[SS2S-Main] Re: temperature measurement

  • From: Grant Saviers <grants2@xxxxxxxxxxx>
  • To: sugarshot@xxxxxxxxxxxxx, richard.rocketry@xxxxxxxxx
  • Date: Mon, 24 Mar 2014 08:19:30 -0700

A lot depends on which DATAQ unit as there are specific models for thermocouple measurements.

However, my guess is you have a one with a single ended high (0 to 5v?) inputs. So the few millivolt output of the thermocouple will have significant errors. Generally, an IN-amp (instrumentation amplifier) with a gain of 100 or more is used to amplify the signal and eliminate other error sources such as high common mode signals. The general properties of in-amps are high gain, low bandwidth, high common mode rejection, low input offset voltages, and very low offset drift. A dozen components plus the IC can make a decent preamp for each channel. The preamp design needs to fit the input specs of the DATAQ - single ended or differential inputs.

A basic tutorial and simple preamp optimized for K thermocouples you can build easily is on page 3 of http://www.analog.com/library/analogDialogue/archives/44-10/thermocouple.pdf I use Expresspcb.com for proto boards and the 3 boards for $51 option will easily get you 8 channels of preamp. The AD8495 is in stock for $4.81 at digikey.com

A fairly deep dive on this topic is at http://www.analog.com/library/analogdialogue/archives/43-09/EDCh%202%20other%20linear%20circuits.pdf

Having a reference junction is nice for high accuracy, but probably not needed given what you will measure. The tables and formulas for linearity correction are dependent on the thermocouple type and for a K type in this application not needed.

Grant Saviers

On 3/24/2014 7:32 AM, Richard Nakka wrote:
For an upcoming motor test, we would like to use thermocouples
(k-type?) to measure motor casing and nozzle temperatures.
We would like to use an 8-channel DATAQ unit for the data acquisition.

We understand there are challenges to this due to small voltage
signal, non-linear temperature-voltage relationship and the need (?)
for a reference junction compensation.

Question to list members: Can anyone give advice on how to do this?
What kind of amp would we need? Could anyone be of assistance in
designing/building such?

thanks in advance

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