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How do you work?
• Pressure is defined as force exerted on a unit
• Mathematically, we have: P = F/A
where P=Pressure(Pa), F=Force(N) and A=Area(m2)
• Basic unit of pressure in SI units is Pascal (Pa).
– Pascal is defined as force of 1 Newton (N) per square
meter (m2). That is: 1 Pa = 1 N/m2
• Since the Pascal (Pa) is a very small unit (1
Pa = 1.45 x 10-4 PSI)
– it is more common to use units of kPa and MPa,
when we deal with the pressures in typical industrial
Absolute, Gauge and Differential
Pressures • Pressure measurements can be stated as either
– absolute or differential.
• Gauge pressure is the unit we encounter in everyday work (e.g. tire ratings are in gauge pressure).
• A gauge pressure device will indicate zero pressure when bled down to atmospheric pressure (i.e. gauge pressure is in reference to atmospheric pressure – that pressure above atmospheric pressure).
• Gauge pressure is denoted by a (g) at the end of the pressure unit (e.g. kPa(g)).
• Absolute pressure includes the effect of atmospheric pressure with the gauge pressure. It is denoted by an (a) at the end of the pressure unit (e.g. kPa(a).)
An absolute pressure indicator would indicate
atmospheric pressure when completely bled down -
it would not indicate scale zero.
The relationship between absolute pressure and
gauge pressure is:
Absolute Pressure = Gauge Pressure + Atmospheric
• The standard value of atmospheric pressure is
taken as the atmospheric pressure at sea level,
which is 101.3 kPa.
• Note a reading of less than 101.3 kPa(a) indicates
a vacuum condition.
– For example, a typical condenser pressure is 5 kPa(a) or a
vacuum of 96.3 kPa.
• Differential Pressure is the difference of two pressures.
• In order to produce a standard (4 - 20 mA) electronic
signal which represents the pressure in a process,
the pressure must be sensed and a physically
detectable motion or force in proportion to this
applied pressure must be developed.
• To sense the process, we use a pressure sensor.
Pressure Measurement Devices
• Some common sensors include:
– Bourdon Tubes
When a force acts
against a thin
it causes a deflection
of the diaphragm with
its centre deflecting
• If it is fixed at the
air inlet, it can
expand like a
• It expands in both
ways whereas in
expands in one
DIFFERENTIAL PRESSURE DEVICES
Differential Pressure Devices • Used to measure Differential Pressure (that
is, the difference between a high pressure input and a low pressure input) and hence called DP transmitters or DP cells.
• A differential pressure capsule is mounted inside a housing.
• One end of a force bar is connected to the capsule assembly so that the motion of the capsule can be transmitted out of the housing.
• A sealing mechanism is used where the force bar penetrates the housing.
• This seal also acts as the pivot point for the force bar.
A Typical DP Cell
• Provision is made in the housing for high
pressure fluid to be applied on one side of the
capsule and low pressure fluid on the other.
• Any difference in pressure will cause the capsule
to deflect and create motion for the force bar.
• The top end of the force bar is connected to an
electronic motion detector, which via an
electronic system, will produce a 4 - 20 mA
signal that is proportional to the force bar
DP Capsule Construction • The DP capsule is formed by welding two metallic
(usually stainless steel) diaphragms together.
• To provide over-pressurization protection, a solid plate
with diaphragm matching convolutions is mounted in the
center of the capsule.
• Silicone oil is used to fill the cavity between the
diaphragm for even pressure transmission. Most DP
capsules can withstand static pressure of up to 14 Mpa
(2000 psi) on both sides of the capsule without any
• However, the sensitive range for most DP capsules is quite
low. Typically they are sensitive up to only a few hundred
kPa of differential pressure.
DP Transmitter • A DP transmitter is used to measure the gas
pressure (in gauge scale) inside a vessel.
• In this case, the low pressure side of the
transmitter is vented to atmosphere, and the
high pressure side is connected to the
vessel through an isolating valve.
• The isolating valve facilitates the removal of the
• The output of the DP transmitter is
proportional to the gauge pressure of the gas
in the tank, i.e., 4 mA when pressure is 20 kPa
and 20 mA when pressure is 30 kPa.
• The strain gauge is a device that can be affixed to
the surface of an object to detect the force applied
to the object.
• One form of the strain gauge is a metal wire of
very small diameter that is attached to the surface
of a device being monitored.
• For a metal, the electrical resistance will increase
as the length of the metal increases or as the
cross sectional diameter decreases.
• When force is applied as indicated in Figure, the
overall length of the wire tends to increase while
the cross-sectional area decreases.
• The amount of increase in resistance is
proportional to the force that produced the change
in length and area. The output of the strain gauge
is a change in resistance that can be measured
by the input circuit of an amplifier.
• Strain gauges can be bonded to the surface of a
pressure capsule or to a force bar positioned by
the measuring element.
Strain Gauge DP Cell Shown in figure is a
strain gauge that is
bonded to a force
beam inside the DP
The change in the
will cause a resistive
change in the strain
gauges, which is
then used to
produce a 4-20 mA