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LM335 as skin temperature sensor

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mado-wardany

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Hallo Every body
plz help me.....

I'm working on a project that takes the skin temperature and transmit the result to computer.
I used an LM335 (TO-92) as skin temperature sensor and connected directly to PIC16f877 to use the ADC.

Question: is it correct to use the LM335 as skin temperature sensor ?!?!?!?
 

In my opinion, If it is to be attached to the skin for short time, the thermal bonding and temperature loss due to heat capacity of sensor should be considered.
 

You need to consider several things:

1. Only a small proportion of the package will be in contact with the skin, the remainder will act as an absorber or radiator to the surroundings. You probably need to insulate the sensor so it's only source of heat is the skin and as little as possible is gained or lost from the surroundings.

2. The LM335 is only accurate to 1C which in medical terms is huge. You will need to calibrate the reading for best accuracy.

3. Assuming you are measuring human skin temperature and the person is alive, (not for forensic or mortuary tests) you are not interested in temperatures below say 10C or above 40C. This only equates to 0.3V difference in voltage across the LM335. The 16F877 ADC is 10-bit so each step in measurement is 4.8mV or nearly 0.5C. You would do well to amplify the LM335 output so the 0.3V range was spread to nearly 5V and the lowest temperature voltage offset to zero. This will greatly increase the accuracy for very little extra cost.

Brian.
 

Thanks Betwixr
how to calibrate the LM335.
is it calibrate by adding a potentiometer 10 K??????
 

Much easier to calibrate in software. First add or subtract an amount to get the zero reading to actually report zero then multiply by a factor to make a higher temperature read correctly. Then all the values between these points should be accurate.

Brian.
 

Hi

The LM 335 is not accurate to measure medical temperature

You will need to use other sensor (See analog device site) with 0.1 degree sensitivity

All the best

Bobi

The microcontroller specialist
 

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