very low temperature readings (~-80C)

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very low temperature readings (~-80C)

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kael

I need to monitor temperatures in biology lab freezers that are normally at -80 C.
Working JeeNode sensor setup (check) Type-T Thermocouple (check) AD595AQ (amplifier setup) (check)
The AD595AQ outputs 10mV/deg C with 0 V = 0C.
Problem: I need a "dual power supply" with voltages above and below the ground to feed the AD595AQ if I want to read temperatures below freezing.
Can I use a voltage divider on say a 12 volt source, and split that 2/3, 1/3. Then I'll have 12V, 8V, 4V, 0V. Then I can call the 4V ground and I have my "dual supply"?? I'm not an electronics guy, so if that's wrong, please tell me. I hate letting the smoke out of expensive parts.

Posted 14/12/2010 03:52:55
jbeale

You can make different voltages using a divider as you say, but you have to be careful about how much current flows down the divider chain and how much power you are taking out of it. Any current you draw out of your divider will change the output voltage, since your load appears like a resistor in parallel with your divider so it's part of the circuit too. Maybe the AD595AQ only uses a few mA, though?
More to the point, if you want to measure a negative voltage, how do you do that with a JeeNode? Maybe easier to have the JeeNode and the AD595AQ circuit grounds isolated (separate floating power supplies) and switch ground and output signal, so to the JeeNode it appears as a positive voltage. If you use a common ground, then you'll need some A/D input device that can read negative voltages.

Posted 14/12/2010 07:27:03
jcw

There won't be smoke, but I doubt that a resistor-based divider will work (thermocouples, and the AD595, are quite sensitive). As jbeale said, the divider will change voltages once you start drawing current.
If it's an option: two batteries? Solves everything from split-voltage to floating ground to read out negative voltages.

Posted 14/12/2010 13:55:41
amvv

If your temperature sensors work anything like the LM35, you do not really need negative supply, you can use a diode. I used an LM35 successfully like that, see the schematic at:
http://jaagpad.blogspot.com/2008/10/standalone-temperature-logger-with.html
The sensor can put out negative values with respect to its ground, but that ground is in fact around 0.9V above the arduino/jeenode ground. I measured the value of this as REF in order to be able to have a better, more accurate measurement. You deduct the value of this REF from the measurement to get the real value!

Posted 14/12/2010 17:25:59
kael
@JC, awesome I never thought of that. Just to clear that up for me: you mean having 2 cells in series and using the connection between them as the ground for the system?
```
    -9V+..gnd..-9V+
```
Right?
Posted 14/12/2010 22:32:05
jcw

Exactly.

Posted 15/12/2010 03:19:59
kael

Thanks, everyone, for your comments.
1) success with: AA Power Board - Common - 9V batt.
Breadboard this up per AD595AQ datasheet using my "dual power supply" and a T type thermocouple and I get 37 C in my mouth and -17.5 in my home freezer. Seems fine considering the cold junction compensation is not optimal on the breadboard, and with no calibration at all.
2) Reading with xx8 ADC. I haven't done this yet, but the plan is (comments appreciated) to do what I think amvv was saying above.
The amp's ground is +3.3 relative to the jeenode (jn), so the -signal from the amp, X, is obtainable using Vcc(jn) as the reference and subtracting the ADC determined voltage (Vm):
X=Vcc-Vm

Posted 08/01/2011 06:30:05