Temperature measurement range selection: Which instrument is most suitable for measuring temperature in the range of -40°C to 425°C in typical industrial service?

Introduction / Context:
Selecting a temperature instrument depends on range, robustness, and environment. Many field applications need simple, durable, and reasonably accurate indication across sub-zero to several-hundred-degree ranges.

Given Data / Assumptions:

• Target range: -40°C to 425°C.
• Preference for mechanical, line-mounted indicators with minimal maintenance.
• No requirement for non-contact high-temperature measurement.

Concept / Approach:
Bimetallic thermometers use the differential expansion of bonded metals to translate temperature into pointer motion. They are rugged, do not require power, and commonly cover about -50°C to 500°C. Mercury or alcohol thermometers have narrower safe ranges and fragility; radiation pyrometers are for much higher temperatures without contact; a bare thermistor typically cannot withstand 425°C and requires electronics.

Step-by-Step Solution:

Verification / Alternative check:
Vendor datasheets list common bimetal ranges extending to 400–500°C, with various stem and case options for process duty.

Why Other Options Are Wrong:

• Mercury/Alcohol: Range limitations and safety concerns; glass bulbs are fragile.
• Radiation pyrometer: Overkill for this range and requires emissivity considerations.
• Thermistor only: Electronics and material limits preclude 425°C.

Common Pitfalls:
Ignoring mechanical durability and readability in field conditions when choosing an instrument for wide-range service.

Final Answer:
Bimetallic thermometer