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Erratic measurements with LTC2983

Question asked by CarlosSa on Jun 3, 2018
Latest reply on Jun 21, 2018 by LoganC

I am trying to build an engine monitoring system based on the LTC2983.

I used the LTC2983 demo program to generate the code to run tests.

I am using two thermocouples type K, and a diode (actually a BC547 transistor) as the reference for the "cold joint" temperature.

An Arduino Uno is driving the LTC chip.

While the LTC2983 is operational, the readings I am getting are erratic: when placed on a warm environment, temperatures do go up, but they jump around a lot. Same for low (below room temp) temperatures.

Also, it seems I need to recycle power to the Arduino/LTC2983 every now and again, or readings will tend to drift to room temp.

 

Below is the sequence of tests I ran to understand the behavior of the system (note that before each test there was a power recycle)

I also attached a spreadsheet that shows the data generated. On the blue tabs you can find graphs for each of the tests. The shape of these graphs should help understand what the problem is.

I am looking for ideas on how to debug this.

 

For all tests:

Probe A on Chan9/10
Probe B on Chan3/4
Each set of measurements is separated by a 5 sec delay (i.e., print probe A temp, voltage, probe B temp, voltage, "cold joint" temp, voltage - then 5 sec delay, and repeat).

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test 1
Probe A in freezer. Probe B at room temp.
At power up, reading seems reasonable, but after a short while, probe A starts showing room temp (close to probe B and transistor).

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test 2
Probe A brought out of freezer. Probe B at room temp; power up.

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test 3
Probe A on range burner. Probe B at room temp; power up

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For comparison, probes A and B swapped and tests repeated
Probe A has already cooled down to room temp (or almost).

_______________________

test 4
Probe B in freezer (nominal temp -20 C). Probe A at room temp.; power up

_______________________

test 5
Brought probe B out of freezer. Probe A at room temp.; power up

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test 6
Probe B on burner. Probe A at room temp.

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Suggestions, anyone?

 

Thanks in advance

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