Which method of temperature sensing involves a bulb and bellows?

The correct method of temperature sensing that involves a bulb and bellows is fluid expansion. In this system, a bulb is filled with a fluid that expands or contracts in response to temperature changes. As the temperature increases, the fluid expands and pushes against the bellows, translating temperature changes into mechanical movement. This mechanical action can then be used to operate a gauge or switch, making this method highly effective for a variety of temperature measurement applications.

The fluid expansion method relies on the predictable behavior of liquids or gases when subjected to temperature variations, allowing for accurate and reliable readings. This principle is commonly employed in thermometers and other temperature-sensing devices.

The other methods mentioned, such as bi-metal, rod and tube, and capacitive sensing, operate on different principles and mechanisms. Bi-metal sensors use two different metals bonded together that expand at different rates, rod and tube sensors rely on materials that change dimensions with temperature, and capacitive sensing utilizes changes in capacitance due to temperature variations. Each of these methods has its unique applications and is based on distinct physical principles rather than the fluid expansion mechanism.