Vapor-pressure thermometer construction: In a vapor-pressure thermometer, which component functions as the primary sensing element that directly experiences the process temperature?

Introduction / Context:
Filled-system thermometers (liquid, gas, or vapor pressure types) use a sensing bulb connected via capillary to a pressure element. Understanding which part is the primary element clarifies how temperature changes get converted into an indication.

Given Data / Assumptions:

• Instrument: vapor-pressure thermometer.
• Components: bulb, capillary, Bourdon tube (or bellows), linkage, pointer, dial.

Concept / Approach:
The bulb is the primary sensing element. It is immersed in the process and contains the volatile fluid whose saturation pressure varies strongly with temperature. This pressure is transmitted through the capillary to a Bourdon tube or bellows that converts pressure into motion for the pointer (a secondary element).

Step-by-Step Solution:

Verification / Alternative check:
Instrument handbooks classify sensing bulbs as primary elements irrespective of the downstream pressure-to-motion transducer type.

Why Other Options Are Wrong:

• Pointer/Dial: Indication only, no sensing.
• Bourdon tube: Secondary conversion from pressure to motion.
• Capillary: Transmits pressure; not the sensor itself.

Common Pitfalls:
Assuming the Bourdon tube is primary because it “moves”; in filled systems, the bulb sets the pressure signal.

Final Answer:
Bulb