unit 1 Temperature measuring Device ppt.pptx

ppt of temp. measuring devices

1. Temperature and its measurement (principle of measurement, various instruments, etc.). The Zeroth law of thermodynamics provides the basis for the measurement of temperature -The height of mercury column in a thermometer, therefore, becomes a thermometric property . There are other methods of temperature measurement which utilize various other properties of materials that are functions of temperature, as thermometric properties. Six different kinds of thermometers, and the names of the corresponding thermometric properties employed are given below: Thermometer Thermometric property 1. Constant volumes gas Pressure (p) 2. Constant pressure gas Volume (V) 3. Alcohol or mercury-in-glass Length (L) 4. Electric resistance Resistance (R) 5. Thermocouple Electromotive force (E) 6. Radiation (pyrometer) Intensity of radiation (I or J)

2. Measurement of Temperature A thermal condition that is dependent on the body's internal or molecular energy can be used to represent temperature measurement. Temperature Measuring Instruments These instruments may be classified in two broad categories: 1. Non-electrical methods: ( i ) By using change in volume of a liquid when its temperature is changed. ( ii ) By using change in pressure of a gas when its temperature is changed. ( iii ) By using changes in the vapour pressure when the temperature is changed. 2. Electrical method: ( i ) By thermocouples. ( ii ) By change in resistance of material with change in temperature. ( iii ) By comparing the colors of filament and the object whose temperature is to be found out. ( iv ) By calculating the energy that radiation absorbs.

3. Measurement of Temperature The thermometers may also be classified as follows: 1. Expansion thermometers ( i ) Liquid-in-glass thermometers (ii) Bimetallic thermometers. 2 . Pressure thermometers ( i ) Vapour pressure thermometers (ii) Liquid-filled thermometers (iii) Gas-filled thermometers. 3. Thermocouple thermometers 4. Resistance thermometers 5. Radiation pyrometers 6. Optical pyrometers.

4. Measurement of Temperature 1. Expansion Thermometers: A mercury-in-glass thermometer is a type of thermometer where mercury fills a glass bulb and extends into the stem. It is suitable for temperatures from 38.9°C to 600°C. The scale is engraved on the glass stem, and an expansion bulb is provided for expansion. Other liquids like pentane, ethyl alcohol, and toluene can be used, with a dye added for better reading.:

5. Measurement of Temperature ii) Bimetallic thermometers. In a bimetallic thermometer differential expansion of bimetallic strips is used to indicate the temperature. It has the advantage over the liquid-in-glass thermometer that it is less fragile and is easier to read. In this type of thermometer two flat strips of different metals are placed side by side and are welded together. Many different metals can be used for this purpose. Generally one is a low expanding metal and the other is high expanding metal. The bimetal strip is coiled in the form of a spiral or helix. Due to rise in temperature, the curvature of the strip changes. The differential expansion of a strip causes the pointer to move on the dial of the thermometer.

6. Measurement of Temperature 2. Pressure Thermometers: In pressure thermometers liquids, gases and vapours can all be used. The principle on which they work is quite simple. The fluid is confined in a closed system. In this case the pressure is a function of the temperature, so that when the fluid is heated, the pressure will rise and the temperature can be indicated by Bourdon type pressure gauge. In general, the thermometer consists of a bulb which contains bulk of the fluid. The bulb is placed in the region whose temperature is required. A capillary tube connects the bulb to a Bourdon tube, which is graduated with a temperature scale. Pressure thermometers are discussed below:

7. Measurement of Temperature ( i ) Vapour pressure thermometer ( ii ) Liquid-filled thermometer.

8. Measurement of Temperature iii ) Gas-filled thermometers. The temperature range for gas thermometer is practically the same as that of liquid filled thermometer. The gases used in the gas thermometers are nitrogen and helium . Both these gases are chemically inert, have good values for their co-efficient of expansion and have low specific heats. The construction of this type of thermometer is more or less the same as mercury-thermometer in which Bourdon spring is used. The errors are also compensated likewise. The only difference in this case is that bulb is made much larger than used in liquid-filled thermometers. For good performance the volume of the bulb should be made at least 8 times than that of the rest of the system. These thermometers are generally used for pressures below 35 bars.

9. Measurement of Temperature 3. Thermocouple Thermometers: 4. Resistance thermometers

10. Measurement of Temperature 5. Radiation pyrometers: 6. Optical pyrometer:

11. Numerical

12. Numerical

13. Numerical Example 1.2 A Platinum resistance thermometer has a resistance of 2.8 ohm at 0 degree Celsius and 3.8 ohm at 100 degree Celsius. Calculate the temperature when the resistance indicated is 5.8 ohm (Ans t = 300 degree Celsius) Dec 2018 Example 1.3

14. Numerical Example 1.4 A gas is contained in a vertical, frictionless piston–cylinder device. The piston has a mass of 4 kg and a cross-sectional area of 35 cm2. A compressed spring above the piston exerts a force of 60 N on the piston. If the atmospheric pressure is 95 kPa , determine the pressure inside the cylinder. Answer: 123.4 kPa