Most industrial RTDs have what temperature coefficient?
A. Negative
B. Variable
C. Constant
D. Positive
View Answer
Answer: D
Explanation:
Most industrial RTDs are made from platinum, which has a positive temperature coefficient (PTC). This means the resistance of the RTD increases as temperature increases.
For a standard Pt100 RTD:
- 0°C → 100 Ω
- 100°C → 138.5 Ω
- 200°C → 175.9 Ω
This positive relationship between resistance and temperature is the fundamental operating principle of RTDs.
Positive Temperature Coefficient
When temperature rises:
Resistance increases
When temperature falls:
Resistance decreases
Why the Other Options Are Incorrect
Negative
- Characteristic of NTC thermistors, not RTDs.
Variable
- The coefficient is defined by standards such as IEC 60751 and is not considered variable for a given RTD type.
Constant
- The resistance changes with temperature, so the effect is not constant.
Because industrial RTDs exhibit an increase in resistance with increasing temperature, they are classified as devices with a positive temperature coefficient.
Therefore, Option D (Positive) is the correct answer.