Table of Contents
- How not to guess if the thermocouple is bad
- Step by step procedure for testing a thermocouple with a multimeter
- What reading you should get if the thermocouple is good. The millivolt thermocouple range.
- The thermocouple testing procedure.
- Lots of resource and related links to help you learn more
How to Test a Thermocouple with a MultiMeter? What is the thermocouple millivolt range for a good thermocouple? What is the thermocouple test procedure? So you have found the pilot light will not stay lit when you try to light the pilot light. You know you have gas, and everything seems to be fine so what is the problem? That will help with a furnace thermocouple test. It also is good for water heater thermocouple testing.
How to Test a Thermocouple with a MultiMeter
You ask a friend, and the friend tells you the thermocouple is bad. So, the next question, “How to Test a Thermocouple with a MultiMeter”?
So how do you test the thermocouple to see if it is bad or not? You will need a multimeter that will read millivolts (most meters read small voltage like millivolts, which is 1/1000th of a volt) and a source of fire such as a lighter or small torch. How to Test a Thermocouple with a Multi-Meter step by step:
The thermocouple fits securely in the pilot burner (pictured above)
How Do I Know My Thermocouple is Bad?
MultiMeter Thermocouple Test Procedure
- First, remove the thermocouple.
- Secondly, turn on the meter and set it to Ohms. Touch the two leads together from the meter, and you should get a reading close to zero. With the leads apart, you will read infinity. That is simply a test to check the meter for continuity. Finally, NOW TURN THE METER TO VOLTS.
- Thirdly, start the flame and put the tip of the thermocouple in the fire.
- Next, ensuring the tip is nice and hot from the flame, take one lead from the meter and put it on the shaft of the thermocouple and the other lead and put on the end of the thermocouple where it makes contact with the gas valve.
- Finally, if the reading is less than 25 millivolts replace the thermocouple as most standing pilot gas valves need 25 or more millivolts to keep the pilot flame lit. The thermocouple millivolt range should be above 25 millivolts. If so then it is good.
How to Test a Thermocouple with a MultiMeter
If you test the thermocouple with the multimeter and you get a reading above 25, then you have another issue. Importantly, always make sure the tip of the thermocouple is directly in the pilot flame. If the tip is in the flame, you have held the nob on the gas valve down for longer than a minute, the pilot flame is strong and steady, and the pilot still does not stay lit, then you likely have a problem with the gas valve. Lastly, Gas valves cannot be repaired as per any manufacturer. You need a new gas valve. Finally, Good luck!
How a Thermocouple Works | Thermocouple Testing Procedure Understanding
A thermocouple is a safety device for a standing pilot gas system. Originally, it had a ratio of copper and nickel and was labeled a Coppel (Copper/Nickel) element. Thomas Seeback discovered it in 1821. In the early 1900s, Honeywell applied it to the safety gas valve.
A standing pilot gas valve has two inner valves, one for the pilot light and one for the main valve. If the pilot valve is closed the main valve would not open. The thermocouple was used to hold the pilot valve open. Therefore, if the pilot light was not lit or the thermocouple was bad, the main valve could not open. That provided a means of proving the main burners would ignite when the main valve opened.
The tip of the thermocouple is called the hot junction and is where the two dissimilar metals join. A weld joins the two metals. The other part of the thermocouple tip is called the cold junction. From the cold junction, a stem runs in length from the main body of the thermocouple tip to the gas valve.
When the heat hits the hot junction or tip of the thermocouple, it produces millivolts. This fractional voltage energizes a small solenoid designed for millivolts. As long as the solenoid is energized, the pilot valve inside the gas valve remains open. That allows the main valve inside the gas valve to feed the main burners with gas.
Conclusion | How to Test a Thermocouple with a MultiMeter
Since this is gas and there is a possible safety issue if you feel uncomfortable doing this call a professional. Plumbers and HVAC service companies are skilled and knowledgeable with standing pilot gas systems. They can troubleshoot the problems and replace the thermocouple if necessary. If you are interested in other examples of troubleshooting standing pilot systems including thermocouples here are few resources for you:
Gas Valve Troubleshooting | How To Light a Pilot Light | Why Does My Pilot Light Keep Going Out
How to Test a Thermocouple with a MultiMeter
Hi. I have an un-vented gas fireplace. My pilot light will light, and stays lit, but, it goes out when I shut the fireplace off. This is happening with increasing frequency. So, I’m not sure if I have a thermocouple or thermopile issue. When the pilot is on, the fireplace turns on readily when I flip the wall switch. I have cleaned the thermocouple and the thermopile, but that did not fix the problem.
It is likely a thermopile although there are some ventless gas logs that use thermocouples also. You have to be careful when cleaning it. Never use anything abrasive to do so as you could damage the thermopile or thermocouple. If you haven’t done so make sure to test it as instructed on this page. Also make sure the tip of it is directly in the fire. If not then that could lead to problems. Lastly, replace it. They are cheap enough. They are available here.
My analog multi-meter cannot discern sub-unit readings, I need to get a digital one that does. Anyway, as you suggested, I have ordered a thermopile that is compatible with my unit.
The thermocouple is soldered to a bracket that the thermocouple/thermopile/ignitor/gas nozzle are affixed to. Everything is in alignment to the flame. Since I cannot locate a thermocouple that looks like the one on this assembly, I would have to modify the bracket so that it might receive an ‘aftermarket’ thermocouple. Not sure I want to do that. We’ll see if the thermopile fixes the issue.
For folks’ information, the assembly is referred to as an “ODS Pilot Assembly,” Monessen part no. 14D0473.
As an update, cold weather set in yesterday, I have been using the gas log fireplace more often, and running it hotter, than usual and since cleaning the pilot has begun to stay on more reliably. The fireplace did, however, shut itself off (pilot stayed on) once. This further supports a thermopile issue.
Thank you for your service. Best wishes.
No, you need to find that particular part or one specifically for gas logs. ODS is an oxygen depletion sensor. I wouldn’t modify anything with a fossil fuel system especially something that is ventless. The ODS system may be the problem. It’s function is to measure the oxygen level in the room. If the air or oxygen decreases the pilot becomes erratic and lifts off of the thermocouple or thermopile. And you do know there is a difference between a thermopile and a thermocouple? They are not the same. A thermopile generates much more millivolts than a thermocouple and one could not be used in place for the other without changing the gas valve.
Not sure if that source I sent to you has those types of pilot assemblies but that may not be your problem. If it has an ODS as you describe then the place you are speaking of is likely too tight and does not allow enough infiltration air into the house or cabin or whatever to supply you with combustion air to burn the gas logs. It is consuming too much air. Air that you need to live. At this point I advise you to call in a professional to check things out. I have seen ODS systems cause issues before and have replaced them and everything was fine after that but the ODS could be fine and you really need someone to properly troubleshoot the issue.
my gas stops flowing to the pilot after a few minuets on my ventfree logs my flame is not lifting off of the thermocouple good voltage reading on thermocouple
If the thermocouple is testing good then you likely have a bad gas valve. I would recommend a professional do the work in replacing the gas valve. Gas valves cannot be repaired and in some municipalities require a licensed professional to install.
Your btu chart is not necessarily right. Altitude and gas value affect it. In our area at 5600 ft our gas value is only 825 btu per cubic ft.
Okay, thanks for letting us know. I did know that I just didn’t take it into account when I made the tool. So, back to the drawing board on making adjustments for altitude. We did add the following: *At sea level. Adjustments are made for higher altitudes. Manufacturers/Contractors derate gas appliances in elevations over 2000 feet. Additionally, gas companies may add additional gases.
Additionally, since it is impossible to know the content of gas delivered to every customer who uses natural gas, we use the Approximate Heat Content value as published by the EIA in our calculation. We did have this addendum on our calculator when we published the calculator originally.
“Calculation is based on the Approximate Heat Content (BTU Value) of Natural Gas as published by the Energy Information Administration for 2017”.
Thanks Richard. I appreciate your help.
I have a furnace. Currently it is out of order. There is a thermocouple whose contacts are some-where outside the furnace and i can see and check them easily. Is there any method to check this thermocouple with Digital |Multi Meter ? without Turing on the flame or fire ?
you need to apply heat to the tip of the thermocouple to get a reading on your meter as explained in the article.
Robert,
You are a freaking jerk. This guy writes a helpful article and makes one small mistake and you call him a idiot. Your a buffoon. I’m sure you write a lot of helpful articles. Don’t you.
Richard,
Thanks for the info. I have a tank top propane heater that won’t stay lit. The thermocouple is putting out 26mV so I would think that would be good. The valve will stay open if I connect a AA battery(I know I am putting way too much voltage). But,It didn’t burn it up(only tested for a few seconds). Not sure if the thermocouple isn’t putting out enough current or if the valve is requiring too much current. Any thoughts?
Thanks,
Jamie
Replace the valve. Valves are not repairable. And I would not advise using a battery to test the valve. As you said, you put way too much voltage on it. If the thermocouple is putting out 26 mV then something is wrong with the valve. Gas valves are subject to corrosion like anything else. Natural gas or even LP is not 100% pure and has a slight moisture content. This causes corrosion problems sometimes. Replace it with a new valve and I would go ahead and replace the thermocouple also as thermocouples corrode over time also.Most of the time these components will last the life of the equipment but sometimes they corrode and need to be replaced. Good luck!
Yes you should have a pilot adjustment try turning the pilot a little higher
Thanks for proof reading and ensuring all our articles are idiot proof. This planet needs more geniuses like you around. BTW, it never exactly said what you wrote but may have implied it for those who lack common sense.