Pressure
Measurement
Muhajir Ab. Rahim
School of Mechatronic Engineering
Universiti Malaysia Perlis
Introduction
• Pressure is define as force divided by
area. (exerts on solid, gas, liquid)
P = F/A
F = force in newton/ lb
A = area in m2/ in2
• The unit of pressure is in Pa, psi, atm, bar, torr
1 psi = 6895 pa
1 bar = 105 Pa
1 atm = 101325 Pa
1 torr = 133.3 Pa
QUIZ
Convert 2 atm to psi. Given 1 psi = 6897 Pa
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.
Pressure Measurement
• Absolute pressure is the
pressure measured wrt
a vacuum (unit = psia)
• Gauge pressure is the
pressure measured wrt
atmospheric pressure
Zero Pressure
Pressure due to
Atmosphere
(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)
Gauge
Pressure at
point of interest
Absolute
Pressure
at point of
interest
Exercise
Find the absolute pressure, if a pressure gauge
reads 8.3psi, while the atm pressure is 14.7psi.
Pabs = Pat + Pg
= 14.7 + 8.3
= 23 psi
Hydrostatic Pressure
• Hydrostatic pressure is the pressure in a liquid.
• The pressure increases as the depth in a liquid increases, due to its
weight.
ρ = density in kg/m3
• In term of equation, P = ρgh
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
Exercise
If a pool has a water
with a depth of 6 ft.
Find the pressure at
the bottom of the pool
in Pa and psi. (assume
density = 103kg/m3)
Pressure Measuring
Instrument
•
•
•
1.
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)
Moderate Pressure Measurement
- Manometer
- Elastic elements (diaphragm, bellows, capsules,
bourdon tubes, spiral, helix)
Pressure Measuring
Instrument
2.
High Pressure Measurement
- Electrical Resistance pressure gauge
3.
Low Pressure Measurement
- McLeod gauge
- Pirani gauge
- Ionisation gauge
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.
Types of Manometer
U-tube Manometer
Well-type Manometer
Incline-tube Manometer
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.
Elastic Elements
Flat diaphragm
Bellows
Twisted Bourdon tube
Corrugated diaphragm
Capsule
Straight tubes
C-shape
Bourdon tube
Helical Bourdon tube
Spiral Bourdon
tube
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.
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.
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.
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.
How to select?
1.
2.
3.
4.
You must understand the pressure range, accuracy
and the response time of the instrument.
Parameter such as hysterisis and stability can be
obtained from manufacturer’s specification.
Other aspect such as cost and maintenance also
should be considered.
The liquid in the tube must not interact with the
pressure gas. (contamination, evaporates)