Which among the following thermometers is preferred for measuring temperature around 1250° C?

For measuring high temperatures around 1250°C, an optical pyrometer is the preferred instrument. Standard thermometers have specific upper limits: mercury thermometers typically function up to 300°C-500°C [t4], and constant volume gas thermometers are generally used for lower ranges or up to 800K-1000K depending on the gas and bulb material [t3][t4]. Platinum resistance thermometers are highly accurate but are generally limited to approximately 1000°C due to the risk of platinum contamination or mechanical instability at higher temperatures [t4]. In contrast, optical pyrometers operate on the principle of thermal radiation and do not require physical contact with the hot object. They are specifically designed for high-temperature industrial applications, with a typical measurement range extending from 700°C to 4000°C [t4]. This makes them the most suitable and reliable choice for the 1250°C threshold.

Sources

1. [1] https://nvlpubs.nist.gov/nistpubs/jres/106/1/j61man.pdf