Thermocouples are one of the most reliable temperature sensors around. Each typically consists of two metals, i.e., thermoelements, which produce an electromotive force (EMF) when heated. This EMF is a minuscule voltage that correlates back to the temperature.
Overall, thermocouples are simple and cost-effective temperature sensors. Still, they are robust and have various applications in many different industries. But how much else do you know about them? Can you make an informed decision when choosing between thermocouples?
This article will guide you through one of the most important considerations people make when choosing thermocouples; thermocouple calibration.
Thermocouple calibration types refer to the different alloys used to make thermoelements. These should be considered when choosing a thermocouple for your application.
Fortunately, it isn’t that hard; thermocouple calibrations have been standardized based on the alloys used and their pairing. The American National Standards Institute recognizes eight main calibration types. These are listed in standard ASTM E230 as K, J, T, E, N, B, R, and S.
Thermocouple calibration considerations
When choosing the most appropriate calibration for your application, you want to check:
- The size of the wire used to make the thermocouple
- The temperature range being measured in the application
- The age of the temperature sensor
The temperature range being measured
Every thermocouple calibration has an operating range. And within each calibration, the maximum temperature varies depending on the size of the wire used to make the thermocouple.
The table below shows different calibration types with their temperature ranges and tolerances.
|Thermocouple Type||Maximum Temperature in ℃ (℉)||Standard Tolerances (whichever is greater)||Special Tolerances (whichever is greater)|
|E||900 (1650)||±3.1F (1.7C) or ±0.5%||±1.8F (1C) or ±0.4%|
|J||750 (1350)||±4F (2.2C) or ±0.75%||±2F (1.1C) or ±0.4%|
|K, N||1250 (2280)||±4F (2.2C) or ±0.75%||±2F (1.1C) or ±0.4%|
|T||350 (660)||±1.8F (1C) or ±0.75%||±0.9F (0.5C) or ±0.4%|
Size of the wire
The size of the wire can lower the upper temperature limit within a given thermocouple calibration. This is especially true for tube & wire style thermocouples typically made in closed-end protection tubes. Sheathed thermocouples with compacted mineral oxide insulation are exempt from this implication.
For instance, Type K thermocouples have an upper temperature limit of 1260°C. This is the limit for an 8-gauge wire. However, when the thermocouple is made from a 24-gauge wire, the recommended upper limit drops to 870°C.
The table below puts this into better perspective:
|Upper Temperature Limit for Various Wire Sizes (Awg), °C (°F)|
|Thermocouple type||No.8 gauge||No. 14 gauge||N0. 20 gauge||No.24 Gauge||No.28 gauge||No.30 gauge|
|T||370 (700)||260 (500)||200 (400)||200 (400)||150 (300)|
|J||760 (1400)||590 (1100)||480 (900)||370 (700)||370 (700)||320 (600)|
|E||870 (1600)||650 (1200)||540 (1000)||430 (800)||430 (800)||370 (700)|
|K and N||1260 (2300)||1090 (2000)||980 (1800)||870 (1600)||870 (1600)||760 (1400)|
|R and S||1480 (2700)|
Thermocouples degrade over time. The rate of this degradation depends on calibration and the environment in which they are used. Generally, they allow will oxidize and corrode over time.
For instance, Type K thermocouples can withstand higher temperatures for longer periods. But at very high temperatures, the alloy is prone to hysteresis, i.e., a change in the sensitivity of the thermocouple as it cycles through a sequential range of temperatures.
On the other hand, Type J degrades faster than other calibrations since it has an iron leg.
Even with all this knowledge, it’s still hard to make an informed decision on the type of calibration you should choose. So, why not leave it to the experts? Contact Rama Corporation for guidance. We are a heating elements manufacturer with decades of experience in temperature sensors and other heating elements.