In a humidstat, the sensing element is typically made from what type of material?

The correct choice for the sensing element in a humidistat is a hygroscopic membrane. A hygroscopic membrane is a material that can absorb moisture from the air, making it highly suitable for measuring humidity levels. When humidity changes, the membrane expands or contracts, which can then be used to trigger a signal for controlling humidity in a space.

This property of hygroscopic membranes allows for precise and responsive control in HVAC systems, making them essential components in maintaining comfort within built environments. The use of materials like plastic film, carbon steel, or copper alloy does not provide the same level of sensitivity and reliability in sensing moisture, as these materials do not respond to humidity changes in the same manner as hygroscopic membranes.