Electricity + Control August 2017

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21 Electricity + Control AUGUST 2017
TEMPERATURE MEASUREMENT
Alternative measures
Measuring
Element
Connec-
tion
Method
Tolerance
Class
Measuring
Error
in ºC
Pt100 2-wire B 5,25
2-wire A 4,65
4-wire B 1,05
4-wire A 0,45
Pt1000 2-wire B 1,47
2-wire A 0,87
4-wire B 1,05
4-wire A 0,45
Example: Measuring error at 150°C, cable length 10 m, conduc-
tor cross-section 0,22 mm2
A further possibility to substantially decrease the influence of
the cabling is to increase the conductor cross-section. With a
cross-section of 0,5 mm2 the line resistance is only 0,036 Ω/m
or 0,1°C/m. Both options (3/4-wire connection or increasing the
cross-section) lead to a higher cost in the cabling, which can
be problematic, especially in cost-sensitive markets such as ma-
chine building. As a compromise between cost and accuracy,
for smaller cable lengths, a class A, 2-wire connection Pt1000
measuring element can be offered.
Conclusion
• The highest measurement accuracies are only achievable
with a Pt100 in a 4-wire connection
• A Pt1000 measuring element in class A also offers good
measurement accuracies in a 2-wire connection and repre-
sents an economical alternative to 3- or 4-wire connections
for machine building
Bibliography
[1] Pt100, Pt1000 or NTC – which is the right measuring ele-
ment?
[2] Bourdon tube pressure gauge – operating principle.
[3] Analogue output signals of pressure sensors 1: 4 … 20 mA.
<<Author>>
Roland Sachs is a Product
Manager for WIKA,
Germany.
+27 (0) 11 621 0000