eFunda: Introduction to Thermocouples
engineering fundamentals Thermocouples: Introduction
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Overview
The Thermocouple is a thermoelectric temperature sensor which consists of two dissimilar metallic wires, e.g., one chromel and one constantan, coupled at the probe tip (measurement junction) and extended to the reference (known temperature) junction.

The temperature difference between the probe tip and the reference junction is detected by measuring the change in voltage (electromotive force, EMF) at the reference junction.

The absolute temperature reading can then be obtained by combining the information of the known reference temperature and the difference of temperature between probe tip and the reference.

Three Wire Layouts of Typical Thermocouples

Common Specifications
Common commercially available thermocouples are specified by ISA (Instrument Society of America) types. Type E, J, K, and T are base-metal thermocouples and can be used up to about 1000°C (1832°F). Type S, R, and B are noble-metal thermocouples and can be used up to about 2000°C (3632°F).

The following table provides a summary of basic thermocouple properties. For more detailed specifications of individual thermocouples, please consult the sensor manufacturer.
ISA Material
(+ & -)
Temperature Range
°C
(°F)
Sensitivity@
25°C (77°F)
µV/°C
(µV/°F)
Error* App.**
E Chromel & Constantan
(Ni-Cr & Cu-Ni)
-270~1000
(-450~1800)
60.9
(38.3)
LT:±1.67°C(±3°F)
HT:±0.5%
I,O
J Iron & Constantan
(Fe & Cu-Ni)
-210~1200
(-350~2200)
51.7
(28.7)
LT:±2.2~1.1°C(±4~2°F)
HT:±0.375~0.75%
I,O,R,V
K Chromel & Alumel
(Ni-Cr & Ni-Al)
-270~1350
(-450~2500)
40.6
(22.6)
LT:±2.2~1.1°C(±4~2°F)
HT:±0.375~0.75%
I,O
T Copper & Constantan
(Cu & Cu-Ni)
-270~400
(-450~750)
40.6
(22.6)
LT:±1~2%
HT:±1.5% or ±0.42°C(±0.75°F)
I,O,R,V
R Platinum & 87% Platinum/ 13% Rhodium
(Pt & Pt-Rh)
-50~1750
(-60~3200)
6
(3.3)
LT:±2.8°C(±5°F)
HT:±0.5%
I,O
S Platinum & 90% Platinum/ 10% Rhodium
(Pt & Pt-Rh)
-50~1750
(-60~3200)
6
(3.3)
LT:±2.8°C(±5°F)
HT:±0.5%
I,O
B 70% Platinum/ 30% Rhodium & 94% Platinum/ 6% Rhodium
(Pt-Rh & Pt-Rh)
-50~1750
(-60~3200)
6
(3.3)
LT:±2.8°C(±5°F)
HT:±0.5%
I,O
*: LT = Low temperature range, HT = High temperature range
**: I = Inert media, O = Oxidizing media, R = Reducing media, V = Vacuum
  Constantan, Alumel, and Chromel are trade names of their respective owners.

Pros and Cons

  • Pros:
  - Low cost.
  - No moving parts, less likely to be broken.
  - Wide temperature range.
  - Reasonably short response time.
  - Reasonable repeatability and accuracy.
  • Cons:
  - Sensitivity is low, usually 50 µV/°C (28 µV/°F) or less. Its low voltage output may be masked by noise. This problem can be improved, but not eliminated, by better signal filtering, shielding, and analog-to-digital (A/V) conversion.
  - Accuracy, usually no better than 0.5 °C (0.9°F), may not be high enough for some applications.
  - Requires a known temperature reference, usually 0°C (32°F) ice water. Modern thermocouples, on the other hand, rely on an electrically generated reference.
  - Nonlinearity could be bothersome. Fortunately, detail calibration curves for each wire material can usually be obtained from vendors.

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