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# Thread: How do I test Millivolt valve system - Pilot is on, but main burner not lighting

1. Originally Posted by mnalep
and after I put it back in the 2nd tome the reading was at the normal 750mV. Not sure why it was low the first time?
I assume 750 mV is open circuit voltage. The voltage for a t'pile is related to flame temperature. There's probably a graph somewhere online for the relationship between temp. and voltage.
A blue flame indicates more nearly complete combustion than a yellow flame.

Two things define a voltage source: open circuit voltage and output voltage when current is being drawn.
Voltages sources have a Thevenin equivalent circuit and it's pretty useful to figure out the two values in this equivalent circuit.
Open circuit voltage is only half the story.

You might want to buy a 3 ohm resistor and measure the t'couple voltage output when loaded down with this many ohms. Independent tests of each system component comes in handy.
If the internal resistance of the t'pile is 3 ohms you will get 750/2 = 380 mV across the resistor.

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