WORKING PRINCIPLE OF THERMOCOUPLE AND PRESSURE DETECTORS

 

THERMOCOUPLE



WORKING PRINCIPLE OF THERMOCOUPLE

  • Thermocouple is a temperature measuring device constructed of two dissimilar metal wires joined at one end.
  • When one end of each wire is connected to a measuring instrument the thermocouple becomes a sensitive and highly accurate measuring device.
  • Thermocouple may be constructed of several different combinations of materials.
  • The performance of a thermocouple material is generally determined by using that material with platinum,
  • The measuring junction is normally formed at the bottom of the thermocouple housing.
  • In a typical thermocouple, the leads are encased in a rigid metal sheath.

CONSTRUCTION OF THERMOCOUPLE

  • The most important factor to be considered when selecting a pair of materials is the thermoelectric difference between the two materials.
  • A significant difference between the two materials will result in better thermocouple performance.
  • Constantan is excellent for temperature up to 2000℉.
  • Nickel/Nickel-Molybdenum sometimes replace chromel Alumel and Tungsten-Rhenium is used for temperature up to 5000 ℉.
  • Combination of wire materials used for specialized application are:-
  • Chromel- White gold, Molybdenum-Tungsten, Tungsten- Iridium, Iridium- Rhodium.






PRESSURE DETECTORS IN POLYMER PROCESSING


  • Bellows type pressure detectors
  • Bourdon tube type pressure detectors
  • Strain gauge type detectors


BELLOWS TYPE PRESSURE DETECTORS






WORKING PRINCIPLE OF BELLOWS TYPE PRESSURE DETECTORS

  • The need for a pressure measuring element that is extremely sensitive to low pressure resulted in the development of the metallic bellows.
  • The metallic bellows are most accurate when measuring pressure from 0.5 to 75 PSIG.
  • Bellows can be used to measure pressures over 1000 PSIG, if used in conjugation with a spring.
  • Bellows is a collapsible, seamless metallic unit that has deep folds formed from thin-walled tubing.
  • The diameter of bellows ranges from 0.5 to 12 inch having as many as 24 folds.
  • As the inlet pressure to the instrument varies the bellows will expand or contract.
  • The moving end of the bellow is connected to a mechanical linkage assembly.
  • As the bellows and linkage assembly moves, a direct pressure indication is provided.
  • The flexibility of metallic bellows is similar to a helical, coiled compression spring

BOURDON  TUBE TYPE PRESSURE DETECTORS 




  • Bourdon tube consists of a thin-walled tube that is flattened to produce a cross-sectional area elliptical in shape.
  • The tube is bent lengthwise into an arc of 270-300 degrees.
  • Since the tube is permanently fastened at one end, the tip of the tube traces a curve that is the result of the change in angular position with respect to the center.

STRAIN GAUGE TYPE PRESSURE DETECTORS





  • A strain gauge measures the external force (pressure) applied to a fine wire.
  • This wire is usually arranged in the form of a grid.
  • The pressure change causes a change in resistance due to distortion of the wire.
  • The value of the pressure can be found by measuring the change in resistance of the wire grid.

Method of  Pressure detection

  • The detector senses the pressure of the system & converts it to a mechanical signal. The mechanical signal from the detector is supplied to the transducer.
  • The transducer converts this signal to a usable electrical signal which is proportional to the detected pressure to the circuit.
  • The detector circuit amplify and transmit this signal to the pressure indicator 
  • A pressure indicator provides an indication of the system pressure being measured.

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