Measurement of Pressure Without Primary sensing elements is Possible?

Table of Contents

Primary sensing elements are the primary instrument of pressure sensor. They change its state and indicate pressure at calibrated scale. Primary sensing elements are as follows:

Manometer

Manometer are widely used in process application  in industry . They are primary standard  for pressure measurement. It is used to measure Low vacuum range to approximately 1000  mm of Hg . A wide variety of fluid are used for manometers, their specific gravity range well below from 1.0 to 13.5707 the sp. gr. of Hg. These are  simple time proven construction, high accuracy good repeatability  wide range of fluid. It is used as a primary standard working device. There are general three types of manometers. They are ‘U’ tube manometers, Well Type  manometers and inclined  Manometer.

manometer type

Bourdon Tube

Most common pressure measuring Instrument working based on elastic deformation. Design  of the bourdon mainly in three shape The ‘C’ , the spiral and the helical .They are widely used for local indications and for signal transmission to remote locations also adopted to transducer designs for obtaining electrical output. Bourdon tubes are made of the materials phosphor bronze,  beryllium copper, 4130 alloy steel. 316 &403 ss, K-monel, monel and Ni-span C. The choice of material depend upon  the process medium, pressure range, temp. corrosiveness etc.  the operating range from  30 inches Hg to 100000 psi or higher or special applications

bourdon tube

Bellows and Capsules :-

Pressure measuring elements  are used to measure absolute, gauge or differential  pressure, some time used  in pressure transmitters  (  as the bellow expands due to pressure in turn exert force on lower end of force of pneumatic/ electronic transmitter . The reduction of drift and hysterics allow their use in functions requiring 0.5%  full span accuracy .. They extend  to medium pressure ranges  0-400 psi of DP and 6000 psi in static  pressure range.

bellows

Diaphragm

The operating principle is similar to that of bellows. The applied pressure causing it to  expand it directly in  proportion to the applied pressure. Diaphragm is  a flexible disc with flat or corrugated surface used in measuring absolute pressure.  vacuum gauge pressure. and  similar to the bellows these are used in electrical & pneumatic transducers.  Their pressure ranges extended to 10000 psi with temp. 800 deg. F & also higher range of special applications. The accuracy + – 0.5%  of full span up to pr.15000 psi.   Material of construction includes phosphore-bronze, baryllium-copper, SS Inconel,  Hasteloy  Nickel and also in non metal  Bunna N rubber, nylon Teflon etc non metallic diaphragm used below 1500 psi. Accuracy range  from 0.5% 25% of span.

Diaphragm

Electronic Transducer / Transmitters ;-

Most  electronic pressure sensors incorporate with one of the primary pressure detector used to vary a measurable electrical quantity to produce a proportionately variable electronic signal. Among the electronic  device used to measure the pressure are Strain gauge, Load Cell, inductive, capacitive, piezoelectric transducer linear variable differential transformer.  The electronic detector are more accurate, much  faster response time .Though these are costing   high  but   reduced movement very nearly eliminates drift, frictions  and hysteresis as  compared to  bellow, diaphragm &  bourdon instrument

Strain gauge:

Strain  is define as a deformation or change in shape of a material as a consequence of applied force. A strain  gauge is a device which uses  the change  of  electrical resistance of a  wire  under  strain to measure pressure ie. mechanical  motion into electrical  signal. generally  strain gauge inst. accuracy is +/- 0.1% of full scale span & range few inches H2O  to 20000 psig Accuracy +/-0.5% of full span.


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