Thermocouples
One of the most common industrial thermometer is the thermocouple. It was discovered by Thomas Seebeck's in 1822. He noted that a voltage difference appeared when the wire was heated at one end. Regardless of temperature, if both ends were at the same temperature there was no voltage difference. If the circuit were made with wire of the same material there was no current flow.
A thermocouple consists of two dissimilar metals, joined together at one end, and produce a small unique voltage at a given temperature. This voltage is measured and interpreted by a thermocouple thermometer.
The thermoelectric voltage resulting from the temperature difference from one end of the wire to the other is actually the sum of all the voltage differences along the wire from end to end
Thermocouples can be made from a variety of metals and cover a temperature range 200 oC to 2,600 oC. Comparing thermocouples to other types of sensors should be made in terms of the tolerance given in ASTM E 230.
Base metal thermocouples
Thermocouple | Maximum Temperature (oC) | |
Continuous | Spot | |
Copper-Constantan | 400 | 500 |
Iron-Constantan | 850 | 1,100 |
Chromel-Constantan | 700 | 1,000 |
Chromel-Alumel | 1,100 | 1,300 |
Nicrosil-Nisil | 1,250 | - |
Tungsten-Molybdenum* | 2,600 | 2,650 |
Principle of Working of Thermocouple
1) Seebeck effect: The Seebeck effect states that when two different or unlike metals are joined together at two junctions, an electromotive force (emf) is generated at the two junctions. The amount of emf generated is different for different combinations of the metals.
2) Peltier effect: As per the Peltier effect when two dissimilar metals are joined together to form two junctions, the emf is generated within the circuit due to different temperatures of the two junctions of the circuit.
3) Thomson effect: As per Thomson effect, when two unlike metals are joined together forming two junctions, the potential exists within the circuit due to temperature gradient along the entire length of the conductors within the circuit.
In most of the cases the emf suggested by Thomson effect is very small and it can be neglected by making proper selection of the metals. The Peltier effect play prominent role in the working principle of the thermocouple.
How Thermocouple Works?
Devices Used for Measuring emf within the Thermocouple Circuit
Advantages with thermocouples
- Capable of being used to directly measure temperatures up to 2600 oC.
- The thermocouple junction may be grounded and brought into direct contact with the material being measured.
Disadvantages with thermocouples
- Temperature measurement with a thermocouple requires two temperatures be measured, the junction at the work end (the hot junction) and the junction where wires meet the instrumentation copper wires (cold junction). To avoid error the cold junction temperature is in general compensated in the electronic instruments by measuring the temperature at the terminal block using with a semiconductor, thermistor, or RTD.
- Thermocouples operation are relatively complex with potential sources of error. The materials of which thermocouple wires are made are not inert and the thermoelectric voltage developed along the length of the thermocouple wire may be influenced by corrosion etc.
- The relationship between the process temperature and the thermocouple signal (millivolt) is not linear.
- The calibration of the thermocouple should be carried out while it is in use by comparing it to a nearby comparison thermocouple. If the thermocouple is removed and placed in a calibration bath, the output integrated over the length is not reproduced exactly.
Thermocouple Types
Thermocouples are available in different combinations of metals or calibrations. The four most common calibrations are J, K, T and E. Each calibration has a different temperature range and environment, although the maximum temperature varies with the diameter of the wire used in the thermocouple.Some of the thermocouple types have standardized with calibration tables, color codes and assigned letter-designations. The ASTM Standard E230 provides all the specifications for most of the common industrial grades, including letter designation, color codes (USA only), suggested use limits and the complete voltage versus temperature tables for cold junctions maintained at 32 oF and 0 oC.
There are four "classes" of thermocouples:
- The home body class (called base metal),
- the upper crust class (called rare metal or precious metal),
- the rarified class (refractory metals) and,
- the exotic class (standards and developmental devices).
Instrument | Temperature Range | Accuracy | |
Recommended (oF) | Maximum (oF) | ||
Type J probes | 32 to 1336 | -310 to 1832 | 1.8 to 7.9oF or 0.4% of reading above 32oF, whichever is greater |
Type K probes | 32 to 2300 | -418 to 2507 | 1.8 to 7.9oF or 0.4% of reading above 32oF, whichever is greater |
Type T probes | -299 to 700 | -418 to752 | 0.9 to 3.6oF or 0.4% of reading above 32oF, whichever is greater |
Type E probes | 32 to 1600 | 32 to 1650 | 1.8 to 7.9oF or 0.4% of reading above 32oF, whichever is greater |
Type R probes | 32 to 2700 | 32 to 3210 | 2.5oF or 0.25% of reading, whichever is greater |
Type S probes | 32 to 2700 | 32 to 3210 | 2.5oF or 0.25% of reading, whichever is greater |