5.4 Thermocouple
wires
When making measurements where additional wires have to be connected to the thermocouple leads, care must be
exercised in selecting these wire types, not only when they are claimed to be of the same composition as the
thermocouple involved, but, also, of the same "quality".
Performance results, where high precision is required and in circumstances where some types of thermocouple wire
leads are added to the original installation, should be reviewed carefully for the impact of the choice of the additional wire
leads.
The quality of the thermocouple wire is established by the limit of error to be expected with its use.
There are three recognized levels of quality:
−
Special or Premium grade
−
Standard grade
−
Extension wire grade
The error limits determining the grade quality differ from thermocouple type to thermocouple type, reflecting the degree of
difficulty in maintaining the precise levels of purity of the metal used.
The table below summarizes the error limits for Premium and Standard grades, while the Extension grade wire is
characterized by limits of error exceeding those in the table.
Errors up to ±4°C may be experienced when using the Extension grade thermocouple wire for J and K thermocouples.
Limit of Error of thermocouples
The range indicated is the temperature limit for the indicated errors. Cold junction at 0 °C
Tc
Class 1
Class 2
Class 3
type T
0.5°C (-40 to +125°C)
1°C (-40 to 133°C)
1°C (-67 to 40°C)
0.004 . T (T >125°C)
0.0075 . T (T >133 °C)
0.015. T (T <-67°C)
T range
-40 to +350°C
-40 to +350°C
-200 to 40°C
type E
1.5°C (-40 to 375°C)
2.5°C (-40 to 333 °C)
2.5°C (-167 to +40°C)
0.004.T (T >375°C)
0.0075.T (T >333°C)
0.015.T (T <-167°C)
T range
-40 to 800°C
-40 to 900°C
-200°C to 40°C
type J
1.5°C (-40 to 375°C)
2.5°C (-40 to 333 °C)
2.5°C (-167 to +40°C)
0.004.T (T >375°C)
0.0075.T (T >333°C)
0.015.T (T <-167°C)
T range
-40 to 750°C
-40 to 750°C
-----
type K & N
1.5°C (-40 to 375°C)
2.5°C (-40 to 333 °C)
2.5°C (-167 to +40°C)
0.004.T (T >375°C)
0.0075.T (T >333°C)
0.015.T (T <-167°C)
T range
-40 to 1000°C
-40 to 1200°C
-200°C to 40°C
type R & S
1°C (0 to 1100°C)
1.5°C (-40 to 600 °C)
4°C (600 to +800°C)
1 + 0.003 (T-100)
0.0075.T (T >600°C)
0.005.T (T>800°C)
(T >1100°C)
T range
0 to 1600°C
0 to 1600°C
----
type B
1°C (0 to 1100°C)
1.5°C (-40 to 600 °C)
4°C (600 to +800°C)
1 + 0.003 (T-100)
0.0075.T (T >600°C)
0.005.T (T>800°C)
(T >1100°C)
T range
-----
600 to 1700°C
600 to 1700°C
Note
: Specially selected "premium grade" wires are available on request
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