56
•
At 100
∘
C the slope of thermocouple J is 55
𝜇
V
•
At -100
∘
C slope is 30
𝜇
V .
•
So the error is 2 time greater then 100
∘
C to -100
∘
C.
15.3.1 Example of Accuracy Calculation
You make a welding between -50
∘
C and +50
∘
C with a J thermocouple with compensation of cold welding.
𝑃 𝑡 = 𝑃 𝑙 + 𝑃 𝑠 + 𝑃 𝑚
(15.1)
Linearization accuracy
𝑃 𝑙 = +0.25
∘
𝐶
Compensation of cold welding
𝑃 𝑠 = +0.5
∘
𝐶
Used caliber
10mV - (see previous example)
Accuracy for voltage measurement
0.1 % ∗10𝑚𝑉 + 10𝜇𝑉 = 20𝜇𝑉
Slope of J thermocouple
50𝜇𝑉 /
∘
𝐶
Accuracy Pm
𝑃 𝑚 = 20/50 = 0.4
∘
𝐶
Total accuracy
𝑃 𝑡 = 0.25 + 0.5 + 0.4 = 1.15
∘
𝐶
15.4 Accuracy Class - Class Index
This is one of the most important concepts of the CEI recommendation; it tends to shorten the list of specifications. To
do so, it introduces the concept of PREDICTIVE CLASS that depends on the C CLASS INDEX.
The normalized values of the class index are: C = 0.1, 0.25, 0.5 and 1.
The intrinsic error (in the reference conditions) does not C% (the manufacturer may also specify this limit of
the intrinsic error as an absolute value (ex. + 5
𝜇
V) for the first calibers).
The variations (of the measured value) with the variations of one of the influential variables in the nominal range of use
do not exceed:
•
C % for the position for the magnetic induction with external source and parasite voltages
•
0.5 C% for the power supply source
•
0.3 C% according to the class index at ambient temperature (0.15% for the 0.25 class).
In addition, the insensitivity range should not exceed:
•
C% in the reference conditions
•
1.5C% for the maximal resistance of the external measurement circuit
•
2C% for parasite voltages
And the excess should not be more than 2C% (4C% within the limits of the power supply source). %
