SOLUTION

A thermoelectric pyrometer instrument can be used for the measurement of a temperature above 1500°K.

A pyrometer is a device for measuring very high temperatures and uses the principle that all substances emit radiant energy when hot, the rate of emission depending on their temperature. The measurement of thermal radiation is, therefore, a convenient method of determining the temperature of hot sources and is particularly useful in industrial processes. There are two main types of the pyrometer, namely the total radiation pyrometer and the optical pyrometer.

Pyrometers are very convenient instruments since they can be used at a safe and comfortable distance from the hot source. Thus applications of pyrometer are found in measuring the temperature of molten metals, the interiors of furnaces, or the interiors of volcanoes. Total radiation pyrometers can also b used in conjunction with devices that record and control temperature continuously.

Total radiation pyrometers are used to measure temperature in the range of 700°C to 2000°C.

Optical pyrometers may be used to measure temperatures up to, and even in excess of, 3000°C.

Measurement of temperature using the thermometer

1. Liquid thermometer:- 
   1. Low temperature: as low as −30°C (freezing point of mercury is −39°C) can be measured using mercury and as low as -100°C can be measured by using alcohol (Remains liquid upto-130°C) in glass thermometers.
   2. High temperature: A mercury thermometer (boiling point of mercury is 357°C) can be used to measure temperature upto 300°C. The range can be increased to 600°C by filling the space above mercury with some inert gas. A gallium boiling point 1700°C thermometer made of quartz can be conveniently used for measurement of temperature upto 1000°C.
2. Gas thermometers
   1. Low temperature: For measurements of low temperatures upto −250°C constant volume hydrogen thermometers are used and for measure merits below -250°C upto -270°C hydrogen is replaced by Helium. The Helium. at the pressure below its vapor pressure can be used to measure temperature down to 1 K.
   2.  High temperature: For high-temperature measurement upto 500°C the standard hydrogen gas thermometer with platinum-iridium bulb can be used, and for the temperature upto 1100°C porcelain bulb is used. Using nitrogen in place of hydrogen in the Rhodium bulb, the temperature as high as 1600°C can be measured.
3. Resistance thermometers
   1. Low-temperature range: The platinum resistance thermometer is suitable for measurement upto -190°C. A lead thermometer may be used to measure temperature as low as -250°C and below these temperature constants and manganin thermometers are used. Phosphor bronze can be used to measure still lower temperatures with great accuracy.
   2. High-temperature range: Platinum resistance thermometers can be used to measure temperature upto 700°C with the coil enclosed in the glass bulb and lead made of copper and upto 1300°C with the coil enclosed in porcelain with platinum leads.
4. Thermoelectric-thermometer
   1. Low-temperature measurement: Thermocouples of Iron-constantan and copper constantan are used to measure temperature as low as 18 K and gold-platinum and silver platinum thermocouples are used to measure below 18 K with high accuracy.
   2. High-temperature measurement:- Iron constantan or copper constants are used to measure temperate upto 300°C and thermocouples of nickel-iron are used to measure temperature opts 600°C. The measurement range of nickel-nichrome thermocouples is from 600°C to 1000°C. Between 1000°C and 1600°C Platonism Rhodium thermocouple is used.