Thermocouples

Chapter One – What is a Thermocouple?

A thermocouple is a transducer that converts thermal energy into electrical energy and is constructed by joining wires made from dissimilar metals to form a junction. Voltage is produced when the temperature at the junction changes.

The concept of the thermocouple is based on the Seebeck Effect, which states that if dissimilar metals are joined at a point they will generate a small measurable voltage when the temperature of the point of connection changes. The amount of voltage depends on the amount of temperature change and the characteristics of the metals.

The structure of a thermocouple consists of two insulated wires connected to a measuring device with coaxial sheathing that is separated by insulated material. Thermocouples serve as a safety and monitoring gauge for various types of equipment.

The process of a thermocouple can be seen in the image below where the temperature is raised at the junction of the wires on the left, and the change in temperature is displayed on the gauge at the right.

Chapter Two – How Does a Thermocouple Work?

When the two wires of a thermocouple are joined to form a junction, one of them is connected to the body of the thermocouple and measures temperature. It is referred to as the hot or measuring junction. The second junction is attached to the body of a known temperature and is the reference junction. A thermocouple measures an unknown temperature and compares it to a known temperature.

The idea of a thermocouple is based on three principles of effect discovered by Seebeck, Peltier, and Thomson.

Seebeck effect:

The Seebeck effect happens when two different or unlike metals are joined together at two junctions and an electromotive force (emf) is generated at the two junctions, which is different for different types of metals.

Peltier effect:

An emf is generated in a circuit when two dissimilar metals are joined to form two junctions due to the different temperatures of the two junctions of the circuit.

Thomson effect:

The Thomson effect is when heat is absorbed along the length of a rod whose ends are at different temperatures. The temperature of the heat is associated with the flow of current to the temperature along the rod.

How a Thermocouple Works

The circuit of a thermocouple is shown in the image below, where A and B are two dissimilar wires that are joined to form a junction. The two junctions are at different temperatures to generate the Peltier emf in the circuit, which is the function of the temperatures of two junctions.

Electrons carry heat and electricity. If a piece of copper wire is heated at one end, the electrons will move along the wire to the cooler end and create a temperature gradient along the wire. The heat has been changed into energy. This same principle, as discovered by Volta and Seebeck, applies to a thermocouple.

If the temperature of the junctions of a thermocouple is the same, an equal and opposite EMF will be generated at the junctions, and the current flow will be zero. If the junctions have different temperatures, the EMF will not be zero, and current will be flowing through the circuit much like the heat flowing through the copper wire. The flow of the EMF through the circuit depends on the metals and the temperature of the two junctions, which is measured by a meter.

The EMF in the thermocouple circuit is very small, in millivolts, and requires a highly sensitive instrument for determining the generated EMF. Galvanometers and voltage balancing potentiometers are normally used with the potentiometer being used the most often.

A potentiometer, also known as a pot or potmeter, measures potential difference by comparing an unknown voltage to a reference voltage. It can provide high precision measurements. It is defined as a three terminal variable resistor and acts as an adjustable voltage divider.

A galvanometer measures very small electric currents. They are used to measure null deflection or zero current.

For a thermocouple to make an absolute measurement, it must be referenced to a known temperature, such as freezing, on the other end of the sensor cable. The hot junction is the measuring assembly, while the cold junction, as seen in the diagram below, is the reference where a cold junction compensation chip is located. The cold junction temperature may vary but provides a reference. The cold junction can be fixed by immersing it in water to maintain a constant temperature.

Ambient air can influence the reference temperature. It can be calibrated and adjusted by a reference junction compensation device.

Chapter Three – Types of Thermocouple

The difference between thermocouples is determined by the types of alloys used to produce their wires. The choice of the type of metal wire depends on the range of temperatures to be measured, the environment, and their mechanical strength. There are three types of connecting points for thermocouples – exposed, ungrounded or insulated, and grounded.

1. Exposed junction

   In an exposed junction, the conductors are outside the thermocouple sheath. They have a very rapid response to temperature changes but can be easily damaged. The best use for exposed junction thermocouples is where a fast response is required, and there is a low risk of damage to the sensor.

2. Grounded junction

   In a grounded junction, the conductors are connected and welded to the external sheath, which forms a sealed connection. Since the wires are connected to the sheath, they are not insulated and may be affected by EMF’s. They are used to measure temperatures in corrosive environments and are the most common form of connection.

3. Ungrounded junction or insulated

   An insulated junction is insulated, usually with magnesium oxide, from the protective sheath, which protects the sensor from EMF’s. The response time of an ungrounded thermocouple is slower than a grounded or exposed junction types. They are used to protect sensitive electronics from feedback voltages.

Thermocouple Sheath Material

A thermocouple can be enclosed in a sheath to protect it from the atmosphere and reduce the potential of corrosion. Sheaths can be stainless steel, Inconel, and Incoloy. Inconel and Incoloy are registered trademarks for Special Metals Corporation and are nickel alloys. The temperature range of the various types of sheaths can be seen in the chart below.

Thermocouple Insulation

Vinyl

Vinyl is low cost, has good flexibility, fair electricals, and is a general purpose material.

Irradiated PVC

Irradiated PVC has moderate cost, heat resistance, good electrical and physical properties, excellent abrasion protection, and is solder resistant.

Teflon

Teflon has high cost, a high temperature rating, excellent chemical resistance, and electrical properties but has poor cut through resistance.

Tefzel

Tefzel has high cost, good electrical and physical properties, and excellent chemical resistance.

Kynar

Kynar has high cost, poor electrical properties, and excellent physical properties.

Kapton

Kapton has excellent physical, electrical, and mechanical properties over a wide variety of temperature ranges and is used in applications where there is extreme heat and vibration. It maintains its mechanical properties in the harshest of conditions.

Polyethylene

Polyethylene has low cost, excellent electrical properties, high flammability, and is stiffer than vinyl.

Irradiated Polyethylene

Irradiated Polyethylene has moderate cost, excellent electrical, physical, and chemical properties, id flame retardant, and is resistant to environmental stress cracking.

Rulan

Rulan is the same as polyethylene but is flame retardant.

Polysulfone

Polysulfone has low cost, good electrical properties, excellent physical properties, and good handling properties with poor chemical resistance.

Fiberglass

Fiberglass excellent for high temperature applications and suitable for use with ambient temperatures where there is a possibility of hot spots.

Ceramic

Ceramic is used in commercial ovens and furnaces and can monitor ambient temperatures of fireboxes, kilns, and grills. Its temperature range is -58o F to 2200°F.

Conductor Jacket

Conductor Jacket can be put over the primary insulation and is necessary when additional mechanical protection is needed. A jacket for vinyl Insulation Is nylon with polyethylene used for vinyl or nylon insulation. A conductor jacket acts as a mechanical barrier and prevents shorting.

Types of Thermocouples

Thermocouples come in different types for a variety of applications and use a system of letters to identify each type. There is a wide range of thermocouple types with their own characteristics and temperature ranges. The difference between each type is determined by their durability, temperature range, resistance, and applications.

Base Metal Thermocouples:

Types E, T, N, J, and K are the most common of the thermocouples and have base metals of iron, copper, nickel, and other such metals. Below is a description of each.

Type E:

Type E thermocouples have chromel, a nickel and chromium alloy, as a positive leg and constantan as the negative leg. They have a temperature range over -330o F to 1600o and have excellent EMF versus temperature values. Type E can be used in sub-zero temperatures and have colorings of red or purple.

Type J:

Type J thermocouples have iron for the positive leg and constantan as the negative one. They are used in oxidizing, vacuum, inert, and reducing atmospheres. Type J thermocouples have to be closely monitored since the iron leg can rust. Their temperature range is 32o F to 100o F and have a red or white color.

Type K:

Type K thermocouples have chromel for the positive leg and alumel for the negative. Alumel is an alloy made of mostly nickel with low percentages of aluminum, silicon, and manganese. Type K thermocouples are used in inert or oxidation environments with a temperature range of -300o F to 2300o F. Their color coding is red or yellow.

Type N:

The type N thermocouple is nicrosil, a nickel chromium alloy, as the positive leg and nisil, a nickel, silicon, and magnesium alloy, as the negative leg. These types of thermocouples have a temperature range 32o F to 2300o F and have a color coding of red or orange.

Type T:

Type T thermocouples have copper as the positive leg and constantan as the negative one with a temperature range of -330o F to 700o F and color coding of red or blue.

Noble Metal Thermocouples:

Noble metal thermocouples, or platinum thermocouples, are types B, R, S, and P with precious metal elements. They are accurate at very high temperatures and have a long lifespan of use.

Type B:

The Type B thermocouple is used in extremely high temperature applications and has the highest temperature limit of all of the thermocouples with exceptional accuracy and stability. Its temperature range is 2500o F to 3100o F.

Type R:

The Type R is used for high temperature applications and has a higher percentage of Rhodium than the Type S, which makes it more expensive. Type R has the same performance as Type S and can be used for low temperature applications because of its stability and high accuracy. It has a temperature range of -58o F to 2700o F.

Type S:

The Type S is used for very high temperature applications in the BioTech and Pharmaceutical industries. It is used for low temperature applications due to its accuracy and stability. It has a temperature range of -58o F to 2700o F.

Type P:

The Type P has the same curve at high temperatures as Type K and can be used in oxidizing atmospheres with a temperature range up to 2300°F. A Type K extension wire is used to connect a Type P thermocouple to the measuring instrument.

Chapter Four – Uses for a Thermocouple

Thermocouples are widely used temperature sensors because of their wide temperature capabilities, ruggedness, and low cost. They are found in home appliances, industrial processes, electric power generation, furnace monitoring and control, food and beverage processing, automotive sensors, aircraft engines, rockets, and spacecraft.

Their small size and fast response, as well as their ability to endure shocks and vibrations, makes them perfect for temperature control and measurement.

Below is a description of a few of the applications for thermocouples.

Thermocouple Applications

Food Production:

Thermocouples are perfect for the food industry because they supply accurate readings in a few seconds. Food products can be checked in any phase of production. Food production thermocouples are a two piece unit with a handheld readout unit and detachable probe. In the tip of the probe are two wires connected to each other. Flat headed probes measure surface temperatures, needle probes take internal measurements and the air temp of ovens.

Extruders:

Extruders require high temperature and pressure. The sensor tip has to be positioned in the molten plastic under high pressure conditions. The thermocouple measures the temperature and is directly installed into the process. These units have high degree of accuracy, with a rapid response time, and can have a type K thermocouple probe.

Furnace:

A pilot light is responsible for igniting the furnace burner. The thermocouple shuts off the gas supply when it does not sense a flame and prevents the furnace from receiving gas when the pilot is out. It restricts gas from building up in a furnace and makes the system much safer.

Molten Metal:

A molten metal thermocouple can be used in a non-ferrous metal environment to measure temperatures up to 1250° C. They monitor and control the temperature of liquid metals during melt preparation, holding, degassing, and casting operations.

Gas Appliances:

A thermocouple, on a gas appliance, signals the gas valve that the pilot is lit so it will remain open. The thermocouple is positioned in the middle of the pilot flame. It detects the heat of the flame and generates the voltage that keeps the gas flowing. If the flame goes out, the thermocouple voltage disappears and closes the gas valve.

High Pressure Boiler:

Finding the right instrumentation for high pressure applications is very difficult because of the high temperatures and heavy vibrations. Resistance thermometers (RTDs) and thermocouples are the commonly used temperature sensors for high-pressure industrial applications. Thermocouples have been found to be the better choice.

There are two configurations of thermocouples for high pressure applications, which are pictured below:

• Conventional

  Conventional is where the thermocouple assembly is either threaded or flanged.

• Lens-ring

  With Lens-ring the thermocouple is mounted in a metallic lens-ring gasket that is compressed between two flanges to create a seal. The lens-ring gasket creates the seal. It is used for mounting and access to the process for measuring.

  

  
  

Chapter Five – How to Test a Thermocouple

Though thermocouples are very reliable and durable, they can fail over time and need to be checked. Regardless of the fact that there is a very wide variety of thermocouples, all of them operate on the same basic principle of two connected wires with one of the wires being the reference and the other being the hot wire.

The testing of the efficiency of a thermocouple involves the use of a multimeter. Below is a description of a multimeter and how to test one type of thermocouple.

Multimeter Readings

Multimeters come in multiple forms and styles. Regardless of the variations, there are some basic symbols that are displayed on all types.

• Volts V
• Amps A
• Ohms Ω

There are also prefixes that may be displayed as well.

• μ - The Greek letter Mu for micro or one millionth
• m - milli or one thousandth
• k - kilo or one thousand
• M - Mega or one million

Multimeters have settings for measuring AC and DC currents.

Some multimeters have a continuity beeper that sounds when the meter detects a closed circuit. A continuity check shows the presence of a complete path for current flow. The image below is of a multimeter with continuity beeper.

Testing a Thermocouple

Step 1:

The multimeter should be able to read ohms, the opposition to current flow in an electrical circuit. Conductors offer little resistance, while insulators have high resistance. Silver, copper, gold, and aluminum are examples of conductors and are metals found in thermocouple wires. The multimeter for testing a thermocouple has to be very sensitive since thermocouples produce millivolts.

Step 2:

Depending on the type of test, the thermocouple will be removed or remain installed. If an open or resistance test is being performed, the thermocouple is removed. For the closed circuit test, an adapter is required for insertion of the thermometer.

Step 3:

There are three types of tests that can be performed.

• Open circuit:
• Closed circuit:
• Resistance:

Step 4:

For the resistance test, the thermocouple is removed from the application, and the ohms option is selected on the multimeter. One lead is placed on the side of the thermocouple, while the other at the end that is inserted into the application. If the thermocouple has proper continuity, a small resistance reading should be visible on the multimeter.

Step 5:

For the open circuit test, the thermocouple is removed from the application and the multimeter is set to millivolts. One lead is placed on the side of the thermocouple, while the other is placed at the opposite end. The end that is placed in the application should be heated. The millivolt reading should be within the acceptable range.

Step 6:

The closed circuit test requires a thermocouple adapter. The adapter is placed inside the application, and the thermocouple is screwed into the adapter. One lead is attached to the screw from the adapter, and the other one is attached to the exposed end of the thermocouple. To get the reading for the thermocouple, the application is activated. The multimeter will be read in millivolts. If the thermocouple fails this test, it has to be replaced.

Chapter Six – How to Read a Thermocouple

Thermocouples are a cost effective method for measuring a wide range of temperatures with accuracy. They are used in boilers, water heaters, ovens, and airplane engines.

When preparing to read a thermocouple, it is necessary to understand a thermocouple reference table. Every type of thermocouple has its own reference table. The one below is for a portion of the table for a Type K thermocouple.

The first column on the left of the table is the temperature in increments of ten. The portion of the table to the right is intermediate distances, increments of one, between the temperature ranges. On the above table, -280o is the third entry from the top of the table. If the temperature reading on the thermocouple is -284o, to read the table and find the millivolts, you go to -280o and go over to the right for the number under the number 4. The numbers to the right, on the table, are the millivolts for that temperature.

Reference junctions on a thermocouple may change temperatures, which can lead to inaccurate readings. The reference temperature can be fixed or made constant by immersing it in water. In most cases, a reference junction compensator adjusts for the ambient temperature changes. The image below is a simplified representation of a compensation calculator.

Thermocouple Laws

Homogenous Materials:

A homogeneous wire is physically and chemically the same throughout its length. A thermocouple circuit, made with the same wire, will not generate an emf, regardless of a change in temperature and thickness. For a thermocouple to work properly, it must have two different metals to generate voltage.

Intermediate Materials:

The sum of the emfs, with two or more different metals, is zero if the circuit is at the same temperature. The addition of different metals to a circuit will not affect the voltage the circuit creates. The added junctions are to be at the same temperature as the junctions in the circuit. For example, a third metal such as copper leads may be added to help take a measurement. This is why thermocouples may be used with digital multimeters or other electrical components. It is also why solder may be used to join metals to form thermocouples.

Successive or Intermediate Temperatures:

A thermocouple, of two different metals, produces an emf, when the metals are at different temperatures. A thermocouple calibrated with a reference temperature to be used with another reference temperature can have extra wires, with the same thermoelectric characteristics, added to the circuit and not affect the emf.

Conclusion

• A thermocouple is a transducer that converts thermal energy into electrical energy and is made by joining dissimilar metals or semiconductors to form a junction.
• The concept of the thermocouple is based on the Seebeck Effect, which states that if dissimilar metals are joined at a point they will generate a small measurable voltage when the temperature of the point of connection changes.
• A thermocouple measures an unknown temperature and compares it to a known temperature called the reference.
• Thermocouples are widely used temperature sensors because of their wide temperature capabilities, ruggedness, and low cost.
• The difference between thermocouples is determined by the types of alloys used to produce their wires.