-
MSA 72 Griddle pilot won’t stay lit
I have an MSA 72 griddle where the pilot won’t stay lit, looks like a traditional thermopile type safety valve but I don’t recognize this type of thermocouple and I have no idea how it works.
Vulcan 00-412788-00020 20″ Thermocouple
The 2 other pilot valves run just fine, this one you light and the moment you let go of the safety button or the pilot lighting button or whatever its called, it dies.
I have the top propped up right now and the front cover is off, any immediate help would be much appreciated.
-
17 Replies
-
Olivero, that is not a thermocpile. It is a thermocouple. The difference is that a thremocouple is a single bond that will make 50 millivolts, were as a thermopile is multiple thermocoulples and generate 750 milivolts. It has 15 thermocouples welded in sieries. Now this is where you use your DVOM to test the output and continuity of the system. Understand that when the curciut is closed to energise the coil in the gas valve the voltage drops. With a thermopile, it is flexable and will drop to roughly 450 millivolts at the valve. However thermocouples are more sensitive to correct pilot flame. Measure the coil resistance to be sure you have continuity and the thermocouple that it is making correct voltage open. They commonly have high resistance where the tube connects to the valve The leaded center gets oxidized and the tube outer as well. They have to be clean and free of foreign materialand oxidation.
-
Don’t forget the basics:
And if you don’t have one, I highly reccomend a Pilot test tap like this;
and the service manual:Â
-
Everything fixbear posted.
Â
Be aware that a good pilot flame is VERY important. Blast the pilot out with a strong shot of nitrogen before doing anything. The air into the pilot (and main burners) is a vacuum cleaner of greasy air, so it will foul over time.
Â
If you don’t have the thermocouple adapter, it’s not unusual. It can be easily be replicated for future use with a simple part from HD or Lowe’s. If you don’t have or a place to purchase one, (sorry fixbear) for expedience sake – just simply install a new thermocouple in there. I know it’s a PITA to get to, but with some experience, it gets easier.
Â
If you want instructions on replicating that adapter, let me know.
Â
That BASO pilot safety valve (PSV) requires 3mV minimum to remain shut. If you’re sure the thermocouple isn’t the problem, then a new PSV will be needed. That’s not unusual. As a practical matter, I always install a new thermocouple with a new PSV anyway. They’re cheap, but DO use one from Vulcan instead the junk from a big box store.
Â
I’m so pleased to have an actual technician asking for help here instead of the usual cra…uh – stuff.
-
ectofix wrote:
Â
I’m so pleased to have an actual technician asking for help here instead of the usual cra…uh – stuff.
Your not alone on that.Â
Â
This question makes me wonder what size wire is used in the BASO valves. Years ago I tried to wind a new coil for my 1955 GE 450 amp gas drive welder. GE did provide the specs on the coil because they no longer supported it and had gone out of the welder business in the sixties. First problem was even finding 48 gauge wire in a long enough roll to make the coil. Then discovering the impossibility of hand winding that fine of a wire without breaking it. Not possible! Had to ship the clean armature to a company in New Jersey.
Â
But I’m getting off track here. Has anyone tore apart the valve and determined what gauge wire it has to only need 50 MV to hold in the valve. Granted, there is no pull in force needed, just a light holding force. And I’ve never measured how much amperage a BASO valve draws. But it sure has to be minuscule.
-
I find it very awesome how you guys take these types of conversations and escalate them to NASA technical status, lol.
Â
Turned out to just be the thermocouple, changed it out and it worked. Found one we had on hand for something else.
Â
Apparently this is one of the oldest types of flame monitoring there is, yet I’ve never encountered it until now. I thought it was some type of pressurized bulb or something going into the valve.
-
It’s been around for as long as I can remember. And I started working in the fifties. It’s accually called a coaxial themocouple. Years ago before they started using Teflon for the insulation of the wire we would lose them often from shorting due to expansion and contraction. But today’s are really good comparatively.
Â
They also make a woven glass heat insulator to go over the tube to protect the internal insulation.
-
Makes sense, I called in one of our more experienced techs and when he saw it, he laughed at me because I told him, “I’ve never seen this before, I don’t know how it works” and he told me the same thing, it’s been around forever.
Â
So how exactly do they work? He said they are not salvageable, not like a flame rod where you can sometimes get away with just cleaning them, apparently this one is dead when its dead.
Â
Is it just 2 dissimilar metals?
-
Yes. Dissimilar metals.
Â
Oliver, read THIS:
Â
That’s a tiny excerpt from an old training manual I have from the military. The thermocouple uses HEAT to generate an EMF (electromotive force), which is explained on the page labelled 1-29 (page 2 in the PDF).
-
FWIW: That’s also the same concept which some temperature probes operate from. Mainly type K (like your digital VOM uses) and type J thermocouples, which are used for some electronic temp controls.
Obviously the thermocouples used to sense a flame and ones used to read a temperature are quite different.
Â
The other two types of electronic temperature probes in common use are RTDs (resistance-temperature detectors) and thermistors. Both of those work from a relationship between temperature creating a resultant resistance.
-
Coaxial flame sensing thermocouples have come very long way as manufacturing technology has improved. I remember the cap tube on them used to be about 3/16 diameter. And since they didn’t have the heat conductive compounds of today they would take almost a minute to generate enough power to hold in the safety. My hot water heater only takes about 5 seconds.
Â
Most failures come from internal shorts and weld failure at the bond. Also they do still have iron as a wire in some, but that being so brittle is has become unfavorable to high end ones Tight radius of the coaxial tube with high heat variation like under a griddle causes a lot of movement inside. This in turn makes wear at a short.
Â
So, the only way to test them is not with a ohm meter, but to apply heat and measure voltage.Â
Log in to reply.