Principles of temperature measurement
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The document discusses various temperature measurement principles, including different temperature scales such as Celsius, Fahrenheit, Kelvin, and Rankine, along with their practical applications. It also details several types of thermometers, including mercury, resistance, thermocouple, and platinum resistance thermometers, highlighting their working principles, advantages, and disadvantages. The importance of accurate temperature measurements and suitable thermometer selection for different temperature ranges and applications is emphasized.
Principles of temperature measurement
- 2. Different scales and their relations: How Temperature scales are Designed When pressure approaches zero, all real gas will behave like ideal gas, that is, pV of a mole of gas relying only on temperature. Therefore we can design a scale with pV as its argument. Of course any bi jective function will do, but for convenience's sake linear function is the best. Therefore we define it as "Thermometers and the Ideal Gas Temperature Scale“ The ideal gas scale is in some sense a "mixed" scale. It relies on the universal properties of gas, a big advance from just a particular substance.
- 3. Practical Applications Celsius scale: It is used for all scientific work everywhere, and for all ordinary temperature work everywhere but in the United States. Fahrenheit scale: Most scientific and industrial processes now use Celsius instead of the traditional Fahrenheit scale, and Fahrenheit is only used primarily in three areas : 1)Atmospheric temperatures as publicly announced. (Celsius equivalents when in written forms) 2)Baking and cooking temperatures (to avoid confusion with older publications and users) 3)Body temperatures for medical uses (again, to conform to older information sources
- 4. Practical Applications Kelvin scale: Kelvin scale starts at absolute zero, where all motion ceases and entropy is zero. All 'action' takes place above this state of inactivity. Using degrees Celsius won't work in the ideal gas law. If we give a negative temperature in Celsius. Using PV = nRT, we would get a negative value for either P, V, or n. we can't have a negative volume, pressure or number of moles. Rankine scale: A few engineering fields and scientist measure thermodynamic temperature using the Rankine scale
- 5. Different types of Thermometer: Mercury thermometer 1. The physical quantity that is used to determine the temperature of a body by means of a mercury thermometer is the length of the thread mercury, or to be more exact, the volume of mercury. 2. When the temperature increases, the volume of the mercury increases too. 3. The sensitivity of a mercury thermometer can be increased by a. reducing the diameter of the capillary tube. b. increasing the size of the bulb.
- 6. Resistance thermometer 1. Thermometers which use liquids inside the glass are not suitable to be used for measuring a wide range of temperature. e.g temperature ranging from -250 degree celcius to about 700 degree celsius. 2. A suitable thermometer which is used for the above range of temperatures is a resistance thermometer. 3. Therefore, tis thermometer is very accurate.
- 7. Thermocouple thermometer 1. An electromotive force (e.m.f) will be produced in a thermocouple when there is a temperature difference between the hot junction and the cold junction. Once this happens, a current will flow. 2. This thermometer is very sensitive and responds towards slight change in temperature. 3. A thermocouple thermometer is a very sensitive thermometer which is suitable for measuring temperatures ranging from -250 degree celsius to 1600 degree celsius.
- 8. Platinum Resistance Thermometer: Working Principle They are slowly replacing the use of thermocouples in many industrial applications below 600 °C, due to higher accuracy and repeatability. Common RTD sensing elements constructed of platinum copper or nickel have a unique, and repeatable and predictable resistance versus temperature relationship ( R vs T) and operating temperature range. The R vs T relationship is defined as the amount of resistance change of the sensor per degree of temperature change.
- 9. Construction These elements nearly always require insulated leads attached. At temperatures below about 250°C PVC, silicon rubber or PTFE insulators are used. Above this, glass fiber or ceramic are used. The measuring point and usually most of the leads, require a housing or protective sleeve, often made of a metal alloy which is chemically inert to the process being monitored. Selecting and designing protection sheaths can require more care than the actual sensor, as the sheath must withstand chemical or physical attack and provide convenient attachment points.
- 10. Applications Sensor assemblies can be categorized into two groups by how they are installed or interface with the process: immersion or surface mounted. #Immersion sensors take the form of an SS tube and some type of process connection fitting. They are installed into the process with sufficient immersion length to ensure good contact with the process medium and reduce external influences. A variation of this style includes a separate thermo well that provides additional protection for the sensor. # Surface mounted sensors are used when immersion into a process fluid is not possible due to configuration of the piping or tank, or the fluid properties may not allow an immersion style sensor.
- 11. Advantages and Disadvantages: Advantages: 1. With this thermometer we can conveniently measure the temperature over a wide range of -200c to 1200c, as in the case of gas thermometer. 2. It is extremely accurate over the whole of this range. With certain modifications, it can read temperature correctly up to 0.002c within the range -190c to600c. 3. It has a large size, because of which it can be conveniently used for measuring the average temperature of a large volume.
- 12. Disadvantages: 1. On account of the low thermal conductivity of the sheath enclosing the thermometer and also of the large thermal capacity of the thermometers wire itself, it takes a long time to acquire the temperature of the bath in which it is placed. 2. Because of its large size, it is not suitable for measuring temperature at a point. 3. It is rather a costly instrument because highly pure platinum is necessary for its construction.