Pressure measuring instruments

Classification of pressure measuring instruments based on construction and working principles

Four basic types of pressure measuring instruments are;

1. Liquid column elements:

  • Barometer
  • Manometer –u tube, enlarge leg well inclined leg

2. Elastic element gauge:


3. Electrical transducers:

  • Resistance and inductance type

4. Force –balanced devices:
  • Dead weight gauge
  • Ring gauge
  • Bell gauge
                                                                                                                                                             

BAROMETER:

Barometer is used to measure atmospheric pressure. Atmospheric pressure is the pressure exerted by the air surrounding the earth that goes on decreasing away from the earth surface.

Working principle:
Barometric liquid balances the atmospheric pressure against vacuum and pressure head reading is obtained in the absolute units.

Construction and working:

Barometer has a glass tube closed at one end and opened at the other; the length of the tube must be greater than 76.2 cm. the tube is first completely filled by mercury and open end is temporarily plugged. Then the tube is inverted so that plugged end is immersed in a mercury pan. When the plug is removed, the mercury in the tube drops b y a certain amount, creating a vacuum at the top of the tube and then reading ‘h’ is noted. The reading ‘h’ is proportional to atmospheric pressure acting on mercury in the pan. Note that this atmospheric pressure reading is in absolute units.

We have stated that vacuum is present the top of the tube above mercury, but actually there is vapor pressure of mercury acting on mercury pressure ’P’ kg/cm2 is given by P= 6.66 X 10-3h
                                                                                                                                                  

MANOMETER:

The device used to know about the pressure difference in the pipeline, it is a simple in construction, basic law of physics are applied  for calculation of the pressure drop. It is a glass or metal tube with a 'U' bend providing with two legs. Manometeric fluids as mercury or carbon tetrachloride etc., where the density should be higher than the fluid which flow through the pipe, manometeric fluid will be filled in the tube for the value, the two legs are connected to the points on which we are interested to calculate the differential pressure, when this done the fluid which flow in the pipe or tube will enter into both the legs, the pressure on the leg will differ showing the deflection of height in the manomertic fluid.
Measuring device glass tube mercury differential pressure manometer
U-Tube Manometer

Principle: all manometers work on the effect of hydrostatic pressure exerted by a liquid column. In manometer unknown pressure is determined by balancing it against some known pressure or vacuum.
Construction and working:
The U-tube manometer consists of glass U-tube partially filled with a suitable liquid like water, mercury etc. one of the arms or legs of the manometer, is connected to unknown pressure tap to be measured while other is connected to other pressure tap or it is left open to atmosphere.
When there is difference of pressure between two arms of the manometer, liquid levels in the two arms of the manometer, liquid levels in the two arms do not match. This level difference in the two arms of the manometer represents differential pressure (P1-P2). The static balance equation is
  • P2-P1=h ρ g
h=height difference
ρ=mass density of manometer liquid
If the fluid over manometer liquid has appreciable density, then static balance equation can be written as:
  • P2-P1= h (ρm - ρl) g
h= height difference
ρm = mass density of manometric liquid
ρl = mass density of fluid over manometric liquid
                                                                                                                                                            

INCLINED –LEG MANOMETER: 
The construction is very similar to enlarged leg manometer except that small diameter tube is inclined to the vertical axis.
When pressure P1 and P2 are applied then liquid rises in the tube,the level of manometer ic liquid inside the tube is measured from zero level along the inclined tube which represents the differential pressure (P1 – P2) the static balance equation can be written as
  • P2-P1= ρd sinα[1+(A1/A2)]
α=anlge of inclination of the inclined leg
d= height difference measured

Advantages:
  • Due to inclined leg, the manometer reading gets amplified. Hence it can be used for measurement of low pressures of which cannot be measured by other manometers.
  • By reducing angle α, the scale length and hence the sensitivity can be increased.
                                                                                                                                                             

MANOMETRIC LIQUIDS:

Desirable properties of good manometric liquid should have
  • Low-freezing point
  • High boiling point
  • Non wetting characteristics
  • Low surface tension
  • Chemically inert
  • Clear visible interface
  • Ability to maintain density at various temperatures
MANOMERTIC FLUIDS USED IN PRACTICE ARE:
  • Mercury: mercury has low freezing point(-38F) and high boiling point (675F) but it corrodes many metals and it is poisonous and expensive.
  • Water with colouring agents: colour agents reduce surface tension of pure water, that reduce the capillarity effect in manometer.
  • Benzene, Kerosene, CCl4,toluene etc.. to make CCl4 visible a small amount of iodine crystals can be added.

Calibration:
Manometer is subjected to known differential pressure and the corresponding height difference is noted. The calibration curve can be prepared by plotting height difference versus differential pressure. This curve can be used to get the differential pressure for certain height difference.
  • Sources of error:Temperature effect: rise in temperature causes decrease in manometric liquid density that affects the calibration which leads to an error.
  • Capillary rise: to avoide capillary rise effect, the tube diameter should be over 10mm otherwise capillary rise results in error in pressure reading.
  • Meniscus shape: For water the free surface is concave while for mercury free surface is convex. The level of manometric liquid should be noted at the center of the meniscus.
Advantages and limitations:
Advantages: 
  • Simple inexpensive construction
  • High accuracy and sensitivity
  • Can be used for low pressure measurements
  • Desired span can be obtained just by using suitable manometric liquids
  • Pressure range of manometers is 3 to 100KPa.
Limitations:
  • No over range protection
  • Requires larege space
  • Non portable
  • Leveling is required
  • Condensation of test liquid affects the reading.
                                                                                                                                                  

ELASTIC PRESSURE TRANSDUCERS
Transducers are a device that converts one form of energy into some other form. These pressure gauges have an elastic element that converts pressure signal into proportional mechanical displacement. In this article we study Bourdon gauge, bellow gauge, diaphragm gauge and capsule gauge.
                                                                                                                                                  

BOURDON PRESSURE GAUGE:
Principle: E.Bourdon introduced Bourdon tube in 1852 as a curved or twisted tube having non circular transverse section, according to Bourdon theory a tube having internal cross section that is not a perfect circle if bent or distorted has the property of changing its shape with internal pressure variation, this cause the free end deflection of the tube which can be taken as the measurement of change in pressures inside it.
C shaped, serial and helical type bourdon pressure gauges
Bourdon gauges

Construction: Bourdon pressure gauges use different types of Bourdon springs as C-shaped Bourdon tube which if formed by winding the tube to form a segment of a circle having arc- length of about 270 degrees. In spiral type, number of turns is wound in the shape of a spiral about a common axis. In helix type number of turns is wound in helix form. In these figures ‘P’ indicates direction of application of pressure, while ‘T’ indicates tip travel for rise in pressure


We study C-shaped Bourdon tube gauge as it consists of a C-shaped Bourdon tube, tip, adjustable link, segments lever, sector, pinion, spring, and pointer. A C-shaped Bourdon tube is a thin walled tube having a non circular or nearly elliptical transverse section as one end of the tube soldered or welded to a socket at the base through which pressure is fed inside the tube while the other end is sealed by a tip. Adjustable link, segment lever sector or pinion are connected to the tip, that convert linear motion of the tip into proportional rotary motion which is given to the pointer that  moves on the scale calibrated in terms of pressure. A hair spring is connected to the spindle on which sector is mounted, that provides the necessary tension for meshing sector and pinion thus eliminating any backlash.
Under range protection is particularly required for gauges having partial ranges (like 20to 50psi).
Bourdon tube material: a bourdon spring can be made of any metal or alloy that exhibits satisfactory elastic properties. Material used are- brass, phosphor bronze, monel, beryllium, copper, stainless steel etc.

  Pressure range:
C-shaped tube
0 to 1,00,000 psi
Gauge pressure
0 to 12,000psig
Absolute pressure
0 to 100 psia
Vacuum
0 to 30’’ Hg

Working:



When fluid under pressure to be measured enters the bourdon tube, its cross section tries to become more and more circular that caused straightening of the tube. Since one end of the tube is fixed straightening cause the free end to deflect that is called a tip travel. The amount of tip travel for given rise in pressure is a function of tube length wall thickness cross section geometry and elastic module of the tube material. This linear tip travel is guided and amplified by adjustable link and segment lever and then it is given to sector and pinion arrangement. Sector and pinion convert the amplified tip travel into proportional rotary motion of the pointer connected to the pinion. The pointer defection can be read on the scale calibrated in terms of pressure.
 Helical and spiral types bourdon tubes have many numbers of turns hence the tops movements for give change in pressure is more than that for single turn C- shaped tube.
Gauge pressure measurement: when unknown pressure is fed inside the bourdon tube and its out side is exposed to atmosphere, the reading would be in gauge units.
pressure transmitter used in pressure vessel with pressure sensorAbsolute pressure measurement: when unknown pressure is fed inside the tube and its outside(instrument case)is evacuated, then reading would be in absolute units.
Vacuum measurement: procedure is similar to gauge pressure measurement. Bourdon vacuum gauge have poor accuracy.
Calibration:
Bourdon gauge are calibrated using dead weight tester or by comparison calibration.

Advantages, limitations, Applications:
Advantages:
1.low cost and simple construction
2.wide pressure range
3. high accuracy in relation to low cost
Limitations:
1.Low spring gradient
2.Susceptibility to shock and vibration
3.Bordon tube material possess some hysteresis in a pressure cycle, hysteresis can be kept minimum by proper heat treatment and by using proper materials.