Mechanical temperature measuring devices and their applications

Mechanical temperature measuring devices, their operating principle, applications, advantages and disadvantages.

1. MECHANICAL TEMPERATURE MEASURING DEVICES PRINCI P LES OF OERATION AND P ROPERTIES 03-Feb-15 Mechanical Temprature Mesurement 1

2. Temperature Measurement Temperature measurement in today’s industrial environment has a wide variety of needs and applications . It is a very critical and widely measured variable for most mechanical engineers. The industry has developed a large number of sensors and devices to handle this demand . 03-Feb-15 Mechanical Temprature Mesurement 2

3. Types of Temperature Measurement Temperature can be measured in a diverse array of sensors. All of them infer temperature by sensing some change in physical characteristic. Four basic ty p es of temperature measurement include: 1) Mechanical 2) Thermojunctive 3) Thermoresistive 4) Radiative 03-Feb-15 Mechanical Temprature Mesurement 3

4. Mechanical Temperature measurement In the absence of electronics, early temperature sensors were based on the mechanical effects of temperature. While the mechanical temperature sensing technologies are decades or centuries old, they are still in widespread use. T hey meet the needs of many common applications in a reliable and cost-effective manner. 03-Feb-15 Mechanical Temprature Mesurement 4

5. P r inci p le of o peration A change in temperature causes some kind of mechanical motion, typically due to the fact that most materials expand with a rise in temperature . Mechanical thermometers can be constructed that use liquids , solids, or even gases as the temperature-sensitive material . The mechanical motion is read on a physical scale to infer the temperature . 03-Feb-15 Mechanical Temprature Mesurement 5

6. Mechanical temperature measuring devices The basis of the thermometer’s operation is the thermal expansion of a working fluid(mercury in this case). The volume of mercury changes slightly with temperature; the small change in volume drives the narrow mercury column a relatively long way up the tube . The space above the mercury may be filled with nitrogen or it may be at less than atmospheric p ressure, a partial vacuum. 1) Liquid-in-Glass Thermometer: 03-Feb-15 Mechanical Temprature Mesurement 6

7. The scale is calibrated to read temperature directly . The range of a thermometer and it reading accuracy is dependent on the size of the hole, the length of the tube and the fluid in the thermometer . Typically the smaller the reading increment , the less range it will have . The accuracy of a thermometer is greatly dependent on the manufacturing process, but also can be affected by usage. 03-Feb-15 Mechanical Temprature Mesurement 7

8. Applications:   Aircrafts  use thermometers and hygrometers to determine if atmospheric icing conditions exist along their  flight path. They are also applied in meteorological and oceanographic applications. These measurements are used to initialize weather forecast models.  Used in household, hospitals, etc., for body temperature measurement. They are also used in indoor climate control systems. 03-Feb-15 Mechanical Temprature Mesurement 8

9. Advantages: Simplicity in use & low cost. Portable device. Checking physical damage is easy. Power source not require . Repeatable, calibration does not drift. Disadvantages: Can not be used for automatic recording. Time lag in measurement. Range is limited to about 300 °C . 03-Feb-15 Mechanical Temprature Mesurement 9

10. 2) Pressure thermometer: A pressure thermometer operates by the expansion of a gas or a liquid. Its ty p es are: 1) Liquid Pressure Thermometer 2) Vapour Pressure Thermometer A pressure thermometer measures temperature indirectly by measuring pressure . The gauge is a pressure gauge , but is typically calibrated in units of temperature instead . 03-Feb-15 Mechanical Temprature Mesurement 10

11. 1) Vapor pressure thermometers The saturated vapor pressure of a volatile liquid is used as a measure of the temperature. These thermometers are very sensitive but the scale is not linear. The measurement accuracy is 1%. The typical temperature range is from - 20° C to 280° C depending on the nature of the gas. 03-Feb-15 Mechanical Temprature Mesurement 11

12. 2) Liquid pressure thermometer: It’s a temperature measuring device whose operation is based on thermal ex p ansion of a liquid. The insides of the thermo-sensor cylinder, capillary, and Bourdon tube are filled with liquid which changes volume with varying temperature . The liquid in the thermo-sensor cylinder is expanded and contracted as the temperature changes, and the pressure change in the Bourdon tube that is transmitted through the capillary is indicated as a temperature .   Liquid filled  thermometers have  scales with  divisions from 10° to 0.01°C .  The measurement accuracy depends on the depth of  immersion  of the thermometer in the medium to be  measured . 03-Feb-15 Mechanical Temprature Mesurement 12

13. Liquid pressure thermometer 03-Feb-15 Mechanical Temprature Mesurement 13

14. Applications: Suitable for most heating and ventilation applications. Used in dairy , brewing and food industries . A common application of this type of thermometer is measurement of outside temperature from the inside of a building . Advantages: Simple structure, high mechanical strength. Low cost, requires no external energy.  Disadvantages: T emperature range is limited, usually 80-400   ℃. S low response time. 03-Feb-15 Mechanical Temprature Mesurement 14

15. 3) Bimetallic Thermometer: The temperature is measured by means of a bimetal system inside the temperature sensor . The bimetal is made from two metal strips, permanently joined together, each metal having a different thermal expansion coefficient . This causes the strip to deflect in proportion to the temperature variation. The actual bimetal system consists of a bimetal strip that is either helically or spirally wound, depending on the size of the sensor and the temperature range to be measured. 03-Feb-15 Mechanical Temprature Mesurement 15

16. 03-Feb-15 Mechanical Temprature Mesurement 16 Bimetal Helix

17. Any temperature variation causes the bimetal to rotate an attached spindle and it is indicated by a pointer on a dial. B imetal thermometers are available for temperature ranges from -70 to +600 ° C Ranges as high as 1000° F are possible, however ranges around the 500° F value are more common. Dial 03-Feb-15 Mechanical Temprature Mesurement 17

18. Cross section of a Bimetallic thermometer 03-Feb-15 Mechanical Temprature Mesurement 18

19. Applications: C ommon application of bimetallic strips is in household thermostats. Its also used in circuit breakers. Advantages: Robust, easy to use and cheap . Can be used at higher temperatures. Power source not required Disadvantages: Not very accurate . Limited to applications where manual reading is acceptable . Not suitable for very low temperatures 03-Feb-15 Mechanical Temprature Mesurement 19

20. 4) Sealed Bellows: The sealed bellows type is filled with a gas, vapor or liquid. It responds to change in temperature by variation in volume and pressure causing expansion or contraction . Gas filled  bellows can make the sensor sensitive to both external pressure and temperature. 03-Feb-15 Mechanical Temprature Mesurement 20

21. Liquid Filled bellows can function in a number of transducer ap p lications. Applications: Used in fuel and air density controls. Used in p assive actuation in a vacuum or pressure chamber. Used as transducers. 03-Feb-15 Mechanical Temprature Mesurement 21

22. 5) Bulb and Capillary Sensor: Bulb and capillary elements are used where temperatures are to be measured in ducts, pipes, tanks or similar locations remote from the controller . The bulb is filled with liquid, gas or refrigerant depending on the temperature range required . Expansion of the fluid in the heated bulb exerts a pressure which is transmitted by the capillary to the diaphragm and there it is translated into movement. 03-Feb-15 Mechanical Temprature Mesurement 22

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24. The linear movement is directionally proportional to the temperature. Advantages: Typically inexpensive Easily understood wiring Disadvantages: Sensing bulb must be p erfectly horizontal, which is difficult to set. Diaphragm case must be mounted in a separate, warmer location for effective operation. 03-Feb-15 Mechanical Temprature Mesurement 24

25. Thank You 03-Feb-15 Mechanical Temprature Mesurement 25