A thermocouple looks like a tube of metal that is smaller than a regular straw. It has a wire coming out one end with a connector for different applications. It can vary in appearance depending on the type and its purpose.
Full Answer
How to tell if a thermocouple is bad?
MultiMeter Thermocouple Test Procedure
- First, remove the thermocouple.
- Secondly, turn on the meter and set it to Ohms. ...
- 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 ...
What is the function of a thermocouple?
The thermocouple is a device that is used to measure the temperature of a specific point. The thermocouple consists of two dissimilar metals connected together to create two junctions, the one junction is connected to the body whose temperature is to be measured and called hot or measuring junction.
How do you change a thermocouple?
- Choose the new thermocouple based on the size and mounting style of the old one. ...
- Detach the old thermocouple as described above.
- Remove the new thermocouple from the packaging and uncoil it to the appropriate shape.
- Attach the new thermocouple to the pilot assembly and the gas valve.
- Turn the furnace back on and test it to see if the pilot stays lit.
How to change a thermocouple on a fireplace?
- Turn off the gas supply.
- Unfasten the three nuts that hold the thermocouple and the two gas tubes to the valve.
- Detach the thermocouple from the burner.
- Install the new one the same way that you removed the old one.
- Turn the gas back on and follow the lighting instructions on the water heater.
What is a thermocouple?
e. A thermocouple is an electrical device consisting of two dissimilar electrical conductors forming an electrical junction. A thermocouple produces a temperature-dependent voltage as a result of Seebeck effect, and this voltage can be interpreted to measure temperature. Thermocouples are widely used as temperature sensors.
How does a thermocouple work?
A thermocouple can produce current to drive some processes directly , without the need for extra circuitry and power sources. For example, the power from a thermocouple can activate a valve when a temperature difference arises. The electrical energy generated by a thermocouple is converted from the heat which must be supplied to the hot side to maintain the electric potential. A continuous transfer of heat is necessary because the current flowing through the thermocouple tends to cause the hot side to cool down and the cold side to heat up (the Peltier effect ).
Why is chromel wire green?
They operate very well in oxidizing atmospheres. If, however, a mostly reducing atmosphere (such as hydrogen with a small amount of oxygen) comes into contact with the wires, the chromium in the chromel alloy oxidizes. This reduces the emf output, and the thermocouple reads low. This phenomenon is known as green rot, due to the color of the affected alloy. Although not always distinctively green, the chromel wire will develop a mottled silvery skin and become magnetic. An easy way to check for this problem is to see whether the two wires are magnetic (normally, chromel is non-magnetic).
Why do thermocouples lose homogeneity?
As thermocouples age in a process, their conductors can lose homogeneity due to chemical and metallurgical changes caused by extreme or prolonged exposure to high temperatures. If the aged section of the thermocouple circuit is exposed to a temperature gradient, the measured voltage will differ, resulting in error.
Why are thermocouples used in furnaces?
The thermoelectric coefficients of the wires in a thermocouple that is used to measure very high temperatures may change with time , and the measurement voltage accordingly drops. The simple relationship between the temperature difference of the junctions and the measurement voltage is only correct if each wire is homogeneous (uniform in composition). As thermocouples age in a process, their conductors can lose homogeneity due to chemical and metallurgical changes caused by extreme or prolonged exposure to high temperatures. If the aged section of the thermocouple circuit is exposed to a temperature gradient, the measured voltage will differ, resulting in error.
What is the sensitivity of a K thermocouple?
Type K ( chromel – alumel) is the most common general-purpose thermocouple with a sensitivity of approximately 41 µV/°C. It is inexpensive, and a wide variety of probes are available in its −200 °C to +1350 °C (−330 °F to +2460 °F) range. Type K was specified at a time when metallurgy was less advanced than it is today, and consequently characteristics may vary considerably between samples. One of the constituent metals, nickel, is magnetic; a characteristic of thermocouples made with magnetic material is that they undergo a deviation in output when the material reaches its Curie point, which occurs for type K thermocouples at around 185 °C.
What is the purpose of thermocouple wire?
While a thermocouple wire type is often described by its chemical composition, the actual aim is to produce a pair of wires that follow a standardized#N#E ( T ) {displaystyle scriptstyle E (T)}#N# curve.
Thermocouples – the basics
A thermocouple is an extremely simple device used to measure temperature.
Thermoelectric effect
All thermocouples work the same way. They generate a small voltage when they are exposed to heat.
Thermocouple working
A thermocouple works based on the movement of the electrons in its metal wires due to the heat difference between the hot and cold junctions.
Type-K thermocouple
Let’s look closely at a Type-K thermocouple. A Type-K thermocouple is probably the most commonly used thermocouple in industrial applications because it responds predictably across a very wide range of temperatures (say around -330 °F to around +2460 °F).
Summary
So hopefully you’ve enjoyed your walk through the world of thermocouples. While the science of how electrons move through metal can seem complex to the average person, taking advantage of simply connecting two different metal wires together to make a temperature measurement is pretty straightforward and inexpensive.
What is a thermocouple made of?
The thermocouple is made up of a tube, a bracket, and wires. The tube sits above the bracket, a nut holds the bracket and wires in place, and underneath the bracket, you will see the copper lead wires that connect to the gas valve on the furnace.
How Does a Thermocouple Work?
The thermocouple responds to changes in temperature by producing an electrical current that causes the gas valve that supplies the pilot light to open when the temperature is high or to close when there is no direct heat source.
How to test a thermocouple?
Step 2: Open circuit test of the thermocouple 1 Before the test, turn off the gas supply because you must first remove the thermocouple. 2 Remove the thermocouple by unscrewing the copper lead and connection nut (first) and then the bracket nuts. 3 Next, take your meter and set it to ohms. Take the two leads from the meter and touch them—the meter should read zero. Once this check is done, turn the meter back to volts. 4 For the actual test, turn on your flame source, and place the tip of the thermocouple into the flame, leaving it there until it is quite hot. 5 Next, attach the leads from the multi-meter to the thermocouple: put one on the side of the thermocouple, and attach the other lead at the end of the thermocouple that sits in the pilot light. 6 A working thermocouple will give a reading of between 25 and 30 millimeters. If the reading is less than 25 millimeters, it should be replaced.
How many millimeters does a thermocouple read?
A working thermocouple will give a reading of between 25 and 30 millimeters. If the reading is less than 25 millimeters, it should be replaced.
How to inspect a furnace thermocouple?
How to Inspect Your Furnace’s Thermocouple. You will need a wrench, multi-meter, and a flame source, like a candle or a lighter, to perform the test.
How to tell if a thermocouple is failing?
Once you have located the thermocouple, do a visual inspection. You are looking for a few things. The first is signs of contamination on the tube , which can include discoloration, cracks, or pinholes.
Can you check the thermocouple on a gas furnace?
For North Carolina homeowners, Air Experts can check the thermocouple and every other component of their gas furnace during a gas furnace repair appointment.
How does a thermocouple work?
The job of the thermocouple is to keep the pilot lit. It does this by sending a tiny electric current to a sensor on the gas valve signaling the valve to stay open.
What is the job of a thermocouple?
The job of the thermocouple is to keep the pilot lit. It does this by sending a tiny electric current to a sensor on the gas valve signaling the valve to stay open. The thermocouple can do this because, in its probe, it contains two different metals, and a voltage is generated when one of them is heated. This is the result of a phenomenon known as ...
How to test pilot flame?
You can test this yourself using a multimeter, but you may need a helper to keep the pilot flame burning while you do it. Start the test by unscrewing the thermocouple from the gas valve. Set the multimeter to read millivolts, then start the pilot and have your helper hold in the gas control knob to keep it burning.
What is a thermocouple in a water heater?
A gas water heater in Boise, Baton Rouge, Buffalo or Boston uses a thermocouple that includes a probe attached to a copper tube that screws into a port on the gas valve.
Why won't my thermocouple stay lit?
This is the result of a phenomenon known as the Seebeck Effect. Nothing lasts forever, and when the thermocouple wears out, the pilot won't stay lit and your appliance won't work. The pilot can go out for a number of other reasons, however, so if you're having trouble getting the pilot to stay lit, the thermocouple may not be at fault.
How long does it take for a thermocouple to go out?
If the flame lights and goes out when you release the gas control knob after holding it in for the recommended 20 to 30 seconds , that's the sign of a thermocouple malfunction. Before jumping to the conclusion that the thermocouple is at fault, look at the pilot flame.
Why is the pilot flame hot?
The pilot flame must be big and hot enough to heat the thermocouple to the temperature it needs to generate enough voltage to signal the gas valve. Sometimes the problem is simply that the thermocouple probe is too far from the flame.
Overview
A thermocouple is an electrical device consisting of two dissimilar electrical conductors forming an electrical junction. A thermocouple produces a temperature-dependent voltage as a result of the Seebeck effect, and this voltage can be interpreted to measure temperature. Thermocouples are widely used as temperature sensors.
Principle of operation
In 1821, the German physicist Thomas Johann Seebeck discovered that a magnetic needle held near a circuit made up of two dissimilar metals got deflected when one of the dissimilar metal junctions was heated. At the time, Seebeck referred to this consequence as thermo-magnetism. The magnetic field he observed was later shown to be due to thermo-electric current. In prac…
Practical concerns
Thermocouples ideally should be very simple measurement devices, with each type being characterized by a precise curve, independent of any other details. In reality, thermocouples are affected by issues such as alloy manufacturing uncertainties, aging effects, and circuit design mistakes/misunderstandings.
A common error in thermocouple construction is related to cold junction compensation. If an err…
Types
Certain combinations of alloys have become popular as industry standards. Selection of the combination is driven by cost, availability, convenience, melting point, chemical properties, stability, and output. Different types are best suited for different applications. They are usually selected on the basis of the temperature range and sensitivity needed. Thermocouples with low sensitivitie…
Thermocouple insulation
The wires that make up the thermocouple must be insulated from each other everywhere, except at the sensing junction. Any additional electrical contact between the wires, or contact of a wire to other conductive objects, can modify the voltage and give a false reading of temperature.
Plastics are suitable insulators for low temperatures parts of a thermocouple, …
Applications
Thermocouples are suitable for measuring over a large temperature range, from −270 up to 3000 °C (for a short time, in inert atmosphere). Applications include temperature measurement for kilns, gas turbine exhaust, diesel engines, other industrial processes and fog machines. They are less suitable for applications where smaller temperature differences need to be measured with high accura…
See also
• Heat flux sensor
• Bolometer
• Giuseppe Domenico Botto
• Thermistor
• Thermoelectric power
External links
• Thermocouple Operating Principle – University Of Cambridge
• Thermocouple Drift – University Of Cambridge
• Two Ways to Measure Temperature Using Thermocouples
Thermocouple data tables: