A thermometer is an instrument used to measure temperature. It consists of a metal or glass tube containing a liquid, usually mercury, that expands and contracts with temperature changes. The expansion and contraction of the liquid cause a marked scale on the tube to indicate the temperature.
Types of Thermometers
1. Bimetallic Thermometer
A Bimetallic Thermometer is an instrument used to measure temperature. It consists of two strips of different metals that are bonded together, usually steel and copper. When the temperature changes, the two metals expand or contract at different rates, causing the strip to bend and giving an indication of the temperature.
Range: Temperatures ranging from - 40 °C to 500 °C can be measured with this type of thermometer.
Construction
In this, a spring of helix shape is made from the spring of two metals. The outer end of this spring is connected to the shell and the other end is connected to the pointer by a rod. This pointer rotates on a circular scale. To protect it from corrosion and wear and tear, it is covered with a thermal wall.
Working Principle
When metals are heated, their temperature increases, due to which the rise in ionization increases the length, the increase in width is negligible to the increase in length.
If one end of a metal spring is kept stationary and heated, the other end moves in a circular motion due to an increase in temperature. When the thermal wall is exposed to a hot temperature, the heat from the thermal wall is transferred to the spring, causing the free end of the spring to move, moving in a circular path.
Advantages
This allows for more accurate readings than other types of thermometers, as the two metals compensate for any errors caused by the environment or the measuring instrument.
Bimetallic thermometers are easy to use and require little maintenance.
It can also be used in a wide range of temperatures, ranging from extremely cold to very hot.
They provide an excellent choice for many applications where accuracy and reliability are essential.
Disadvantages
Bimetallic thermometers are sensitive to changes in ambient temperature and can be affected by external magnetic fields.
These thermometers require frequent calibration and maintenance to maintain accuracy.
2. Liquid Expansion Thermometer or Glass Thermometer
These types of thermometers are used to measure temperature values ranging from -130°C to 560°C.
Construction
Mercury thermometer has a bulb in which mercury is filled, the bulb is closed to a metal wall which prevents it from breaking and bursting, and reservoirs are made on top of the bulb, in which if the temperature is high the expansion of mercury exceeds the limit.
The excess mercury goes into the reservoirs and the thermometer is saved from breaking.
Working Principle
This thermometer is used for temperature display as well as for non-ionic expansion of a liquid because if the temperature of a liquid is increased at constant pressure, its volume increases.
When the metal well is brought in contact with a hot object, the heat is transmitted to the mercury through the metal wall, due to which the mercury expands and rises in the capillary and radiates the temperature on the appropriate scale.
The scale calibration of the Hg thermometer is not completely linear because of the following reasons.
(i) Expansion and contraction occur in the bulb of the thermometer due to temperature changes.
(ii) The volume expansion coefficient of mercury changes with temperature.
(iii) N2 gas is filled above the mercury to increase the mercury boiling point at a certain pressure and when the mercury column increases, then the pressure of the gas increases above it, which hinders the expansion of mercury.
3. Thermo Couple Thermometer
This thermometer is based on the Sievek effect. When thick wires of two different metals are joined together to form a closed circuit. Keeping both the junctions of this circuit at different temperatures, then e.m.f. is produced and current starts to be generated in the circuit. This arrangement made of two metals is called a thermocouple.
If the cold junction is kept at 0 °C and the hot junction at t °C, then e.m.f E = at + bt².
If the graph is drawn in this e.m.f and t, then the value of e.m.f increases as the temperature of the hot junction increases and E decreases more at a particular temperature, this temperature is called neutral temperature.
Now by increasing the temperature of the hot junction more and more, the value of K starts decreasing and finally, it becomes zero. This temperature is called reversal temperature.
Range: With this, temperatures from -200°C to 160°C can be measured. Thermocouples of different metals have to be taken for different temperatures.
Advantages
It gives an accurate measurement.
These can be used easily.
It is cheap.
Its electrical torque can be increased in various ways.
Disadvantages
It is used in very few Paras which is less than 350°C. Because the temperature measurement of the pair is a very small voltage change.
It does not contain mathematical formulas for calculating the temperature of different ranges.
4. Resistance Thermometer
Its use is used to measure temperature from 200°C to 659°C. The resistance of a metal wire increases as the temperature increases. It has been observed experimentally that the resistance of a platinum wire normally increases with an increase in temperature. If the resistance of a pt wire increases with an increase in temperature, if the resistance of a platinum wire is Ro at 0 °C and Rt at t °C, then Rt = Ro (1+∝t)
where ∝ = resistance is the temperature coefficient.
Construction
In this, the pure platinum wire is doubled and wrapped on a sheet of mica. Put this leaf in the silk tube at the lower end. The ends of the platinum wire are joined to the copper wire. This copper connector is connected to the screw on the lid of the connector ebonite after passing through several sheets of mica.
There are two other wires in the same way to neutralize the effect of the resistance of the wire. The lower ends of which are connected and the upper ends are tightened in the screws on the ebonite lid.
Working Principle
First of all, by keeping the lower end of the thermometer in ice, the 0°C resistance of Platinium is measured by Wheaton Rotu, then keep it in steam whose temperature is to be determined if the resistance at 0°C and 100°C is Ro and R 100 respectively. yes then
Rt = Ro (t + ∝t) for
R100 = Ro(1 + ∝100)
∝ = R100 - Ro/100Ro ..........1
Nou as die weerstand by onbekende temperatuur t Rt is t = Rt – Ro/Ro∝ .........2
Substituting the value of ∝ equation 1 and equation 2 in.
t = Rt - Ro/Ro (R100 - Ro/100Ro )
t = (Rt - Ro)/(R100 - Ro) x 100
In this way we find the value of t.
Advantages
Its use is simpler than that of a gas isotherm.
It gives a very accurate measurement of temperature.
With this temperature can be measured from -200°C to 1200°C.
