Bimetallic thermometer | Pressure thermometer | Working and Consideration |Advantages and Disadvantages | Diagram | error in pressure thermometer :
Working of Bimetallic thermometer :
1) Different two materials having a different coefficient of the thermal expansion rigidly joint together, one on other form a bimetallic strip.
2) When bimetallic strip is fixed at one end heated from free end then it bends in the direction of material having low thermal coefficient of expansion.
3) The bending movement of free end is connected to pointer which moves over calibrated scale.
4) Always Bimetallic strip is wound in the form of helix or in spiral form.
5) Its one end is fixed the e permanently to outer casing to form stopper end is pointer. and other connected to pointer.
6) A typical bimetallic thermometer used between – 40°C to 320°C consists of Invar (36 % Ni, 64% Fe) has low coefficient of expansion when joined to Ni-Mo
Fig. A bimetallic thermometer strip
Advantages of Bimetallic thermometer strip :
1) Its construction is simple.
2)Compact and robust construction.
3) No maintenance is required.
4) Over loading can be tolerated.
5) Its cost is low.
Disadvantages of Bimetallic thermometer strip :
1) Material is subjected to creep at high temperature.
2 ) Speed of response is low.
Fig. Bimetallic helix thermometer.
Pressure thermometer :
There are two types of pressure thermometer :
a) Liquid pressure thermometers.
b) Vapour pressure thermometers.
a) Liquid pressure thermometers :
These are also called fluid-expansion thermometers. These are most economical, versatile and are widely used in industries. The principle of the operation is illustrated is in the Fig. A bulb containing a liquid is immersed in the environment. The bulb is connected by means of a capillary tube to a pressure measuring device, like a bourdon tube pressure gauge. An increase in temperature causes the liquid to expand, thereby increasing the pressure on the gauge.
Ideally, the tube should serve simply as a connecting link between the bulb and the indicator. But the liquid filled tubes are also temperature sensitive, and would introduce error. This can be reduced by increasing the bulb size. But increasing the bulb size to higher extent reduces the time response of the system. The system with a bulb, tube and a pressure gauge can be calibrated directly.
Fig. Liquid pressure thermometer
Advantages of liquid pressure thermometer :
1) The instrument can be used and hence measurement of temperature at a far distance is possible provides sufficient driving force to indicate the readout.
2) These instruments are comparatively less costly and are simple.
3) Tubes can be as long as 50 meters.
b) Vapour pressure thermometers :
These are similar to liquid filled thermometers, but contain only partially filled bulbs. They Use the principle that the vapour pressure of the liquid depends on the temperature, and most fluids partially filled in an enclosed chamber will create a definite vapour pressure.
When fluids reach the boiling point, the vapour pressure will be equal to the total pressure above the liquid surface and can be used as the measure- of temperature. The construction of a simple vapour pressure thermometer is illustrated in Fig.
The bulb is partially filled so that when the bulb senses the temperature vapour be formed in the remaining space in the bulb, and passed through a capillary tube to the indicator with a bourdon tube. The vapour created under the test temperature be used to move a calibrated scale and indicates the temperature. Such as thermometers are useful in the range of – 6° C to 350° C.
Fig. Vapour pressure thermometers.
Reasons for static error in the pressure thermometer.
1. If the location of bulb is improper.
2. If the immersion of bulb is incorrect.
3. If there is change in the ambient pressure.