ETAS
Hardware Description
ES620.1 - User Guide
18
3.6
Measurement Accuracy
While measuring temperatures with thermocouples the total measurement
inaccuracy is determined by different parts. These parts are:
• Basic accuracy
T
g
• Temperature drift of thermocouple inputs
T
d
• Inaccuracy depending on the internal resistance of the thermocouple
T
i
• Inaccuracy of cold junction compensation
T
cjc
• Temperature drift of cold junction compensation
T
dcjc
The maximum total inaccuracy is the sum of all parts.
T =
T
g
+
T
d
+
T
i
+
T
cjc
+
T
dcjc
The amount of the parts depends on the type of thermocouple. The figures are
listed in table “Thermocouple Inputs” on page 30.
Two examples show you how to calculate the maximum total inaccuracy.
Example 1
A thermocouple of type K with an internal resistance R
i
of less than 50
is
used to measure a temperature T
m
of 200 °C (473.15 K; 392 °F) at an ambient
temperature T
a
of 25 °C (77 °F).
T
g
= ( | T
m
- T
0
| * 0.025 %) + 0.25 K
T
g
= ( | 473.15 K - 298.15 K | * 0.025 %) + 0.25 K
T
g
= 0.29 K
T
d
= 0.005 K/K * | T
a
- T
0
|
T
d
= 0.005 K/K * | 25 °C - 25 °C |
T
d
= 0.00 K
T
i
= 0 because the internal resistance is less than 50
T
cjc
= 1 K
T
dcjc
= 0.008 K/K * | T
a
- T
0
|
T
dcjc
= 0.008 K/K * | 25 °C - 25 °C |
T
dcjc
= 0.00 K
The maximum total inaccuracy is:
T =
T
g
+
T
d
+
T
i
+
T
cjc
+
T
dcjc
T = 0.29 K + 0.00 K + 0.00 K + 1 K + 0.00 K
T = 1.29 K
In this example the maximum total inaccuracy is determined by the inaccuracy
of the cold junction compensation.
