 degC span runs from 0 to 5 volts. That scales to 714 mv/degC, or 7.14 mv/.01 degC. A 10 bit A to D can just resolve that (4.9 mv/bit).

    The accuracy figure is going to be expensive and require additional hardware.
     
  3. AlGeorge

    Thread Starter New Member

    Nov 16, 2009
    5
    0
    Hi thanks for info. From what Ive been able to ascertain Thermistors provide the best accuracy. Some items here may be suitable.
    http://www.amwei.com/news.asp?news_id=79
    These devices are providing a 1k2 swing in the 35-40degC range/240Ω/deg/>1% accuracy.

    However, I am still most interested in Infrared, as the desired temperature is underneath the skin.
    Any contributors on infrared?
    Al.
     
    Last edited: Nov 16, 2009
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Thermistors are a reasonable choice for a limited range. They are no too linear, but the deviation in 7 degrees should be negligible. There will be a published curve to insure negligibility.

    No opinion on IR.
     
  5. AlGeorge

    Thread Starter New Member

    Nov 16, 2009
    5
    0
    Thank you. There is very little info on i/r. Obviously it must be somewhere - take for example the in-ear thermometers I guess they would use i/r. They must be pretty accurate too.
    Kind regards,
    Alistair.
     
  6. AlGeorge

    Thread Starter New Member

    Nov 16, 2009
    5
    0
    True, I looked at that. It certainly seems the best option, however Digikey is expensive. I did contact the maker of the product but they havent replied I guess my few thousand quantity is not sufficient for them. It must be cheap.
    Al.
     
  7. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
    2,040
    287
    I think you're going to have the best results with a thermocouple wire.
     
Loading...