Temperature Measurement of Furnaces and Metals.

Introduction

In all melting practices and in a number of different situations, some equipment are necessary for measuring temperatures. A scientific and reliable method of measuring temperature is by an instrument called pyrometer.
A pyrometer is a device that is used for the
temperature measurement of an object. The device actually tracks and measures the amount of heat that is radiated from an object.

There are two types of pyrometers.

• Thermo-electric pyrometers
• Optical pyrometers.

Thermo-electric Pyrometers.

These may be employed at any temperature (a) up to about 1200°C for those made of 'base-metals' and (b) to 1500°C for 'noble-metals'.

Base metal couple is made from (a) nickel chromium wire with nickel-aluminium workforce temperatures up to 1200°C and (b) nickel chromium with constantan wire up to 800°C. Base metal couples oxidize more readily than noble metal couples. They have, therefore, much shorter life.

Noble-metal couple : consist of one wire of pure platinum and other of an alloy of platinum and rhodium. This is used for temperatures up to 1500°C. Wires in the noble-metal couple do not oxidize readily.

Operation of thermo-electric pyrometers is based on the principle that when two wires of different metals are joined to form a complete electric circuit, and one junction is at a higher temperature than the other, an electric current flows in the circuit.
The magnitude of the current depends upon the metal used and the temperature difference of the junctions. In the practical application of this principle, an electric meter is connected to two wires, which are welded together to form the hot junction. The two wires which form the junction are known as 'thermocouple'.

The essential parts of the thermo-electric pyrometer are;

• Thermocouple
• Galvanometer and
• Wires which lead from thermocouple to galvanometer.

The galvanometer is so calibrated that instead of indicating electrical units, it reads in degrees of temperature.

Optical pyrometers

For measuring the temperature of a final bed or other body which may be at or above a red heat thermo-couple is frequently not the best means either because : * The temperatures are too high, or * The position is inaccessible.

Sunder such circumstances, optical pyrometers are used. These may be employed at any temperatures up to about 4000°C. They are frequently used by the steel smelters for ascertaining the furnace temperature while melting steel.

principle

It compares the intensity of light being emitted from the furnace with that of some standard source. This instruments calibrated so that it reads in degrees of temperature. Instead of in units of electric current.

Operation

The telescope is sighted on the hot object and focused. The lamp filament will then be seen against the red -hot through an adjustable resistance and is gradually increased until the tip of the filament just disappears into the background.

Reference

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