Pressure measurement id Mitesh Kumar

1. Pressure
measurementMITESH KUMAR
Rall no-10300513026
Applied Electronics & Instrumentation Engg.
Haldia Institute Of Technology

2. Density
D=M⁄V
So for greater density mass will more in the
substance in unit volume. And greater density
will give more hydrostatic(head) pressure than
lower density.
Density always define in given temperature
because density of a liquid decreases as
temperature increases.
This temperature is here reference temp.

3. Specific gravity
• Some time density can define in terms of Specific
gravity.
• Specific gravity is the ratio of the density of a
particular liquid to the density of water at a
reference temperature.
problem:
Water has a density of 1,000 kg/m3 at 50 °F. The
density of gasoline is 660 kg/m3 at 50 °F.
calculate Specific gravity of gasoline?

4. • Ullage Pressure is pressure that is exerted on the surface of a
liquid.
• In an open tank, atmospheric pressure (the pressure exerted by
the Earth’s atmosphere) is the pressure on the surface.
• A gauge pressure measurement is sufficient for level
measurement.
• In a closed tank, it is common practice to fill the vapor space with
buffer gas. This is done in order to protect the products inside the
tank or to prevent them from evaporating into atmosphere.
• This buffer gas will exert a pressure on the column of liquid that
must be subtracted from the measurement of the height of the
liquid column otherwise error will occur.
• Differential pressure measurement, where the low side reference
leg is connected to the vapor space will allow the head pressure to
be subtracted out.
Ullage Pressure

5. Factors for gas
• Factors for gas’s exert pressure
⇨ Gas container volume
⇨ Gas temperature.
There is ideal gas law relating it’s pressure
temperature and volume.
PV= nRT

6. Relations between
gas pressure and volume
Boyel’s law : It is stated that that the pressure and volume
of a gas have an inverse relationship, when temperature is
held constant.
P1V1= P2V2
A fixed amount of gas is transferred to a larger container,
the pressure will decrease in proportion to the increase in
container volume.
So we can conclude that gas can be compressed, the
pressure of a gas increases proportionately as the volume
of the container in which it is held decreases.

7. Need of pressure measurement
• There are four common need
⇨ Safety
⇨ Process efficiency
⇨ Cost savings
⇨ Measurement of other process variables

8. Safety
• Pipes, tanks, valves, and other equipment used with pressurized
fluids in process industries are designed to withstand the stress of a
specific range of pressures.
• Accurate pressure measurement and precise control help prevent
pipes and vessels from bursting.
• In addition, pressure measurement and control help minimize
equipment damage, reduce the risk of personal injury, and prevent
leaks of potentially harmful process materials into the environment.
• Pressure measurement used to control the level and flow of
process materials helps to prevent backups, spills, and overflows.
• By monitoring the pressure in the process, actions can be taken to
prevent (or minimize) an environmental release or personal injury/
exposure.

9. Cost saving
• The equipment used to create pressure or vacuum in
process industries (e.g., pumps and compressors) uses
considerable energy.
• Because energy costs money, a precise pressure
measurement can save money by preventing the
unnecessary expense of creating more pressure or
vacuum than is required to produce required result.

10. Process efficiency
Most common example making paper from paper bulb.
The piece of paper on which these words are written
was created from a pulp solution gone through a paper
machine at a specific pressure , If the pressure had
gone above or below the set point (required range),
the result would have been scrap instead of a usable
sheet of paper.
So, The efficiency of a process is directly related to the
quality of the product being produced.

11. Measurement of other variables
Pressure transmitters are frequently used to measure..
⇨ Temperature measurement
⇨ Level of fluid in a tank
⇨ Flow rate measurement through a pipe
⇨ Density of a substance
⇨ Liquid interface measurement

12. Types of pressure measuring sensors
• Mechanical type Instruments
⇨ Manometers- Comparing the unknown to
known pressure.
⇨ Metal diaphragm or bellows or capsules
⇨ Bourdon tubes.
• Electro-mechanical type Instruments
• Vacuum sensors.

13. U-Tube Manometer

14. Explanation
• Let the density of the fluid whose pressure being measured be ρf
and that of the manometer liquid be ρm.
• Equilibrium of the manometer liquid requires that there be the
same force in the two limbs across the plane AA.
• We can write : P+ ρfgh= Pa + ρmgh
• Finally ,
P-Pa =(ρm- ρf )gh
for manometer liquid most commonly used liquid is mercury(Hg)
and basically used for high pressure measurement
2nd common liquid is water and basically used for low pressure.

15. Contd..
• When measuring pressures close to the atmospheric
pressure in gases, the fluid density may be quite
negligible in comparison
with the manometer liquid density.
• Rewriting : P – Pa≈ ρm gh
for mercury manometer :
ρm=13600kg/m3
ρf=1kg/m3

16. Contd..
Advantages :
• Manometers include simple and time proven construction.
• High accuracy.
• Good repeatability.
• Wide range of filling fluids and use as primary standard or as
working device.
Disadvantages :
• Include lack of portability.
• Need of leveling, the hazardous condition existing when
mercury is used as the filling fluid and exposed to the atmosphere
and the reading error due to the meniscus on small diameter tubes.

17. Well type Manometer
• Sometimes a well type manometer is used and it operates in
the same manner as the U-tube manometer, except that
construction.
• In the well type manometer, one of the legs of the U-tube is
substituted by a large well such that the variation in the level
in the well will be negligible and instead of measuring a
differential height, a single height in the remaining column is
measured.
• The advantage of the well type design is that relatively large
pressure differences may be measured with enough
manometer liquid being available for doing so.

18. Contd..

19. Well type inclined manometer
In case the measured pressure difference is small one may use
an inclined well type manometer

20. Summary
Advantages :
• Very simple
• No calibration required.
Disadvantages :
• slow response – for fluctuating pressures not useful, useful only for
slow varying pressures.
• For the "U" tube manometer two measurements must be taken
simultaneously to get the h value. This may be avoided by using a tube
with a much larger cross-sectional area on one side of the manometer
than the other.
• For very accurate work the temperature and relationship between
temperature and density of the manometric liquid must be known.

21. Elastic type pressure sensors
Elastic members are also used for measurement of
pressure up to 700 MPa.
⇨ Bourdon tube or pressure spring.
⇨ Bellows elements
⇨ Diaphragms.
Bellows and Diaphragms can be useable up to 3-6
Mpa but bourdon tubes for very high range.

22. Bourdon tube
• Bourdon tube pressure gages are extensively used for
local indication and signal transmission to remote
location.
• There are three types of bourbon elements
⇨ c-Type
⇨ spiral
⇨ Helical

23. C-type
• Bourdon gages are purely mechanical devices utilizing the
mechanical deformation of a flattened but bent tube.
• The motion is against a spring torque such that a needle
attached to the shaft indicates directly the pressure
difference.

24. C- Type working
The Bourdon tube is a metal tube of
elliptic cross section having a bent
shape.
• The inside of the tube is exposed to the
pressure to be measured. The outside of
the Bourdon tube is exposed to a second
pressure, usually the atmospheric.
• The Bourdon tube is held fixed at one
end (the end connected to the pressure
source) and the other end is connected
by linkages to a spring restrained shaft.
• A pointer is mounted on the shaft. The
needle moves over a circular scale that
indicates the pressure.
The position of the needle is determined
by a balance between the Bourdon tube
developed torque acting on the shaft and
the torque due to the shaft mounted
spring that opposes its movement.

25. Note
• The commercial Bourdon pressure gauges have near elliptical
cross section and tube generally bend into a C- shape or arc
length of about 27 degrees.
• The materials used are commonly Phosphor Bronze, Brass and
Beryllium Copper

26. Parts of Bourdon tube

27. Contd..

28. Mounting position

29. sensitivity

30. Spiral type
The spiral bourdon element is used when the free-end movement
of the C-type is not great enough to provide the needed motion.
• Since greater movement of the free end is attained with the
spiral element, it is not necessary, in most cases, for mechanical
amplification, so better accuracy is obtained.
• Spiral tubes are made by winding the tube with its flattened
cross section in a spiral form of several turns.
• As pressure is applied to the spiral, it tends to uncoil, producing
the relatively long movement of the tip end whose motion can be
used for indication or transmission.

31. Contd..

32. Helical type
The helical bourdon element is similar to the spiral element,
except it is wound in the form of a helix.
• It increases the tip travel considerably, producing even greater
amplification than the spiral element.
• Usually a central shaft is installed within the helical element,
and the pointer is driven from this shaft is installed within the
helical element, and the pointer is driven from this shaft by
connecting links.
• This system transmits only the circular motion of the tip to the
pointer and hence, is directly proportional to the changes in the
pressure

33. Contd..

34. Need for various type
• The types are varied for specific uses and space
accomodations,for better linearity and larger
sensitivity.
• How ?
The displacement of the tip varies inversely as the
wall thickness and depends upon cross-sectional
form of the tube. But importantly length of the
arc.
Therefore displacement of the of the tip may be
increased by increasing the length of the arc of
the tube without changing the wall thickness.

35. Factors need to consider
• Nonlinearity
• Hysteresis
• Static error
• Elevation error

36. Non linearity
• Because of the compound stresses developed in the tube, actual
tip travel is nonlinear in nature.
• However for a small travel of this can be considered to be linear and
parallel to the axis of the link.
• The small linear tip movement is matched with a rotational pointer
movement.
• This is known as multiplication, which can be adjusted by adjusting
the length of the lever. A shorter lever gives larger rotation for the
same amount of tip travel.
• The approximately linear motion of the tip when converted to a
circular motion with a link-lever and pinion attachment, a one-to one
correspondence between them may not occur and a distortion
results. This is known as angularity which can be minimized by
adjusting the length of the link

37. Hysteresis , Static error and Elevation
error
•Hysteresis:
• Like all elastic elements Bourdon tube element also has some
hysteresis in a given pressure cycle. By proper choice of
material and its heat treatment, this may be kept low.
• Static error:
• It generally appears because of the change in elasticity due to
change in temperature. With temperature rising, elasticity
decreases and hence, indication would increase.
• Materials like Ni-span C, which consists of highest amount of Ni.
Lowest amount of carbon is virtually free from this change and
hence is suitable for low static error.
• Elevation error occurs if the tubing connecting the pressure
measuring element and the pressure point is partially filled

38. Advantages and Disadvantages
• Simple construction
• Very low cost
• Improved design at high pressure for maximum
safety
⇨ Disadvantages :
• They are susceptible to shock and vibration due
to their large overhang.
• They are subjected to hysteresis

39. Dead Wight tester

40. working
• A Bourdon pressure gage may be calibrated by the use of a dead
weight tester, a schematic of which is shown in Figure.
• The dead weight tester consists of an arrangement by which a
piston may be allowed to float over a liquid (usually oil) under
internal pressure and a force in the opposite direction imposed
on the piston by weights placed as indicated in the figure.
• The oil pressure is changed by the pumping piston. The pressure
is calculated as the weight placed on the piston divided by the
cross section area of the piston (the piston is to be oriented
with its axis vertical).
• The gage under test experiences the same pressure by being
connected to a side tube communicating with the oil.

41. limitations
• The fiction between the cylinder and the
priston.
• The uncertainty in the area of the priston

42. Typical dead weight tester supplied by
WIKA, Australia