This document discusses various types of pressure measurement. It defines pressure and units like pascals and atmospheres. Static pressure is exerted by stationary fluids while dynamic pressure results from moving fluids. Absolute pressure is measured against a vacuum and gauge pressure against atmospheric pressure. Hydrostatic pressure increases with depth in liquids. Common pressure measurement instruments include manometers, elastic elements like bourdon tubes, and electrical resistance gauges. Low pressures are measured using McLeod, Pirani, and ionization gauges. Selection depends on the pressure range, accuracy needed, and other factors like cost and maintenance.

1. Pressure Measurement
• Submitted by: Preeti choudhary
• M.Sc.(Applied Physics)

2. Introduction
• Pressure is define as force divided by area.
(exerts on solid, gas, liquid)
• The unit of pressure is in Pa, psi, atm, bar, torr
P = F/A
F = force in newton/ lb
A = area in m2
/ in2
1 psi = 6895 pa
1 bar = 105 Pa
1 atm = 101325 Pa
1 torr = 133.3 Pa

3. Static and Dynamic Pressure
• Static pressure is the pressure of fluid or gases that
are stationary or not in motion.
• Dynamic pressure is the pressure exerted by a fluid
or gas when it impacts on a surface or an object due
to its motion or flow.

4. Pressure Measurement
• Absolute pressure is the
pressure measured wrt a
vacuum (unit = psia)
• Gauge pressure is the
pressure measured wrt
atmospheric pressure (unit =
psig)
• Atmospheric pressure is
the pressure on the
earth’s surface due to the
weight of gases in the
earth’s atmosphere (14.7psi)
Zero Pressure
Pressure due to
Atmosphere
Gauge
Pressure at
point of interest
Absolute
Pressure
at point of
interest

5. Hydrostatic Pressure
• Hydrostatic pressure is the pressure in a liquid.
• The pressure increases as the depth in a liquid increases, due to its
weight.
• In term of equation, P = ρgh
ρ = density in kg/m3
g = acceleration due to gravity
(9.8m/s2
)
h = depth in liquid in m
P = pressure in Pa
P = ρwh
ρw = weight density in lb/ft3
h = depth in liquid in ft
P = pressure in lb/ft2

6. Pressure Measuring
Instrument
• The techniques for pressure measurement is depending on
pressure level. (moderate, very high, very low)
• Very high pressure level is higher than 1000 atm
• Very low pressure level is below than 133 Pa (1 torr)
1. Moderate Pressure Measurement
- Manometer
- Elastic elements (diaphragm, bellows, capsules, bourdon
tubes, spiral, helix)

7. Pressure Measuring
Instrument
2. High Pressure Measurement
- Electrical Resistance pressure gauge
3. Low Pressure Measurement
- McLeod gauge
- Pirani gauge
- Ionisation gauge

8. Manometer
• Manometer is the simplest device for measuring static
pressure.
• It contains water/ mercury or any other suitable fluid in the
manometer tube.
• When a pressure line is connected to one column of
manometer, the fluid in the column will be forced down,
and the fluid in the other will rise.
• By measuring the difference in height of the fluid in the two
columns, the pressure of the inlet can be expressed in
inches of fluid.

9. Types of Manometer
U-tube Manometer Well-type Manometer
Incline-tube Manometer

10. Elastic Elements
• Elastic elements, when subjected to pressure, get deformed.
• Measurement of the deformation, gives an indication of
pressure value.
• The deformation may be measured by mechanical or
electrical means.
• Example of elastic elements are; diaphragms, capsules,
bellows, Bourdon or helical tubes.

11. Elastic Elements
Flat diaphragm Corrugated diaphragm Capsule
C-shape
Bourdon tubeStraight tubesBellows
Spiral Bourdon
tubeTwisted Bourdon tube Helical Bourdon tube

12. Electrical Resistance
Pressure Gauge
• The concept of operation is based on electrical resistance change in a
conductor when applied directly to a pressure.
• The sensing element consist of a loosely wound coil of relatively fine wire,
and it will be compressed when high pressure applied on it.
• The length and cross section of the wire affect its electrical resistance,
when pressure applied at a rate determined by the bulk of modulus
material.

13. McLeod Gauge
• It compresses the low pressure gas so that the increased pressure can be
measured.
• The change in volume and pressure can then be used to calculate the
original gas pressure, providing that the gas not condensed.

14. Pirani Gauge
• It consist of platinum filament and thermocouple enclosed in a chamber.
• The pressure measurement is based on the relation of heat conduction and
radiation from a heating element to the number of gas molecules per unit
volume in the low pressure region.

15. Ionization Gauge
• It can be used to measure pressure down to about 2 torr.
• The gas is ionized with a beam of electrons and the current is
measured between two electrodes in the gas.
• The current is proportional to the number of ions per unit
volume, which also proportional to the gas pressure.

16. How to select?
1. You must understand the pressure range, accuracy and the
response time of the instrument.
2. Parameter such as hysterisis and stability can be obtained
from manufacturer’s specification.
3. Other aspect such as cost and maintenance also should be
considered.
4. The liquid in the tube must not interact with the pressure
gas. (contamination, evaporates)

17. Thank-you