Thermocouples come in two major categories; Base metal thermocouples made from standard metal alloys and Precious metal thermocouples made with Platinum, Rhenium and Tungsten alloys. Precious metal thermocouples can be used at higher temperatures, up to 2500°C in some cases, but they are much more expensive than base metal sensors and not always as accurate. If your application does not exceed 1200°C then base metal thermocouples are the ones to choose.
Which Thermocouple Should I Use:
Type K, Type N, Type J, Type T or Type J?
Base Metal Thermocouple Alloys
Let us look at the options available for the base metal thermocouples; There are five types most commonly used and these are set out in the table below. Some thermocouple types have associated compensating pair combinations which give the same emf-temperature readings but over a limited temperature range and at a significantly lower cost. The suffix -P and -N stand for ‘positive’ and ‘negative’ in relation to the polarity of the thermocouple element.
| Type | Thermocouple Alloys | Temperature Range | Compensating Pair |
|---|---|---|---|
| K | KP – Nickel/Chromium KN – Nickel/Aluminium | -200°C to +1200°C | KPC – Copper KNC – Copper/Nickel |
| J | JP – Iron JN – Copper/Nickel | -40°C to +750°C | - - |
| T | TP – Copper TN – Copper/Nickel | -200°C to +350°C | - - |
| E | EP – Nickel/Chromium EN – Copper/Nickel | -200°C to +900°C | - - |
| N | NP – Nickel/Chrome/Silicon NN – Nickel/Silicon | -200°C to +1200°C | NPC – Copper NNC – Copper/Nickel/Manganese/Iron |
Temperature Considerations
When selecting the right thermocouple for your application you need to consider the temperature range and environment. The table above gives the overall temperature range for all the classes of thermocouple which are available in each type but there are some limits to the actual temperature ranges in each class. Type K, for example, according to standard BS EN 60584 has three temperature classes as follows:
| TYPE K CLASSES | |
|---|---|
| Class 1 | -40°C to 1000°C |
| Class 2 | -40°C to 1200°C |
| Class 3 | -200°C to 40°C |
In industrial process control applications, many processes run up to 1200°C, which is the temperature metals begin to melt at. So, Type K and Type N are ideal for these.
In plastics processing industries temperatures rarely exceed 700°C so Type J and Type E are perfect for these whilst for food processing industries Type T is the preferred option as cooking temperatures are generally below 350°C.
Environmental Considerations
The environment impacts your decision as some of the thermocouples have vulnerabilities to certain conditions. Although you can place the thermocouple wires inside metal sheaths to protect them, junctions and extension cables can become exposed to the environment and a failure anywhere in the circuit will stop the thermocouple working.
Iron is extremely susceptible to corrosion in damp atmospheres and the Nickel/Chromium alloy in Type KP can be attacked in low oxygen environments when the Chromium is preferentially oxidised causing a change in the composition of the alloy. In severe circumstances the Chromium Oxide swells and cracks the surface of the wire, much like rust on an Iron wire, exposing more metal for corrosion.
If you are concerned about external electrical noise affecting the mV readings Type E, Nickel/Chromium vs Copper/Nickel, has the greatest emf output of any combination and is, therefore, less susceptible to electromagnetic interference.
Extension and Compensating Wires
The conductors in the thermocouple measurement circuit must be made of the Thermocouple Alloy until the cable reaches a ‘reference temperature’ junction. Extension grade alloys are available for all the base metal thermocouples. These have the same composition as the thermocouple alloys but are only calibrated over a restricted temperature range, typically up to 200°C.
Compensating alloys are available for Type K and Type N thermocouples and the precious metal thermocouples which provide the same emf-temperature profile as the thermocouple alloys up to 200°C. These are much lower in cost which can be very significant in industrial process plants where thermocouple cable lengths can run into hundreds of metres from the process vessel back to the control room and where the precious metal conductors can cost hundreds of pounds per metre.
Compensating Alloys:
| Type | Thermocouple Alloys | Temperature Range |
|---|---|---|
| K | KP – Nickel/Chromium KN – Nickel/Aluminium | KPC – Copper KNC – Copper/Nickel |
| N | NP – Nickel/Chrome/Silicon NN – Nickel/Silicon | NPC – Copper NNC – Copper/Nickel/Manganese/Iron |
| R | RP – Platinum/13% Rhodium RN – Platinum | RPC – Copper RNC – Copper/Nickel/Manganese |
| S | SP – Platinum/10% Rhodium SN – Platinum | SPC – Copper SNC – Copper/Nickel/Manganese |
| B | BP – Platinum/30% Rhodium BN – Platinum/6% Rhodium | BPC – Copper/Manganese BNC - Copper |
Price and Construction
Price and construction are linked as there may be trade-offs available when selecting the thermocouple. Type N and Type K can operate unprotected in air up to 1200°C but the Iron leg of Type J will always need protecting in a metal sheath to prevent corrosion.
In the heat treatment industry Type K probes are rarely used, instead the glass fibre insulated Type K conductors of the thermocouple cable are simply bared at the ends and spot welded together and then welded to the fabricated metal piece being heat treated to form the measurement junction.
If you need help in selecting the right thermocouple type for your application, please contact the Scott Precision Wire Technical Team who will be able to assist you.