Intruments - Temperature

Question 1

How does an RTD work?

Answer 1

• An RTD’s electrical resistance changes predictably with temperature.
• Most are platinum (Pt) because it’s stable and linear.

The common types:
- Pt100: 100 Ω at 0 °C
- Pt1000: 1000 Ω at 0 °C

Wiring types:

• 2‑wire: simplest, but lead resistance directly adds error.
• 3‑wire: cancels lead resistance assuming two leads are equal (common in industry).
• 4‑wire (Kelvin): best accuracy; measures voltage separately from current path.

Question 2

How does a thermocouple work?

Answer 2

Two dissimilar metals joined at one end produce a voltage due to the Seebeck effect. The voltage is proportional to the temperature difference between the measuring (hot) junction and the reference (cold) junction.

Must compensate for ambient temperature using cold junction compensation. Convert the terminal temperature to a voltage and add it to the measured voltage of the TC.

Question 3

What is a K Type thermocouple?

Answer 3

A Type K thermocouple is made from two dissimilar nickel-based alloys welded together at the measuring junction:

• Positive leg (Chromel)
• Negative leg (Alumel)

Temperature range: roughly −200 to 1 260 °C (commonly used 0 to 1 100 °C).