Filled-system thermometers: which property of the fluid inside the temperature-sensing bulb changes with temperature and provides the measurement signal?

Introduction / Context:Filled-system thermometers (gas-filled, liquid-filled, or vapor-pressure types) couple a bulb and capillary to a pressure element (Bourdon tube, bellows). Temperature changes in the bulb alter the condition of the filling fluid, producing a measurable mechanical motion via pressure.

Given Data / Assumptions:

• Standard industrial filled-system thermometer with bulb, capillary, and pressure spring element.
• Signal variable transmitted to the indicator is mechanical displacement proportional to internal pressure.

Concept / Approach:Although the fluid’s volume and viscosity may vary with temperature, the instrument’s operating principle is that temperature changes cause a pressure change in the sealed system. The Bourdon/bellows converts this pressure into pointer movement. In gas-filled types, P ∝ T (approximately), while in liquid-filled and vapor-pressure types, volumetric expansion and phase equilibrium still manifest as a pressure change at the spring element.

Step-by-Step Solution:Recognize the mechanical transduction path: bulb → pressure change → spring motion.Identify pressure as the variable that is transmitted through the capillary to the gauge element.Select “Pressure.”

Verification / Alternative check:Datasheets specify range in terms of pressure–temperature filling characteristics and list capillary length limits because the signal is a pressure wave, not free expansion volume.

Why Other Options Are Wrong:Volume/viscosity changes may occur locally but are not the transmitted signal; the pointer responds to pressure at the spring element.“All”: Overstates what the measurement relies upon.

Common Pitfalls:Confusing vapor-pressure thermometers (which use saturation pressure vs. temperature) with volumetric glass thermometers; the filled-system industrial design reads pressure.

Final Answer:Pressure