Electricity + Control August 2017

TEMPERATURE MEASUREMENT

Alternative measures Measuring Element Connec- tion Method

Tolerance Class

Measuring Error in ºC

Pt100

2-wire 2-wire 4-wire 4-wire 2-wire 2-wire 4-wire 4-wire

B A B A B A B A

5,25 4,65 1,05 0,45 1,47 0,87 1,05 0,45

Pt1000

Example: Measuring error at 150°C, cable length 10 m, conduc- tor cross-section 0,22 mm 2 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 mm 2 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.

<> Roland Sachs is a Product Manager for WIKA, Germany.

+27 (0) 11 621 0000 greg.rusznyak@wika.com www.wika.com

Electricity + Control

AUGUST 2017

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