What are the different methods of temperature measurement?
The different methods of temperature measurements are
1. Mechanical
2. Electrical.
Mechanical Methods of Temperature measurements
1. Mercury in glass thermometer: - This consist of a glass tube of very fine bore joined to a reservoir at the bottom and sealed at the top. A measured quantity of mercury is enclosed. When the temperature is heated the mercury is expanded much more than the glass and is there for forced to rise up in the tubing. A scale is fixed at the side.
2. Bimetallic thermometer: - Two metals how the coefficient of linear expansion is is ferent are welded and rolled to gather to the desired thickness. The actual movement of a bimetal is its flexibility. With one end fix, a straight bimetallic string deflects in proportional to its temperature.
Bimetallic thermometer are made for precision laboratory work as well as for a verity of industrial purpose.
Advantages of Bimetallic Thermometer
1. Their cost is low.
2. They are tough and cannot easily be broken.
3. They are easily installed and maintained.
4. They have a good accuracy relative to cost.
5. They have a fairly wide temperature range.
Disadvantages
1. They are limited to local mounting.
2. Only indicating type is available.
3. There is always a possibility of calibration change due to rough hand line.
4. Their accuracy is not as high as glass thermometer.
3. Pressure spring thermometer: - There are four classes of pressure spring thermometer.
Class 1. Liquid filed thermometer Class 2. Vapor filed thermometer
Class 3. Gas filed thermometer Class 4. Mercury filed thermometer
Table of characteristics of classes of a tube system
| |||
Filling Liquid
|
Low Limit
|
High Limit
| |
Class 1
|
Liquid other than mercury
|
-300°F
|
600°F
|
Class 2
|
Vapor
|
-300°F
|
600°F
|
Class 3
|
Gas
|
-450°F
|
1000°F
|
Class 4
|
Mercury
|
-48°F
|
1000°F
|
Advantages of Filled System Thermometer
1. They required low maintenance.
2. In a filled system thermometer there is no need for electric power.
3. The point of a display can be located at a considerable distance from the point of measurement.
4. They possess satisfactory time response sensitivity and accuracy for the most industrial application.
5. There cost is low.
6. They deliver enough power to drive not only a pointer or recording pen but also a controller mechanism.
7. Three or more separate systems can be put in a single instrument case.
8. The capillary allows considerable separation between the point of temperature indication.
Disadvantages
The disadvantages of filled system thermometers are
1. They need a large bulb for the sake of accuracy.
2. The entire system usually has to replace in case of failure.
3. The accuracy, sensitivity and temperature SPAN is much lower compared to electrical temperature instrument.
4. Maximum SPAN is not as narrow as in the bimetallic thermometer and electrical system.
5. A thermal system being under pressure cannot be broken without destroying the calibration.
6. Separation of the measuring element and bourdon of more than 75 meters generally is not recommended in these thermometers. It is frequently more economical to employ transmitters for signal transmission beyond 30 meters.
TEMPERATURE SCALE
The common Fahrenheit temperature scale is based on the freezing and boiling points of water, 32°F & 212°F. The centigrade system is also based on the freezing and boiling points of water but has 0° to 100°.
We shall also develop the relationship between the Fahrenheit and centigrade system, the more usual commercial and industrial scale. °F scale has 180 units between the boiling point and freezing point of water while the centigrade scale has 100 units to measure the same temperature range. There for 180 F units equal 100 C units or 1°F = °C. Also 1°C = °F.
Because the f scale starts at 32° instated of 0°, we have to subtract 32 from each reading before multiplying by the factor of .
Thus 150 °F is converted to the centigrade scale as follows.
150 – 32 = 118 °F
Or
|
When converting from °F to °C the number will decrease to slightly more then half of its original value.
To convert from C to F scale the procedure is reversed. For example to change 65.5 °C to °F. We write (65.5) + 32
= 117.9 + 32
= 149.9°F
When converting from °C to °F the number will be almost double in value. This will provide a quick approximate check.
Kelvin
Freezing point 273.15 °K
Boiling point 373.15 °K
The Kelvin scale like the centigrade scale, is also divided into 100 equal divisions. The centigrade can be converted into Kelvin by using the equation °K = °C + 273.15
On the Rankin scale like the Fahrenheit scale is also divided into 180 equal division. On the Rankin scale, the freezing point is 491.7 °R and the steam or boiling point is 671.7 °R. Temperature in F can be converted into Rankin by using the equation.
°R = °F + 459.69
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