Thermopile mv reading: Can it be too high?
Recently I replaced the thermopile on a gas log fireplace(White Rogers Valve). The old TP was reading about 140 with only the pilot running. So, replacement seemed like it would do the trick. I replaced it, and fireplace ran fine. Just for kicks I decided to test the mv on the new TP. I put my meter on and it was reading is about 950mv, almost 1 full volt... I adjusted the pilot as low as it could go (the pilot screw didn't seem to affect the pilot flame at all) and got it down to 920mv. The pilot didn't seem overly large anyway.
After talking to the customer she had the TP replaced a year ago and it had already gone bad. Does this excess of voltage tax the TP to the point where it will deteriorate every year?? Can you have too much voltage out of a TP??
Most RS are rated at 250-750 mv. I don't know that I've ever seen one read that high.
If you waited at least 5 minutes before taking reading 2 and if the pilot flame is observed to be correct, you must have purchased "the fabled golden tp generator"!
It won't hurt the magnet or that circuit as long as you don't get line voltage. If you get line voltage off that tp gen, you might have discovered the first step of solving the energy crisis. If that's the case, email me privately off-line... ;-)
Voltage output is related to temperature. If you are getting over 900 mv then overheating may be the reason it is failing so quickly. Is this unit burning LP gas? If it is, there may be a natural gas orfice in the pilot assembly causing the TP to overheat and fail.
He said the pilot looked correct, though.
If it's got a NG orifice in there, it's going to be a LARGE pilot flame and easily distinguishable.
The pilot flame should envelope the top 1/3 of the tp gen.
SOME units have had issues with tc's burning out and they've gone to an industrial jacketed tc instead of the factory-installed. I haven't heard of any tp gens with those issues if the pilot is a lp orifice for lp and it's properly adjusted, other than an occasional bad electrical part. And even that's rare anymore.
Originally Posted by logdoc_rob
First, check mv output with another meter to be sure of reading. Check your parts source. There are TPs that can put out >900mv http://scholar.lib.vt.edu/theses/ava...cted/chap2.pdf
We don't want to get into too much detail on testing: see site rules.
Keep the fire inside the fireplace.
I'm certainly not disputing that there might be tp gens that can produce higher voltages, but we're discussing fireplaces here and a typical 1950-001 which is designed to produce 250-750 mv.
I do agree with your suggestion to check the mv output with another meter, but how many techs truly carry a spare meter on their truck? I do carry a little cheap radio shack flip case mm, but only for an emergency if my Greenlee mm would get damaged while out on a service route.
Use screen name when referencing another member. Not all want their private info in a public forum.
Originally Posted by hearthman
Last edited by Senior Tech; 11-15-2009 at 04:07 PM.
Did you measure the output while it was open-circuited or while it was connected to the gas valve?
Originally Posted by jwgagnon
Both & calling for heat
Originally Posted by WhoIsThat?
There should have been a slight drop in open circuit voltage when connected to the gas valve.
Originally Posted by mike3
The pilot oriface is setup correctly, the house has NG and it is setup for NG. I guess the flame looked a bit on the large side, but not horribly large. The pilot adj screw did absolutely nothing anyway.
I tested when it was connected to the valve. I wouldn't think this would cause an increase in mv but i could try testing with it unhooked.
The thermopile I used was one we use on most all our gas fireplaces, mostly staying around 600mv max.
Not sure if this matters, but the valve location was mounted in the basment and the TP wire was about 5 ft long to reach the terminals on the valve.
In retrospect, I kind of wonder if the valve has some issues. The pilot screw did nothing to the pilot when i adjusted it. Also, while unit was running, the flame was abnormally large for a gas log. (even with the burner covered with sand and embers). I ended up turning the gas shutoff 1/4 shut to make the flame appear more normal.
I'm going back to the house later this week with another multimeter and a pressure checker for the valve. I'll let you guys know what I find out. Thanks for all the imput!!!
Measure the current into the valve and the valve resistance.
Originally Posted by jwgagnon