Question

A Pirani gauge measures vacuum pressure and works on the principle of

• A. change in ionizing potential
• B. change in thermal conductivity
• C. deformation of elastic body
• D. change of self-inductance

Hint:

Thermal conductivity gauges (like Pirani and Thermocouple gauges) are ideal for medium to high vacuum ranges ($10^{-3}$ to $100\text{ Torr}$).
They all rely on heat transfer rates changing with gas density.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
This is a conceptual question regarding pressure measurement in the vacuum range using a Pirani gauge.

Step 2: Key Formula or Approach:
A Pirani gauge is a thermal conductivity gauge used to measure low pressures (vacuum).
We must identify which physical property changes with gas pressure to allow measurement.

Step 3: Detailed Explanation:

• In a vacuum system, as the gas pressure decreases, the number of gas molecules decreases.

• The thermal conductivity of the gas is directly proportional to its pressure in the low-pressure range.

• A Pirani gauge consists of a heated metal filament placed in the vacuum chamber.

• As the pressure changes, the rate of heat loss from the filament to the surrounding gas changes due to the change in thermal conductivity.

• This temperature change alters the electrical resistance of the filament, which is measured using a Wheatstone bridge.

Step 4: Final Answer:
The Pirani gauge works on the principle of change in thermal conductivity, which corresponds to Option (B).