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How to measure negative temperatures with a thermocouple on a battery powered device.

Question asked by jr@freshtemp.com on Aug 26, 2011
Latest reply on Aug 26, 2011 by harryh

I am struggling to figure out the best/most affordable way to accurately measure a thermocouple with a single positive supply (2 AA batteries).  First off, I am trying to measure temps down to -200C (liquid nitrogen), the board itself will be at room temperature (~25 C).

 

My current circuit works OK, but doesn't track well as the battery level changes.  The batteries start at ~3.7V and drop down to 2.5V before failing.  I am currently using the ADA4051-2.  Channel 1 is a differential amplifier with gain ~= 196.  Rf=Rg=332K and R1=R2=1690.  Channel 2 is a level shifter and provides an offset equal to Vref*.5.  I chose .5 because it scales well over the range 3.7V-2.5V. 

 

For example when the battery is at 3.7V, the offset would be equal to 1.85.  Assume we are measuring liquid nitrogen and the temp is -200C.  Using the <a href = http://srdata.nist.gov/its90/download/type_k.tab>table</a> for a K-Type thermocouple, the voltage into the differential amplifer should be -5.891 mV.  With a gain of 196 we would expect the output of the differential amplifier to be -.005891*196 = -1.1546V.  Because the A/D cannot accept negative voltages we shift this to a level our A/D can read.  I've attached an image to show how the level shifter is wired.  Vref is equal to Vdd * R4/(R4+R3).  Right now I have R3=R4=1K, so Vref = Vdd*.5.  So at Vdd==3.7, Vref = 1.85.  After the level shifter we get -1.1155+1.85 = .6954V.  This is the voltage into the 10-bit A/D.

 

Here is where it starts getting a little tricky. 

 

The Reference of the A/D is whatever the battery level is.  Also the offset voltage is dependent on the battery level.  Without having a precise measurement for the battery level all other measurements fail.  This also leaves some intense tables/math that need to be made if you want to be able to get the actual thermocouple temperature.  Ultimately there are three variables, junction temperature (we use a thermistor and a seperate A/D reading, again simple voltage divider, which allows the A/D output to be independent of battery level), battery level and thermocouple voltage difference.

 

You might ask, why not throw a precision switching regulator at the output of the battery terminals and regulate the board down to 2.x Volts.  My concern with this is battery life.  If the device lasts for 1+ years, will this still be achievable with a regulator?  Also we could use a precision reference IC but this adds some complexity and currently the board is setup to only use the battery level as a reference.

 

The best way would be to have the circuit battery level independent.  One way I thought of was to have the gain of the diff amp change with as the voltage changes using a FET or VCA, but then theres the issue of figuring out what the gain is...

 

Lastly, to use the AD8495 to measure temps of -200C what would the reference need to be?  What would you reccomend?

 

If anyone is interested here is how I determined the actual temp from the A/D output.

 

Say the A/D output in HEX is 160.  160 in DEC = 352.  We used a 10-bit A/D (2^10=1024) and had Vref = 3.00 V so the output of the diff amp was 352/1024*3.00 = 1.03125 V.  Assume for this example the offset is equal to 1 V.

 

1.03125 - 1 = .03125 (output after being amplifed, with offset accounted for)

 

The gain of the amp is 196.

 

Thermocouple output = .03125/196 = .00015943 (look this value up on the table, .15943mV == ~4 degrees C)

 

Determine the junction temperature, we have an onboard thermistor wired into a seperate A/D.  For simplicity lets assume the board/junction temp is equal to 25 C.  At 25C the thermcouple compensation is 1mV.  Remember the thermocouple measures the difference in temp between the board and whatever it is your temping, so you must compensate for the board temperature.

 

Simply add the two (1mV + .15942mv) to get the temperature at the thremocouple probe tip.  So 1.15942mV.  Look this value up and that is the temp you are measuring. (~29C).

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