55
Thermocouple Measurement Accuracy
The thermocouple measurements are treated as voltage measurements.
For a given range of temperature measurement, the software calculates the voltage caliber the following way:
•
Let < T > the absolute value of the max. measurable value, in
∘
C
•
Add 40
∘
C to take the max. cold welding temperature into account
•
Search the corresponding voltage value U in the table of thermocouples
•
Program the caliber with U included in the range.
15.1 J Type Thermocouple Example
You want to program a measurement range between -50 and + 50
∘
C with a J thermocouple:
•
Max. absolute value T = 50
∘
C
•
Add 40
∘
C T + 40 = 90
∘
C
•
Corresponding U voltage according to the tables ThJ U = 4.726 mV
•
Programmed caliber: 10 mV (measurement range: -5mV to +5mV)
The measurement inaccuracies below are max. values: the typical values are half to thrice as much.
The temperature measurement accuracy is the sum of several possible uncertainty causes:
•
Pl: linearization accuracy
•
Ps: weld welding accuracy
•
Pm: measurement accuracy of the equivalent voltage
The total accuracy Pt is then:
𝑃 𝑡 = 𝑃 𝑙 + 𝑃 𝑠 + 𝑃 𝑚
For the recorder:
•
Pl = + 0.25
∘
C for all thermocouples
•
Ps = + 0.5
∘
C for all thermocouples
•
Pm = 0.1% of the voltage c 10
𝜇
V) divided by the slope of the thermocouple in
𝜇
V/
∘
C
15.2 Measurement Accuracy: Pm
The measurement accuracy Pm depends on the voltage caliber of the appliance (see the previous paragraph) and on
the slope of the thermocouple. You shall take the slope at 0
∘
C while knowing that it will vary as a function of the
temperature, but this variation is generally of the second order for the accuracy calculation.
15.3 Voltage of Thermocouple (mV)
Slope of the thermocouples (
𝜇
V)
