tidu271

www.ti.com
5.2 RTD Input Channel
Theory of Operation
5.2.1 Resistance Temperature Detector (RTD) – “Back to Basics"
A Resistance Temperature Detector (RTD) is a sensing element used to measure the temperature as the
element's resistance changes with changes in temperature. RTDs have been constantly in use for many
years to measure the temperature in laboratories and industries. The relationship between an RTD’s
resistance and temperature is highly predictable and allows accurate and repeatable temperature
measurement over wide ranges.
RTDs are constructed in two common configurations – wire wound RTDs and thin metal film RTDs (see
Figure 25 and Figure 26). Wire wound type RTDs are made by inserting helical sensing wire into a packed
powder-filled insulating mandrel to provide a strain-free sensing element. Thin film RTDs are made by
depositing a thin layer of metal (for example, platinum) in a resistance pattern on a ceramic substrate. The
thin metal film RTDs appear to have an advantage over wire wound configurations in terms of
ruggedness, vibration resistance, cost, size, and response.
Figure 25. Wire Wound RTD
Figure 26. Thin Film RTD
The basic principle for RTD operation is that all the metals have positive change resistance with increase
in temperature. Metals having high resistivity, high melting point, and high corrosion resistance are
generally preferred for making RTDs and allow using less of the metal material to achieve high nominal
resistance value.
Table 4. Metals and Their Resistivity
METAL
RESISTIVITY (Ω/CMF)
CMF = CIRCULAR MIL FOOT
Platinum (Pt) 59
Nickel (Ni) 36
Tungsten (W) 30
Gold (Au) 13
Copper (Cu) 9.26
Silver (Ag) 8.8
TIDU271–May 2014
Submit Documentation Feedback
Temperature Sensor Interface Module for Programmable Logic Controllers
(PLC)
Copyright © 2014, Texas Instruments Incorporated
23