Module -1.pdf
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This document provides an introduction to process instrumentation and discusses different types of filled system thermometers. It describes the construction, operating principle, and classification of filled system thermometers. The four main classes described are liquid filled, vapour filled, gas filled, and mercury filled thermometers. The document also discusses sources of error in filled system thermometers and compares their advantages and disadvantages to other temperature measurement instruments.
Module -1.pdf
- 1. PROCESS INSTRUMENTATION (CH255) INTRODUCTION Dr Sourav Poddar Department of Chemical Engineering National Institute of Technology Warangal, TS
- 33. GF = gauage factor = (∆R/R)/ɛ ɛ = strain = ∆l/l Eb =battery voltage
- 34. Hint:
- 73. 𝑆𝑡𝑎𝑡𝑖𝑐 𝑆𝑒𝑛𝑠𝑖𝑡𝑖𝑣𝑖𝑡𝑦 = 𝐶ℎ𝑎𝑛𝑔𝑒 𝑜𝑓 𝑂𝑢𝑡𝑝𝑢𝑡 𝑆𝑖𝑔𝑛𝑎𝑙 𝐶ℎ𝑎𝑛𝑔𝑒 𝑜𝑓 𝐼𝑛𝑝𝑢𝑡 𝑆𝑖𝑔𝑛𝑎𝑙 = ∆𝑞𝑜 ∆𝑞𝑖
- 99. Suppose a reading of 27.5 mV results from the sensor used in the previous problem. Find the temperature that could provide this reading.
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- 116. Basic concepts of response of order instruments
- 174. Bio metallic strip
- 181. INTRODUCTION Filled system thermometer are temperature measuring device which works on pressure or volume change of a gas or changes in vapour pressure of a liquid.
- 183. CONSTRUCTION • The bulb contains a fluid that is sensitive to temperature changes. • The capillary tube connects the bulb to the element that is sensitive to pressure or volume changes. • The pressure-sensitive or volume-sensitive element may be Bourdon tube, a helix, a diaphragm or bellows. • The motion of the temperature or volume-sensitive element couples mechanically to the indicating, recording, or controlling device.
- 184. OPERATING PRINCIPLE • Almost all liquids, gases or vapors expands when heated & contracts when cooled. This phenomenon is utilized to measure temperature by filled system thermometer. • Bulb contains liquid, gases or vapour whose pressure or volume changes in response to a change in temperature. • This variation in pressure is transmitted through capillary tube to the bourdon tube. • The movement of bourdon tube is used to drive a pointer by linkages for indicating temperature.
- 185. CLASSIFICATION • As per the classification made by Scientific Apparatus Makers’ Association (SAMA) of the USA, filled system thermometers are divided into four classes as shown below
- 186. Class - I - LIQUID FILLED • These types of thermometers are completely filled with a liquid other than Mercury & operates on the principle of volumetric expansion of liquid with an increase in temperature. • The filling fluid are usually inert hydrocarbon liquids such as Xylene, Toluene, Alcohol etc. which has coefficient of expansion six times of Mercury so that system has high sensitivity. • Range: -87 to 371oC • Accuracy : + 0.5 of full range • Advantages: (i) Wide temperature span, (ii) Smaller bulb size and (iii) Lower cost • Disadvantages: (i) Compensation of errors is difficult and (ii) It has a shortest capillary.
- 187. Class - II - VAPOUR FILLED • Range: -184 oC below ambient temperature & Above ambient temperature to 343 oC • Accuracy: 1% of differential range • Speed of response : 4 - 5 seconds except when passing through ambient • Advantages: (i) Ambient temperature effect is negligible. (ii) Good response against temperature. (iii) Long capillary length available. • Disadvantages: (i) Nonlinear scale (ii) Difficult to measure ambient temperature.
- 188. Class - III - GAS FILLED • These thermometers operates on the principle of ideal gas law which states that volume of a gas increases with increase in temperature if pressure is maintained constant or vice versa i.e. pv = RT • These thermometers are filled with gases such as N2, H2, He which makes system more sensitive to temperature changes. • As N2 is chemically inert & inexpensive and also has good coefficient of thermal expansion, it is used mostly. • Range: - 268 oC to 760 oC • Accuracy: ±0.5% upto 320oC & ±1% above this range • Speed of response : 4 - 7 seconds • Advantages: (i) Greatest working range of temperature., (ii) No head or elevation effect. • Disadvantages: (i) Larger bulb size, (ii) Generates less deflection force for controlling device.
- 189. Class - IV - MERCURY FILLED • Mercury is a liquid & in this respect a mercury filled thermometer is similar to liquid filled thermometer. • These two are separated due to the unique characteristic of Mercury & its importance as a temperature measuring medium. • Range: -40 to 649oC • Advantages over liquid filled thermometer: (i) Rapid response (ii) High accuracy (iii) Large power for operating elements. (iv) At high temperature, pressure is also high which reduces head or elevation effect error. (v) As mercury is incompressible, ambient temperature effect error is also very less. (vi) Least difficult to compensate.
- 190. SOURCES OF ERRORS • The common sources of errors in the filled system thermometers are : 1) Ambient temperature effect : The capillary tube & bourdon tube are exposed to surroundings & subjected to a variable temperature widely different from that of the bulb, which results as an error in temperature measurement. 2) Head or elevation effect : If the long capillary tube is used & thermometer bulb is placed at a different height with respect to the bourdon tube, then due to elevation difference between bulb & bourdon tube deflecting end, pressure head is created which results as an error in temperature measurement.
- 191. 3) Barometric effect : The tip travel of a bourdon tube is due to pressure difference between atmosphere & filled system pressure. So if any variation in atmospheric pressure from designed pressure, the bourdon tube tip travels & causes an error in temperature measurement. 4) Immersion effect : If the bulb is not properly immersed or head of bulb is not properly insulated, then heat from the bulb may be lost due to conduction from bulb & hence temperature at the bulb may be reduced, so that, it indicates lower temperature than the actual. 5) Radiation effect : This error occurs due to temperature difference between the bulb & other solid bodies around it.
- 192. ADVANTAGES Advantages of filled system thermometer : 1. Simple & low cost. 2. Quite rugged construction & less chances of damage in handling. 3. Self operated, no power requires & generate sufficient power for controller mechanism. 4. Good response, accuracy & sensitivity 5. Vapor actuated thermometer are most widely used as they are less costly & simple. It has good speed of response. 6. It can be used for remote indication.
- 193. DISADVANTAGES Disadvantages of filled system thermometer : 1. Lower accuracy, sensitivity & temperature span compared to electrical temperature instruments. 2. Compensation is required in case of variation in ambient temperature. 3. The transient response is primary dependent on the bulb size & thermal properties of the filled fluid. 4. To get good accuracy, requires large size of bulb results in poor response. 5. In case of damage, entire system has to be replaced. 6. As filled system thermometers don’t produce electrical signals, it is difficult to implement them in process controls that rely on electrical and computerized control.
- 201. Comparison of RTD and Thermistors Characteristic RTD Thermistors Change in resistance with temperature Positive, Resistance increase with increase in temperature Negative, Resistance decreases with increase in temperature Temperature resistance relationship Approximately linear Non-linear Operating range -160°C to 600°C -100°C to 300°C Composition Platinum, Copper, Nickel Metallic oxides like Manganese, Nickel, Copper, Iron, Titanium, Uranium Sensitivity Lower Higher Cost High Low
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