How to test a thermocouple

How to test a thermocouple

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When the pilot light on a gas water heater won’t stay lit, the problem is usually a faulty thermocouple. The thermocouple is a heat-sensitive device that sends an electrical signal to the gas valve to keep the pilot valve open when the pilot is on. It also signals the valve to close when the pilot goes off to prevent unburned gas from escaping. The thermocouple is situated next to the pilot, and replacing it is an uncomplicated job. It involves reaching into the burner compartment, so if you don’t want to do that, you can remove the burner assembly.

Turn the red gas control knob to the “Off” position and shut off the gas-supply valve. The supply valve is off when the handle is perpendicular to the gas line.

Pull the cover off the the outside of the heater to expose the burner compartment. Remove a second cover as well, if your water heat has one.

Unscrew the pilot tube and thermocouple lead from the burner assembly, using a 7/16-inch wrench. The pilot connection is usually on one side of the red control knob; the thermocouple lead is on the other side. Unscrew the burner supply tube, using a 3/4-inch wrench. It should be directly under the knob.

Pull out the burner assembly. Unclip the thermocouple tube and pull the probe out of the fitting that holds it next to the pilot tube. Insert the probe of a new thermocouple into the fitting and adjust it so that it sits 1/8 to 1/4 inch past the pilot tube. Snap the hose onto the burner assembly.

Push the burner assembly back into place under the heater. Reattach the burner, pilot and thermocouple tubes.

Turn on the gas valve and check for leaks by putting a little dish soap around the connections for the burner and pilot. Tighten the connections if you see bubbles. Light the pilot when you are sure the connections aren’t leaking.

Most natural gas-based appliances such as water heaters, stoves, and furnaces use a thermocouple. This part acts as a safety shut-off device to prevent harmful gas leakage if the pilot light goes out. Knowing how to test a thermocouple when an appliance malfunctions is a vital part of the repair process. Even though standing pilot lights have become somewhat obsolete in recent years, due to the advent of direct ignition technology, there are lots of older, fully serviceable equipment still in use today that use a pilot light, and thus have a thermocouple. Testing one with a multimeter is a job most DIYer’s can do with ease.

How to test a thermocouple

Defining a Thermocouple

Before starting the test procedures, a brief description of what a thermocouple is, and how it works is helpful. A thermocouple is a type of electrical device know as a transducer, and in various forms, is found in everything from small toasters to massive industrial kilns. It basically converts the temperature it senses into a certain measure of DC voltage. There are at least two distinctly different metals inside the part that form two separate junctions. The live (hot) junction is connected to the part where temperature needs to be monitored. The base (cold) junction is either kept open, or is connected to a known temperature source as a point of reference. The difference in temperature between the hot and cold junctions is then converted into a measurable voltage.

Let the Testing Begin!

Knowing how to test a thermocouple with a multimeter is an important step in diagnosing why a pilot light goes out, or won’t ignite at all. Before staring the tests, be sure to thoroughly clean the part first, to eliminate any dust or carbon buildup that might have contributed to it’s failure. The equipment needed to perform the first set of basic open circuit checks are simply a multimeter (either digital or analog), and a pair of double-ended alligator clips.

  1. Using a digital multimeter, turn it to the millivolt (mV) scale on the DC voltage range.
  2. With the alligator clip wires in place on the meter lead ends, clip one test wire to the gas valve (source) and the other one on the exposed copper wire leading to the thermocouple.
  3. Now light the pilot light by turning the on/off switch to “pilot”, and then push down on the pilot valve opening button.
  4. Hold everything in place for up to a minute to obtain the proper reading. The meter should show about 30 mV +/-5mV.

In-depth Tests

If the reading is found to be proper, then most likely the thermocouple can be ruled out as a source of the problem. However, before you call the service pros in, and if you are game, there is a second, slightly more involved test using the same multimeter and clips as before. These tests are known as closed cicuit, and will give you a more thorough assessment of how to test a thermocouple, as well as possibly revealing any other underlying issues such as draft/flue problems, fuel pressure, and gas valve failure. The use of a special screw-in adapter available through most HVAC dealers is the only additional piece of equipment you will need. It will probably amaze and confuse them to encounter such a dedicated DIYer!

  1. The first measurement will check the millivolts drawn by the electromagnetic coil inside the thermocouple. With the pilot light off, and the adapter in place on the thermocouple, a reading of 15 mV should be obtained.
  2. The second measurement can now be made with the pilot light on, introducing a certain “load”. This is mainly to determine if the cold junction portion of the unit is being altered by other heat factors. If the flame is proper, the reading should be about the same as before (15 mV).
  3. The final test is to see how the magnet reacts to pilot light blow-out. Maximum drop-out time is around 3 minutes, with 1.5 minutes being the norm. If the part is good, the voltage with drop to 1-2 millivolts, and there will an audible “click” when the magnet valve shuts.

Thermocouple as a Test-Bed

Since most commercial thermocouples are relatively inexpensive, a case could be made for not bothering at all with the above closed circuit tests, and just go ahead and replace it. However, this would be a big mistake, because how you test a thermocouple can lead to a better understanding of why your appliance failed in the first place. When the part is blindly replaced without knowing the real reasons for failure, it isn’t fixing the original problem.

One of the best ways to get to the root of thermocouple malfunction, while at the same time determining the efficiency of your appliance, is to examine the pilot flame, and it’s relation to the tip of the thermocouple. When the flame is adjusted for optimal performance, it should envelop the upper 1/2 to 3/8″ of the unit, and the tip should glow a “dull-red” color. Anything brighter than a soft, blue flame will tend to heat the tip excessively to a “cherry-red” color, and eventually cause the tip to weld together, and the part to fail. Particular attention should be given to the cold junction portion of the the thermocouple, as it is highly susceptible to any higher temperatures resulting from an improperly adjusted flame.

Precautions During Testing

Safety concerns linked to the above tests mostly relate to the nature and volatility of natural gas. It’s a good idea to perform these tests with adequate ventilation in place, preferably with fresh air flow-through. Extra care should taken when reconnecting the thermocouple fittings to prevent any cross-threading. To be sure there are no gas leaks associated with an improperly tightened fitting, you can use soapy water to check for any bubbles after the gas is turned back on.

Finishing Up

Even though they are fairly cheap to replace, it’s important to know how to test a thermocouple to expose any other problem areas beyond the part itself. Knowing how to perform even a simple function test can give you a good idea whether your appliance just needs an inexpensive repair, or a more costly fix is in order. You can then decide what course of action to take, and be better prepared for the eventual repair/replacement that follows. Being able to troubleshoot any appliance these days can be quite a rewarding and empowering experience!

Table of Contents

  1. How not to guess if the thermocouple is bad
  2. Step by step procedure for testing a thermocouple with a multimeter
  3. What reading you should get if the thermocouple is good. The millivolt thermocouple range.
  4. The thermocouple testing procedure.
  5. 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:

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The thermocouple fits securely in the pilot burner (pictured above)

How Do I Know My Thermocouple is Bad?

MultiMeter Thermocouple Test Procedure

  1. First, remove the thermocouple.
  2. 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.
  3. Thirdly, start the flame and put the tip of the thermocouple in the fire.
  4. 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.
  5. 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

How to test a thermocouple

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:

How to Test a Thermocouple with a MultiMeter

Table of Contents

How to Test a Thermocouple for a Water Heater – Everyone who has owned a gas heater including furnaces or water heaters will know that from to time you have to light the pilot light unless you have the electronic ignition that automatically lights the pilot light.

Sometimes when you try to light the pilot it does not want to stay lit. You know you have a problem. Often times the problem turns out to be a bad thermocouple that needs to be replaced. But how do you know if the thermocouple is bad? Additionally, is it the thermocouple that is the problem, or is it something else?

How to Test a Thermocouple for a Water Heater

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Some may be asking what a thermocouple is? A thermocouple is a safety device used in gas heating for many years. A thermocouple is made up of two dissimilar metals. At the end of the thermocouple is a welded end which is at the tip of the thermocouple. The welded end is referred to as a hot junction. The other part of the thermocouple is not welded and this is called the cold junction.

When heat is applied to the hot junction a small amount of voltage is produced at the cold junction. This small amount of voltage is measured in millivolts. One millivolt is equal to 1/1000th of a volt and the thermocouple for a gas heating appliance produces enough millivolts to keep a safety solenoid inside the gas valve open.

How to test a thermocouple

If the flame goes out the voltage goes away and the safety valve closes. With the safety valve closed the main valve or primary valve inside the gas valve will not open. This prevents the main valve from opening and spewing un-ignited raw gas into the atmosphere where it can be potentially explosive.

When the thermocouple goes bad it will not stay lit and the entire furnace will not function as a heater any longer. It is possibly a bad thermocouple or at least in the process of troubleshooting the gas furnace we surmise that the thermocouple is bad and needs to be tested.

Multi-Meter and Milivolts

How to test a thermocoupleTo make certain the thermocouple is bad you have to test it. For testing a thermocouple you first have to remove it from the furnace, water heater, or gas appliance where it is installed. Once it is removed you will need a multimeter that needs to have the ability to read millivolts and the source of the fire.

Usually, a lighter will suffice for the test. Attached alligator clips to your multimeter and attach one to the very end of the thermocouple where the connection is made to the gas valve. Attach the other end to the stem of the thermocouple.

The stem is not the tip or any part of the tip but the area between where the connection is made to the gas valve and the thermocouple tip itself. Light the fire and hold the tip of the thermocouple inside the fire. As the thermocouple tip heats up you should see a response on the multimeter.

Make sure the meter is set for millivolts. You read 25 millivolts all the way up to a maximum of 100 millivolts although a reading that high is unusual. Anything less than 25 millivolts and the thermocouple needs to be replaced.

Conclusion | How to Test a Thermocouple for a Water Heater

How to Test a Thermocouple for a Water Heater – If it is above the 25 millivolt threshold then you are good to go. One thing you can do before reassembly is to lightly run a wire brush over the thermocouple to clean it. Only lightly though as it can easily be damaged.

You reassemble the thermocouple and then replace it back in the pilot burner and restore everything else back to normal as it was before. How to Test a Thermocouple – Light the pilot light and everything should be good again. Good luck.

How to Test a Thermocouple for a Water Heater

Temperature measurement is possible through several means. We commonly use thermometers for measuring temperature. However, for precision measurements, where even a small spike in temperature requires to be recorded, we use advanced forms of measurement devices like thermocouples. Thermocouples are measurement devices that can read very slight temperature changes. Hence, they are widely used in applications where accuracy is of utmost importance. However, like any measurement device, the efficiency of thermocouples will deteriorate overtime under constant use. At this point, the thermocouple needs recalibration.

What is a Thermocouple Calibration?

A thermocouple is essentially a sensor that detects a temperature change. A thermocouple has two dissimilar wires welded at one end and free at the other. When these wires experience a temperature difference, a voltage is produced, creating a potential difference at the junction. This voltage at the junction is measured and correlated with the temperature. Thermocouples are made rugged and robust. They can withstand a wide range of temperatures. However, since the temperature measurement depends on the voltage, thermocouple calibration at regular intervals is necessary to ensure that the device can successfully recognize the voltage. The process of calibration involves comparing the thermocouple’s measurement accuracy against a known and standard reference.

How to Calibrate a Thermocouple?

Calibration of thermocouples requires specialized equipment. There are mainly three ways we can calibrate a thermocouple. Thermodynamic fixed-point calibration: The thermodynamic fixed point calibration is the most accurate way to calibrate a thermocouple. This method involves comparing the thermocouple’s temperature readings against the globally accepted, fixed temperature points of common elements and compounds where their physical state changes. For example, the freezing point of metal like Tin is 231.928 degrees Celsius as per the ITS-90 or the International Temperature Scale developed in 1990. Keeping the reference junction at 0-degrees-Celcius, the thermal EMF (Electromotive Force) from the thermocouple is measured during the fixed point transition where the metal materials transition from a solid to liquid. This EMF is then compared with standard measurement charts to determine the thermocouple’s measurement accuracy. Thermodynamic fixed-point calibration is the most accurate way to calibrate a thermometer. Stirred bath or furnace method: The option between stirred bath or furnace is used based on the temperature requirements. When the temperature is at the desired level, the thermocouple to be calibrated is used to measure temperature along with a known accurate thermocouple. If the thermocouple needs calibration, the two thermocouples will show different readings. This method is also conducted in a lab, but it is less accurate than the thermodynamic fixed-point calibration. Dry Block Calibrator: The Dry block calibrator method uses a dry-block machine. The thermocouple probes are inserted into the dry-block. The metal block is then cooled or heated to a specific temperature, and thermocouple readings are measured. If the thermocouple reads the same temperature set in the dry-block, it doesn’t require calibration. However, if you spot a variance, then calibration may be due.

Do I Need to Calibrate Thermocouples?

Thermocouples are critical components of a system that closely measure a physical property. Such devices are expected to perform without compromise because an error or faulty reading can lead to something catastrophic. In industries, thermocouples may get subjected to varying temperatures all day long for months on end. Generally, annual calibration is advised for every thermocouple. However, for thermocouples that see constant use, calibration should be made at shorter intervals. With constant use, the efficacy of the thermocouple deteriorates over time. So it is important that calibration should be made to ensure that the thermocouple works smoothly.

How Often Should I Calibrate a Thermocouple?

The frequency of calibration really is up to the user. Many thermocouples are calibrated annually, but more frequent calibrations may be required depending on the conditions under which the thermocouple is used. Conditions such as extended high temperature exposure, severe environmental conditions or thermal or mechanical shock conditions can affect thermocouple accuracy.

How Can I Know When Was the Last Time a Thermocouple Has Been Calibrated?

There is no surefire way to determine the last service of a thermocouple from its physical appearance. However, industries keep a log of parts that have undergone calibration and the ones scheduled for future maintenance. Thermocouples should be calibrated when:

  • Their calibration cycle is due.
  • Their performance comes into question.
  • When subjected to unexpected or unusual operating conditions.

Thermocouples should be changed when:

  • The thermocouple exhibits an open circuit.
  • When an ungrounded thermocouple exhibits low electrical isolation from the sheath or housing.
  • If there is any damage to a thermocouple due to mechanical forces.
  • The sheath is compromised due to corrosion or oxidation.
  • When calibration identifies deviation of the accuracy outside of the specifications.

Quality Certifications of Thermocouples

With a temperature change, a standard thermocouple gives a reading that corresponding to the temperature change. However, how can we verify that what we see is reliable? To sort this issue, we adhere to international standards that are put in place to provide a reference for measurements. ISO and NIST are among the most widely renowned standards used to gauge the performance of thermocouples. At Omega Engineering, we provide calibration services that adhere to international standards. Omega uses various fluid baths, furnaces, and calibrators to perform temperature calibrations over a wide temperature range. Each heat source is outfitted with an independent NIST traceable standard. Contact us for your thermocouple calibration requirements and experience top of the line service and unparalleled work quality.

How to test a thermocoupleAfter a long day in the saddle you finally make it to the campground. In the back of your mind, you can hear the shower beckoning. Being the proud owner of a dinosaur RV, your water heater isn’t one of those fancy “flip the switch and go” jobs; rather you have to light the pilot.

You set the burner control valve to “pilot,” and press the pilot light control button and toss some flame down on the end of the pilot light burner. The flame is good—and you wait the obligatory 30 seconds or so, then release the pilot light control button. As if by magic, the pilot light vanishes! So you repeat the process until you’ve burned your fingers with matches, or turned the air blue with some unrepeatable language.

The most common cause of this problem is a faulty thermocouple. A thermocouple is a safety device designed to shut down the gas valve should the pilot light go out. The “business end” of the thermocouple is a metal piece that lies about 3/8 to 1/2 inch into the pilot flame. A metal connector leads back to a threaded connector that screws into the gas valve. In practice, heat produced by the pilot flame is converted by the thermocouple to electrical current. The current holds an electromagnetically controlled valve open, allowing gas to flow. No current, valve shuts, gas stops flowing.

Thermocouples eventually reach an end to their useful life. To test them, you’ll need to take them out of their appliance and use a digital electrical tester with a millivolt scale. It’s an easy job and can save you no end of frustration.

“Mr. Safety” here: It’s a good idea to close your rig’s propane gas valve(s) before you embark on this journey. Since propane gas is highly flammable (and concentrations of it can explode) if you feel the least bit “not up to the job,” get professional help.

You’ll need to identify the thermocouple in your unit. Inside the main burner there will be an assembly containing both the thermocouple and the pilot flame burner. The thermocouple is cylindrical and sits in the pilot flame; hence it will be the topmost of the pilot burner/thermocouple combination.

How to test a thermocoupleTo remove the thermocouple, chase the connecting wire (or sometimes it looks like a tube) back to the gas valve and use the appropriately sized open-end wrench (typically 3/8-inch) to unscrew the threaded end from the gas valve. Gas valve? That’s the metal box with the control knobs on it. Now remove the sensor bulb from the pilot light assembly. You’ll find that on the other end of the wire-like connector coming from the fitting you took out of the gas valve.

Now set your digital voltmeter to the millivolt scale. Connect the positive lead to the button on the end of the threaded fitting. Connect the negative lead to the jacket of the thermocouple. Here’s where it gets tricky: You’ll need to put the sensor bulb in a live flame (a lighter is fine) for about 45 seconds. Get an assistant with asbestos fingers or use a pair of pliers.

How to test a thermocoupleAfter 45 seconds the voltage reading produced by the thermocouple should read a minimum of 12 millivolts; ideally the reading will be around 26 millivolts. If less than the minimum, replace the thermocouple. If the thermocouple readings fall in acceptable value, then you may be looking at a bad gas control valve.

Putting the new thermocouple back in is a simple “reverse order” job. But beware: when replacing a thermocouple, screw the fitting in to finger tight, then wrench turn 1/4 turn more. Any more than that can damage the thermocouple, rendering it useless. Now open your gas valve(s) and fire up. It’s shower time.

Published: June 18, 2014 – by Tim McElwain

How to test a thermocouple

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.







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, 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 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 MVs – normal is 1 to 2 MVs. 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.


A normal set of readings

OC- 30 millivolts

CC- 15 millivolts

CC (load) -15 millivolt

The best way to be able to diagnose these readings is to use MILLIVOLT CHARTS.

Thermocouples from different manufacturers vary as to their dependability. The only thermocouples I recommend are made by Johnson Controls. The K15 and K16 series are the best. If you are having durability, problems then use the K16RA, which is a nickel-plated high ambient or corrosive environment thermocouple. The Husky (K16) will fit most applications and for those that it does not the Slim Jim (K15) will fit.

To repeat the adjustment of the pilot flame to envelope the upper 1/2 to 3/8 of the thermocouple is important, the flame should be a soft blue flame not roaring which will cause the tip to glow a “dull red” versus “cherry red”.

The combustion condition (excessive temperatures) in the chamber is also an issue and this will require a combustion test and draft measurement to insure that excessive temperatures are not being applied to the pilot. In some cases on water heaters, it may be necessary to alter the pilot adaptation to get better quality performance. This however should not be done unless you have had proper training.

The possibility of the equipment operating in a depressurization environment will certainly lead to thermocouple failure. In addition if the equipment is flued together with a “fan assisted” furnace or boiler this can lead to problems. There are solutions to this also but training is required.

The thing that I find is often a problem is the environment in which the equipment is operating. Many times corrosive chemicals and airborne contaminants are being drawn into the air gas mix and a chemical reaction takes place. This again requires attendance at a training session by a professional combustion person to help you to see the various affects this will have.

Lastly, the failure to put all the doors and covers back in place on equipment. The failure to do this will cause an alteration in combustion air and the flame stability is affected.

The design of some equipment is also a problem. When there is high demand for heat (very cold weather) the temperatures that are created in the chamber have an adverse affect on the pilot and thermocouple system. The addition of the K16RA thermocouple can offer some assistance toward extending the life of the thermocouple in this situation.

Insufficient air for combustion and dryers operating in close proximity to equipment also lead to problems.

Last of all and this is not directed at any one in particular but just plain lack of service personnel and installers knowing what they are doing.

I want to test a thermocouple loop by injecting a millivoltage from the field where the thermocouple terminates in its compensating cable back to a substation and a thermocouple/mA converter card with cold junction compensation. Let’s say the field is 30 degrees Celsius and the substation is 10 degrees Celsius..

How to test a thermocouple

To test a thermocouple loop, you’ll need to go back to thermocouple basics, more precisely to the three laws of thermoelectricity. In particular, the law of homogenous materials, the law of intermediate materials, and the law of intermediate temperatures.

How to do a 4-20 mA thermocouple loop test

Here we need the law of intermediate temperatures. This law states that the sum of the electromotive force (EMF) from a thermocouple with its ends at temperatures T1 and T2 and another thermocouple with its junctions at temperatures T2 and T3 equals the EMF from a thermocouple between T1 and T3.

Confused yet? This picture should make testing a thermocouple easier.

Courtesy of HFO Power Plant

Now that you have everything you need to test a thermocouple, let’s move on to a more specific case. Say we want to test a loop by injecting a millivoltage in its compensating cable back to a substation and a converter card with cold junction compensation. Let’s make the field 30 degrees Celsius and the substation 10 degrees Celsius.

You’ll have to sum up the millivolts (mV) for each temperature from the sensor to the input card. You’ll also need the temperature where you have the card.

To get the mV for each temperature, you’ll use a thermocouple reference table. If you can’t find your mV reading on the table, then you just do a little interpolation.

How to test a thermocoupleCalculation for real temperature into input card – Courtesy of SENAI

You’ll find a minimal error margin most of the time.

Want to know more about Thermocouple loops, feel free to get in touch our engineers!

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