Psychrometry instrumentation — Humidity of a gas–vapor mixture can be determined by measuring which property of a hygroscopic fibre sensor?

Introduction:
Industrial humidity measurement uses several transduction principles: dew-point cooling, capacitance, resistance, frequency, and length change of hygroscopic materials. Fibre- or polymer-based sensors are common due to robustness and low cost. This question targets the property most often exploited with hygroscopic fibres for humidity determination.

Given Data / Assumptions:

• A hygroscopic fibre (e.g., treated polymer or organic fibre) whose properties vary with moisture content.
• Measurement is performed electronically in a practical instrument.
• Ambient temperatures typical of HVAC/process areas.

Concept / Approach:

Moisture absorption changes the fibre's ionic conduction/charge carrier mobility, altering electrical resistance. Modern polymer-film or fibre sensors leverage this by forming a resistive (or capacitive) element whose impedance varies predictably with relative humidity. Electronics then convert resistance to RH via calibration curves. While mechanical length or strength may also change with humidity (e.g., hair hygrometers), resistance is the standard electrical property used in many compact sensors.

Step-by-Step Solution:

Verification / Alternative check:

Datasheets for polymer-resistive humidity sensors show monotonic R–RH characteristics; many HVAC transmitters use resistive or capacitive polymer films rather than thermal conductivity or tensile strength changes.

Why Other Options Are Wrong:

B: Thermal conductivity hygrometers exist but are not fibre-based sensors. C: Strength variation is used in mechanical hair hygrometers, not typical electronic fibre sensors. D/E: Humidity can indeed be sensed via resistance; optical reflectivity is not a standard sole basis in fibres.

Common Pitfalls:

Assuming all humidity sensors are dew-point mirrors; many field devices are polymer resistive/capacitive elements sensitive to moisture content.

Final Answer:

electrical resistance