On Tue, 23 Dec 2014 15:23:20 +0000 (UTC), "Danny D."
wrote:

makolber wrote, on Tue, 23 Dec 2014 07:05:32 -0800:

The flame sensor probe acts as a tiny battery and the
controller needs to sense that current. The flame sensor
current is relative to the furnce ground which needs to
be connectd to the AC ground and to the controller ground.

You're the first person to describe *how* the flame sensor
works, so, I appreciate that input, as the first step of
measuring something is knowing how it works.

I had suspected that the DC current through the flame sensor
changes with heat, such that the controller senses that change.

In the same vein, that's the same as saying the resistance
changes, such that the voltage drop across the flame sensor
and ground changes (which, I think, is your battery analogy).

I guess I could remove the flame sensor and put a bunsen
burner on it and measure the resistance change, but, right
now I'm looking up for how to test the current at such low
microamperes, when my Fluke 75 only has a 300ma scale.

Danny,
I don't think you fully understood his battery analogy. It really
generates voltage. Google thermocouple. If you put it in a bunsen
burner, use your Fluke on its DC voltage range and measure the voltage
to see if it is good. (It is also possible you are correct and the
device is a variable resistor that changes with heat, but
thermocouples are very common in gas heating appliances).

Pat