Question 30 QMED04 - Boiler Technician-Watertender

The Correct Answer is D **Why option D ("Bimetallic thermometer") is correct:** A bimetallic thermometer is specifically designed based on the principle of **differential expansion of dissimilar metals** (typically welded together into a strip or helix). When the temperature changes, the two metals expand or contract at different rates, causing the composite strip to bend or coil. This mechanical movement is directly linked to a pointer that indicates the temperature on a **rotary dial scale**. **Why the other options are incorrect:** * **A) Liquid-in-glass thermometer:** This type works on the principle of the volumetric thermal expansion of a single liquid (like mercury or alcohol) confined within a glass bulb and capillary tube. It does not rely on the differential expansion of dissimilar metals and typically features a linear scale, not a rotary dial. * **B) Bourdon tube thermometer:** Also known as a pressure thermometer or filled-system thermometer, this device works by measuring the pressure change of a confined liquid or gas within a bulb as the temperature changes. The resulting pressure change deforms a curved tube (the Bourdon tube), and this mechanical movement drives a pointer. While it often features a rotary dial, the core sensing principle is pressure change due to thermal expansion of a fluid, not the differential expansion of two dissimilar solids. * **C) Thermocouple pyrometer:** This device is an electrical temperature sensor. It works on the **Seebeck effect** (thermoelectric effect), where a voltage (electromotive force) is generated between two wires made of dissimilar metals when their junctions are held at different temperatures. It measures temperature electrically and converts it to a reading; it does not rely on the mechanical movement caused by the differential physical expansion of metals, nor is it classified as a direct reading mechanical thermometer in the same way.