Usually, the pyrometers are divide into two groups. Such as-
Thermocouples and Resistance Pyrometer: The pyrometer has to be directly connected with the place or object whose temperature is going to be measured. This kind of pyrometer with the temperature can be measured up to 1600০C. In this, the temperature is measured by thermocouple or resistance cable. These are also called contact pyrometer.
Optical and Radiation Pyrometer: By such kind of pyrometer, the temperature can be measured from a relative distance through the radiant light and heat waves from the source of heat. This kind of optical and radiation pyrometer can be used to measure up to 4000০C. These are also called distance pyrometer.
What is Thermocouple?
The thermocouple is a system of two different metal wires in which a closed circuit is made by welding its one side and other open sides are connected with an indicator tool. The welded side of the wire is called hot junction and the open sides, which are connected to the junction head is called cold end.
The following thermocouples are used usually. Such as-
Noble metal: 10% radium with 90% pure platinum.
- Iron constantan: almost 60% copper and 40% nickel.
- Chromel Alumel ( 90% nickel and 10% chromium, and Alumel- almost 94% nickel, 2% aluminum, 3% manganese and 1% sodium.
What is Resistance Pyrometer?
With the changes in metal’s temperature, the electrical conductivity of metal is changed and resistance pyrometer works on the basis of it. Among different kinds of a pyrometer, platinum resistance pyrometer can measure temperature with extreme quiddity.
With this, the temperature from 180°C to +1000°C can be measured. In this pyrometer, the platinum wire of equal amount is intertwined like a small coil on the mica stand. This coil is called a bulb, which is enclosed in a metal or refractory material’s jar or in a close-ended protective tube. The ends of the lead wires are extended to the junction head from the coil. Lead wires are connected with resistance measurement tools (such as huitstone-bridge) from junction heads.
It can be assumed that platinum wire is directly proportional to the resistance temperature, that is to say, Rt = Ro (i+αt)
Where Rt = t° C Resistance obtained at a
Ro = O° C Temperature Resistance
α=0° C Temperature Resistance Expansion
Generally, the resistance wire is from platinum, but in some cases, nickel hybrid or palladium used. Contingent upon the proper correction, an exquisite temperature can be measured with this pyrometer.
What is Thermoelectric Pyrometer?
This kind of pyrometer is used more in metallurgical temperature measurement. With it, up to 1600০ C temperature can be measured successfully.
A thermoelectric pyrometer has mainly the following parts-
- A thermocouple, which is consisted of two different metal or metal hybrid.
- Two open-ended connector wire with indicator.
- E.F.M indicator or recorder.
- Touch-free protective resistant tube for elements.
- Tool for protecting cold junction temperature at the specific and proper temperature.
- Junction block.
This kind of pyrometer works on the basis of two following principles-
Peltier Effect: When two wires of different metals are brought in electrical contact, an electromotive force is produced along with the connected part. The value of this electromotive energy depends on the chemical material of the metal and temperature of the junction point.
Thomson Effect: If there is a difference in temperature between the two ends of the single wire, there will be an electromotive force in both ends of the wire. The value of this electromotive energy depends on the chemical material of the wire’s metal, the chemical homogeneity of the wire’s metal and on the temperature difference.
The total EMF (electromotive force) in a thermoelectric pyrometer, which is called the Seebeck effect sometimes, the amount of it is equal to two-Peltier EMF of the cold and hot junction point. Moreover, two Thomson EMF of every single wire, that is to say, it equivalent to the mathematical sum of these four EMF.
Cold junction or reference junction is kept at 0০C or 32০F in the same constant temperature. If the cold junction is kept at a specific temperature, the EMF produced in the pyrometer circuit must be produced by the effect of the temperature of the hot junction. The temperature of hot metal can be measured from the relationship of EMF produced in the hot junction and the temperature. The EMF produced in the hot junction is identified with potentiometer. In addition, from it, the temperature’s amount is found with a special mechanism.