JNTU B.Tech III INSTRUMENTATION AND PROCESS CONTROL Previous Question Paper (September - 2021 - R18)

A bimetallic thermometer operates on the principle of differential thermal expansion of two dissimilar metals bonded together. The sensing element consists of a bimetallic strip, usually formed into a helix or spiral, comprising two metals with significantly different coefficients of thermal expansion (e.g., Invar and brass or steel).

Constructionally, one end of the bimetallic helix is fixed to the thermometer housing, while the other end is connected to a pointer mechanism. When the temperature changes, the two metals expand or contract at different rates, causing the strip to deflect or coil/uncoil. This mechanical movement is transmitted through a linkage to rotate the pointer across a calibrated scale.

Applications include industrial temperature monitoring in HVAC systems, ovens, water heaters, and general-purpose process control where high precision is not critical but ruggedness is required.

The static accuracy of a thermometer is influenced by several design and environmental factors:

• Calibration Error: Deviations from the standard temperature scale during initial manufacturing or due to aging of the sensing element.
• Hysteresis: The difference in reading when the temperature is approached from a higher versus a lower value, often caused by mechanical friction in the linkage or internal stress in the bimetallic element.
• Ambient Temperature Effects: Changes in the temperature of the thermometer housing or the stem can introduce errors if the instrument is not properly compensated.
• Stem Effect/Conduction Error: Heat transfer along the stem or protective well can cause the sensing element to measure a temperature different from the actual process fluid, especially if the immersion depth is insufficient.
• Time Constant/Lag: While primarily a dynamic characteristic, slow response times can lead to static errors if the process temperature is not perfectly stable during measurement.
• Mechanical Wear: Friction in the gears or pivots of the pointer mechanism over time degrades accuracy.
• Thermal Contact Resistance: Poor contact between the thermometer bulb and the medium being measured prevents accurate heat transfer, leading to significant reading errors.