Thermocouple testing

JohnNY8

I've tried to test the voltage generated by a common thermocouple but never quite got a satisfactory reading telling me whether or not it was doing its job.

Where's the proper place to apply the test leads and what's a good and bad reading?

Unknown

New Guy

go over to www.oiltechtalk.com and find "Ask Tim McEwlain a question" in the dicussion area is a complete explanation of testing thermocouples.

If I have some time I will post it here for you.

Unknown

Thermocouples

I am often asked about troubleshooting a thermocouple on gas systems. This will be a permanent reference that will give a step-by-step procedure.

A thermocouple is a device used to satisfy pilot safety on many 24 volt gas systems. The thermocouple is a device made up of two dissimilar metals. They are joined together at the tip (Hot Junction). When heat is applied to that hot junction a small millivoltage is created. This develops because of temperature difference between the hot junction and what is called the cold junction. The flame has to envelop the upper 1/2" to 3/8" of the thermocouple and the tip should glow a "dull red". If the flame is adjusted to a sharp flame it will glow "cherry red" this will cause the tip to be welded and eventually the thermocouple will fail. The flame should be adjusted to a soft blue flame, not roaring or lifting. The normal millivolt output is 25 to 35 millivolts, on some you may even get up to 35.

The other part of this safety pilot system is the electromagnet (power unit). It is if you will the LOAD and we can say the thermocouple is the SOURCE. The electromagnet is made up of a coil of wire and "U" shaped iron core.When the thermocouple is heated and the millivolts generated the coil will be energized and create a magnetic field. The magnetic field will cause the "U" shaped iron core to be magnetized, it in turn will hold open a seat allowing gas to pass through.

When this system malfunctions it typically causes the pilot to go out and the gas will not flow. The first thing that should be done when arriving at a pilot outage situation is to do some visual checks.

1. IS THE PILOT LIT?
2. IS THE PILOT CLEAN? (NOT YELLOW)
3. IS THE PILOT HITTING THE UPPER 1/2 TO 3/8 OF THE THERMOCOUPLE?
4. ARE CONNECTIONS TIGHT?
5. IS TIP DAMAGED?
6. IS THE COLD JUNCTION BEING HEATED BY THE PILOT OR MAIN BURNER FLAME?

Once those things are addressed it is a good idea to take some millivolt readings. It should also be mentioned that many times it is the policy of some to replace the thermocouple on a call and clean the pilot. It is not a bad thing to do, however it is statistically about 85% of the time it is the thermocouple giving the problem. It is the other 15% of the time that taking readings can solve other problems.

You need a multimeter with a DC volt scale as the millivolts generated are DC volts. There are four readings we are going to take they are

OPEN CIRCUIT - this is taken with the thermocouple disconnected and the meter leads attached to the outside of the thermocouple and the other meter lead attached to the tip of thermocouple (the part that was attached to the valve). The pilot-on-off knob will have to be held manually to take this reading. This measures the output of T'couple the readings must be above 17 to 18 millivolts.

* CLOSED CIRCUIT - This measures the millivolts used by the coil in the electromagnet . A rule-of-thumb is this reading should be roughly half of the open circuit. It is taken using an adapter screwed into the magnet and the thermocouple screwed into the adapter.

CLOSED CIRCUIT LOAD - This reading is taken the same as the previous reading except the burner is now on. With a proper flame this reading should be about the same as the previous reading. With a lifting main burner flame or excessive drafts or chimney pull, this reading may reduce from previous reading (flame being pulled away from the thermocouple). With the cold junction being heated this reading may increase. If the "cold junction" is heated excessively it will break down.

DROP OUT - This is the final reading. It requires the pilot to be blown out. It measures the ability of the magnet to hold under reduced MV input. A good unit should drop out below 6 MV's - normal is 1 to 2 MV's. The allowable "drop out" time is 180 seconds yes three minutes. It is more likely to be a minute and half to two minutes. There will be an audible "click" when the magnet shuts down.

* THE CLOSED CIRCUIT READING REQUIRES A SPECIAL ADAPTER THAT SCREWS INTO THE MAGNET ASSEMBLY TO ALLOW CONNECTION OF THE METER. AN ADAPTER CAN BE PURCHASED FROM ANY ROBERTSHAW DEALER THE PART NUMBER IS 10-038 THERMOCOUPLE TEST ADAPTER.

A normal set of readings

OC- 30 millivolts
CC- 15 millivolts
CC(load) -15 millivolts
DO- 1 millivolt

The best way to be able to diagnose these readings is to use MILLIVOLT CHARTS these can not be displayed here but I can provide them if you e-mail me.

I will add a little more to this later but for now this will be a good start.[/b]

jeff_51

hey tim

nice post. I downloaded that one into my favorites file along with the copper bed and the Nash

Bob Harper

one twist

The drop out rate Tim is referring to applies to standard furances, boilers, water heaters and other Cat.1 heating appliances. I deal with fireplaces M-F. Those numbers also apply to vented gas logs, B-vented fireplaces, and ventfree(lung vented) appliances. The dropout rate for sealed combustion direct vent fireplaces, however is 30 seconds per ANSI. This is because you are dumping unburned fuel into a sealed combustion chamber with a continued source of air just waiting on ignition. Luckily, most of these pilots drop out in about 10-15 seconds anyway, unless you are one of those recalled Robertshaw valves where the EPU can stick open. This is all the more reason to test for dropout.
BTW, being in an enclosed combustion chamber, the pilot assy. heats up once the burner lights. Since the entire pilot assy. is heated without the benefit of a nice cooling updraft, the burning pilot millivolts will drop some on a direct vent Fp. However, it should stabilize. If it doesn't, in addition to testing the TC, pilot, inlet gas pressure, valve, etc., you need to consider the venting. If the rise/run allowables are violated or the system doesn't "breathe" properly, the pilot can overheat causing an unwanted pilot outage. Sticking a pilot assy. into a fire and trying to maintain a 400F differential btw hot and cold junctions is not easy in a sealed fireplace.
Bob

Unknown

What Bob is referring to

is called a "quick drop out thermocouple" such as a Honeywell Q335. They have been around for years and their purpose is to give a much quicker drop out for safety reasons. In particular a poor combustion condition which pulls the pilot flame away from the thermocouple allowing it to cool and drop out in 30 seconds or less. On a lot of decorative appliances that are using a 750 millivolt system to operate the equipment. You will notice a thermocouple out to the side of the pilot generator. The thermocouple is connected to the pilot safety circuit. The 750 millivolt generator to the actual valve coil. That way any poor combustion situation will quickly affect the thermocouple and shut down the first line of defense the "pilot safety" which means all flow of gas will cease even though a demand for heat continues.

Rodney Summers

thermocouple testing

Hi M. DeFord
I work in a poultry farm,and we have 24 volts Robertshaw pilot controls on the radiant Lp unit that hang from the ceiling to heat the poultry house. They are Shenondoah brand and the pilot uses a thermocouple to let the pilot burn or turned off as a regular pilot does. Would it be safe to use the procedure you describe to test the ones we think are defective(we have accumulated quite a few)using the testing part number you give. Its not only a matter of saving money,but from where we are it is quite a drive to go and get a few when in an urgent need(in wintertime) it would be much simpler and quicker to use your way of doing.Any idea what brand they are? Thanks
Pierre
pedrofun11@hotmail.com

Unknown

Dave DeFord

the procedure will work fine with any thermocouple operated system.

There are several adapters that all do the same job they are:

Universal Enterprises ATHA1 Millivolt Adapter or Robertshaw 10-038 Test Adapter.

You can purchase them from:

Connolly Distribution

39 River Road

P.O. Box 7189

North Arlington, NJ 07031

800-223-1209 Ask for Jackie

OR

Controls Incorporated at 800-334-5886

e-mail parts@controlsincgas.com