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Topic: Measuring automotive exhaust gas temp with arduino (Read 1 time) previous topic - next topic

Ryuske


wvmarle

Not sure if it's an issue for these sensors, but the low total resistance gives a relative high current in the sensor: over 7 mA (470+200 Ohm / 5V = 7.4 mA). That's producing some 11 mW of heat by itself, and with the sensor being small, this may be enough to heat up the sensor a bit and throw off your measurement.

The solution to this: use a much higher pull-up resistor value and an amplifier circuit (using an OpAmp) to bring the read voltage back up.

Again I don't know whether this is actually an issue in this application but considering the currents you have you should at least make sure it's not.
Quality of answers is related to the quality of questions. Good questions will get good answers. Useless answers are a sign of a poor question.

TonyWilk

Not sure if it's an issue for these sensors, but the low total resistance gives a relative high current in the sensor: over 7 mA (470+200 Ohm / 5V = 7.4 mA). That's producing some 11 mW of heat by itself, and with the sensor being small, this may be enough to heat up the sensor a bit and throw off your measurement.
Yeh, that is a consideration.
We're on 3.3v so it's a bit less (4.8mW) and it's an Exhaust Gas Sensor so I'd expect self-heating to be a minor concern.

However, it would be interesting to see if any effect is measurable on the bench,

Yours,
 TonyWilk

wvmarle

Depends on how accurate you want to be as well.
Going to be tricky to measure on the bench as the effect changes with temperature (low temperature = high resistance = low current).
Quality of answers is related to the quality of questions. Good questions will get good answers. Useless answers are a sign of a poor question.

TonyWilk

Going to be tricky to measure on the bench as the effect changes with temperature (low temperature = high resistance = low current).
Actually, 'on the bench' should be worst-case since low temp == low resistance == high current (it's a Pt sensor)

Yours,
 TonyWilk

Ryuske

So for anyone else who finds this in the future, I figured out this equation, that works well for calculating the degrees of given ohms.

Code: [Select]
degrees = -232 + 1.08ohms + 0.000428ohms^2

It isn't quite perfect (+- a few degrees) but it's pretty good.

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