ETAS
Hardware Description
ES420.1 - User’s Guide
21
The combination of thermocouple cold junction, replaceable splitter cable and
ES420.1 enables temperatures to be measured highly precisely and virtually
independently of environmental influences.
4.4
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 “Sensor Inputs” on page 72.
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.