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Thermisters/Thermocouples

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Bodger

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I am a programming whiz, but an electronic dunderhead ;)

I have a friend that wants to measure temperatures from 100 deg F to 300 deg F (37 deg C to 150 deg C) on 8 different temperature sensors once every 30 seconds.

I want to know which would be better for this application, the primary concern is cost the less money the better.

I really do not know much about either thermocouples or thermistors, which would be better for this range of temperatures and monitor rates? The little I read about thermistors is that they only work good around 350 deg F, which is way above the temperatures we want to measure. Although the challenge of the nonlinear values is easy for me to handle, I can use many numerical methods to get to an accurate reading with a good set of data curves.

In this article:

http://linuxgazette.net/118/chong.html

The author uses the pc parallel port to do a/d conversion, yet it looks like each thermistor in his design uses 2 channels. Can you tell me why he uses 2 channels? Is there a way I could use only 1 channel (I need 8 sensors)?

Thanx

Bodger
aka Julian
 

IanP

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I can't see any reason why you couldn't configure this A/D converter for 8-Channel single ended inputs ..
In this particular case the author is measuring current from the temperature sensor as voltage accross 47kΩ resistor, and the A/D converter is probably configured as 4-channel differential inputs ..

Regards,
IanP
 

e_eja

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i agreed with ianP, why they used diff input for ADC.
and if you want to measeure temp with high resolution you may consider about PCB design otherwise you migh false reading temperature.
regrds
 

Bodger

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Actualy we are going to be taking the temp of meat, high precision is not a problem.

Julian
 

VVV

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Thermistors work very well in the range of temperatures you need.
Perhaps thermistors are better for your application, since with thermocouples you need amplifiers, seeing that the voltage are small.

Plus, with thermocouples you have to be more careful, since all the connections between dissimilar metals between the thermocouple and the amplifier can turn into parasitic thermocouples that affect your measurements.

Also, since thermocouples only measure the temperature difference between the "hot" and the "cold" junction, you need another circuit (the cold-junction compensator) to read the actual absolute temperature of the "cold" junction to be able to calculate the temperature of the "hot" junction (and of the meat).

So it seems that thermistors are better suited to your application, since they will give you higher voltages, which you can convert directly. Since the output voltages are higher, you do not really need to care about the parasitic thermocouple effects. And since the currents are low, the voltage drops across the connecting wires are generally not a problem.

As for the hardware, you can configure the A/D for single-ended operation, giving you 8 channels.
Nonlinearity is present even with thermocouples anyway, even though it is not as bad. But the non-linearity and even calibration (for better accuracy) can be taken care of in software. This can be done for every sensor separately. Usually, a calibration at 2 points is sufficient. The rest will be interpolated, following the table from the manufacturer's datasheet, with the values scaled accordingly by the constant(s) you calculate of-line during the calibration.
 

Bodger

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I appreciate all your input. I am going to get some datasheets on thermistors in the range and see if I can get a good match. Thermocouples definitely sound too difficult for this application.

Julian
 

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