# Milli Volts

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• 04-04-2005, 12:04 AM
wormy
This post got me to thinking...

There's one measurement I've never taken on thermocouples and thermopiles. Guess I've always diagnosed the prob by checking voltage output under various load conditions (no load, pilot only, pilot and main open)
I've never thought to check the resistance.

Hope I dont' forget..... I'll check the resistance on a good thermopile and thermocouple and compare that to one that is weak or bad.
I'm sure it will at least say when one is bad (reads infinity) if it has a broke conductor in it, but wonder what it will say about a weak thermopile/thermocouple.

Winter is almost over, I'll probably not see another floor furnace for another year. Bummer (yeah right... as if I'm excited about crawling under a house hehe)
• 04-04-2005, 03:36 AM
ct2
BWAL2:
( I do use that sort of thing in class. Last I heard, math was a necessary skill.)

I kinda thought you would bring the subject up once in awile

The electricity classes I took were 40 hrs a week for 12 weeks - before we even saw a set of gauges
• 04-04-2005, 08:59 PM
corny
I dont think a resistance measurement on a thermocouple or thermopile would be of much use......They are basically two different conductors welded or twisted together and therfore would have a very low resistance......but Im no expert on this...just seems easier to test one in circuit for the proper voltage output.

Out of curiosity it might be interesting to ohm out a known bad one just to see.....but you better have an ohm meter that is capable of accurate measurements under an ohm or so..... On thermopiles with flexible leads it would help to see if maybe one of the conductors is broken under the cloth covering....but then again...you are gonna check it in circuit and if it dont work then you are just gonna replace it.
• 04-05-2005, 12:06 AM
hearthman
current matter
When assessing thermocouples and thermopiles, ordinarily you can measure the dc voltage output. You'll want to measure the resting and burning millivolts. However, this is only one part of the equation. You'll need to know the specs on the valve for hold in current and drop out current. For instance, a SIT controls 820 NOVA plus valve calls for a hold in or <285mA with a drop out of >125mA. Coil resistance is 0.018Ohms +,- 0.003 Ohms.

However, this is a quick dropout valve with a thermocouple, too. The regular 820 NOVA valve with just a TP only has a hold in of <12mA and a drop out of >4mA. Coil is 10.2Ohms +,- 0.5 Ohms. You need to get the specs.

Voltage alone doesn't tell the whole story. You can have high millivolt output yet it cannot handle that valve. Think of car batteries--they are sold by their cold cranking amps. A TP is also a power source. To measure this in your system, either measure the mA in series or read the mv off TP/TP-TH under load using the T'stat then compare when jumped out. If there is more than about 30 mv variance, the TP is probably bad.

HTH,
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