Thermocouple accuracy is the amount of error which exists in its temperature measurement. It indicates how close the measured temperature value of a given thermocouple is close to the true temperature value. It is called the tolerance or error. A table called the “Initial Calibration Tolerances” tells us what accuracy or tolerance we can expect from a given thermocouple.
Thermocouple accuracy is influenced by several factors including the Thermocouple type, its range of interest, the purity of the material, electrical noise (EMI and RFI), corrosion, junction degradation, and the manufacturing process. Thermocouples are available with standard grade tolerances or special grade tolerances called Class 2 and Class 1 respectively. The most common controlling international standard is IEC-60584-2 while the most common U.S. standard is ASTM E230. Each standard publishes limits of tolerance for compliance.
A typical Initial Calibration Tolerances table for thermocouples is given below:
Thermocouple Type
|
Maximum Temperature
|
Tolerances
|
|
| Standard(whichever is greater) | Special(whichever is greater) | ||
| E | 900°C (1650°F) | ±1.7°C (±3.06°F) or ±0.5% | ±1.0°C (1.8°F) or ±0.40% |
| J |
750°C (1380°F)
|
±2.2°C (±3.96°F) or ±0.75% | ±1.1°C (1.98°F) or ±0.40% |
| K, N |
1250°C (2280°F)
|
±2.2°C (±3.96°F) or ±0.75% | ±1.1°C (1.98°F) or ±0.40% |
| T | 350°C (660°F) | ±1.0°C (±1.8°F) or ±0.75% | ±0.5°C (0.9°F) or ±0.40% |
| B | 1700°C (3100°F) | ±0.5% | - |
| R, S |
1450°C (2640°F)
|
±1.5°C (±2.7°F) or ±0.25% | ±0.6°C (1.08°F) or ±0.1% |
| C, D, G |
2315°C (4200°F)
|
±4.5°C (±8.1°F) or ±1.0% | - |
These tables are provided by manufacturers of thermocouples and tolerances determined from these tables are valid for:
(a) The specified type of thermocouple in the tables provided
(b) The specified temperature range
Standard and Special Tolerance Thermocouples
As can be seen from the table above, special tolerances thermocouples have higher accuracies than standard types. Special tolerance thermocouples are a little more expensive than the standard types and their accuracy is almost twice as good compared to the standard types. They are used in applications where accuracy is one of the critical factors.
The table above is very easy to use. To determine the initial accuracy of a thermocouple, you simply determine the greater of the two tolerances given. Note that standard and special tolerances for thermocouples are determined using the same method.
For example, a type K thermocouple is heated to 300°C. What is the standard tolerance at this temperature? We can see from the tolerance table above that it is either ±2.2°C or ±0.75% whichever is greater. To get the required thermocouple accuracy, follow the steps below:
Step 1
Convert the ±0.75% tolerance to a ± °C value and compare with ±2.2°C
Step 2
Multiply 300°C by ±0.0075 which gives ±2.25°C.
Step 3
We choose ±2.25°C because it is greater than ±2.2°C
So at 300°C, the accuracy of the type K thermocouple is ±2.25°C
