This document discusses different types of pressure transducers and gauges. It describes Bourdon tube gauges, strain gauges, quartz gauges, and piezoresistive gauges. It also discusses U-tube manometers and how they use hydrostatic law to measure pressure differentials. Finally, it provides details on diaphragm pressure transducers, how they work, common materials used, and their applications in measuring low pressures.

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2. 2
Pressure (P ) expresses the magnitude of normal force (F-N) per unit area
(A-m2) applied on a surface(Crowe et al. 2005)
Units: Pa(= N/m2), psi(=lbf/in2), bar (=105 Pa=100 kPa), mbar (=100 Pa=1
hPa),
atm (=101.3 kPa), mmHg (or Torr), inHg, etc.
Note: For every Unit: hUnit=hectoUnit=100 Unit
Where Pabs : Absolute pressure
Patm : Atmospheric pressure
(standard is: 101.3 kPa =14.696 psi=760 mmHg=29.92 inHg)
Pgage : Gage pressure
A
F
Por
A
F
P



gageatmabs PPP 

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Bourdon Gage:
http://www.hydraulicspneumatics.com/FPE/images/sensors1_1.jpg
http://www.efunda.com/DesignStandards/sensors/bourdon_tubes/images/Bourdon_tube_A.gif
Applications: tire pressure, pressure at the top or along the walls of tanks or
vessels
http://www.cpigauges.com/images/gauges/WeldGageStlCsBM400psi.jpg
Principles: change in curvature of the tube is proportional to difference of
pressure inside from that outside the tube

4. 4
Strain Gage
Applications: Sensors for internal combustion engines, automotive, research etc.
Principles: ∆ P  ∆ Resistance  ∆ Voltage

5. 5
Quartz Gage
Applications: measurements with high accuracy, good repeatability, high resolution.
e g. Quartz Clock
Principles: ∆ Pressure  ∆ Charge  ∆ Voltage
http://www.ransohoff.com/images/systems/transducerlgr.jpg
Piezoelectric transducers
http://upload.wikimedia.org/wikipedia/commons/c/c4/SchemaPi
ezo.gif

6. 6
Piezoresistive Gage
Applications: Very accurate for small pressure differentials
e.g. Difference between indoor and outdoor pressure
Principles: ∆Pressure = ∆Charge = ∆Resistance = ∆Voltage
Digital Manometer

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U-tube Manometer
Principles: Hydrostatic Law
∆P=ρ g h
http://www.efunda.com/formulae/fluids/images/Manometer_A.gif

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U-tube Manometer
Air Water ManometerMercury Water Manometer
Applications: air pressure, pipe pressure, etc.

9. 9
Click here to connect to UT’s Interactive Fluids Applets website

10.  Like a diaphragm, bellows are also used for pressure measurement, and
can be made of cascaded capsules. The basic way of manufacturing
bellows is by fastening together many individual diaphragms. The
bellows element, basically, is a one piece expansible, collapsible and
axially flexible member. It has many convolutions or fold. It can be
manufactured form a single piece of thin metal. For industrial purposes,
the commonly used bellow elements are:
 By turning from a solid stock of metal
 By soldering or welding stamped annular rings
 Rolling a tube
 By hydraulically forming a drawn tubing
 Working
 The action of bending and tension operates the elastic members. For
proper working, the tension should be least. The design ideas given for a
diaphragm is applied to bowels as well. The manufacturer describes the
bellows with two characters – maximum stroke and maximum allowable
pressure. The force obtained can be increased by increasing the diameter.
The stroke length can be increased by increasing the folds or
convolutions.
10

11.  For selecting a specific material for an elastic member like
bellows, the parameters to be checked are:
 Range of pressure
 Hysteresis
 Fatigue on dynamic operation
 Corrosion
 Fabrication ease
 Sensitivity to fluctuating pressures
 Out of these hysteresis and sensitivity to fluctuating
pressures are the most important ones. Hysteresis can be
minimized by following a proper manufacturing technique.
For instance, a diaphragm when machined from a solid
stock shows less hysteresis compared to the one produced
by stamping. The same technique could be adopted for
bellows as well. In the latter case, the dynamic nature of the
variable is likely to induce resonance quickly depending on
the natural frequency of the system. The natural frequency
is calculable from the dimensions of the system and the
gauge.
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12.  For strong bellows, the carbon steel is selected as the main
element. But the material gets easily corroded and is
difficult to machine. For better hysteresis properties you can
use trumpet bass, phosphor bronze, or silicon bronze. Better
dynamic performance can be achieved by using beryllium
copper. Stainless steel is corrosion resistive, but does not
have good elastic properties. For easy fabrication soft
materials are sought after.
 All bellow elements are used with separate calibrating
springs. The springs can be aligned in two ways – in
compression or in expansion when in use. Both these types,
with internal compression springs or external tension
springs, are commercially known as receiver elements and
are used universally in pneumatic control loops. The figures
below show the compressed and expanded type. Spring
opposed bellows are also shown below. The open side of a
bellows element is usually rigidly held to the instrument
casing and because of the rigid fixing, the effective or active
length of the bellows element is smaller than its actual
length. This device is used in cases where the control
pressure range is between 0.2 to 1 kg/cm2.
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14.  A diaphragm pressure transducer is used for
low pressure measurement. They are
commercially available in two types – metallic
and non-metallic.
 Metallic diaphragms are known to have good
spring characteristics and non-metallic types
have no elastic characteristics. Thus, non-
metallic types are used rarely, and are usually
opposed by a calibrated coil spring or any
other elastic type gauge. The non-metallic types
are also called slack diaphragm.
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15.  Working
 The diagram of a diaphragm pressure gauge is
shown below. When a force acts against a thin
stretched diaphragm, it causes a deflection of
the diaphragm with its centre deflecting the
most.
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16.  Diaphragm Gauge
 Since the elastic limit has to be maintained, the
deflection of the diaphragm must be kept in a
restricted manner. This can be done by
cascading many diaphragm capsules as shown
in the figure below. A main capsule is designed
by joining two diaphragms at the periphery. A
pressure inlet line is provided at the central
position. When the pressure enters the capsule,
the deflection will be the sum of deflections of
all the individual capsules. As shown in figure
(3), corrugated diaphragms are also used
instead of the conventional ones.
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18.  Diaphragm Pressure Transducer
 Corrugated designs help in providing a linear deflection and also increase the
member strength. The total amount of deflection for a given pressure
differential is known by the following factors:
 Number and depth of corrugation
 Number of capsules
 Capsule diameter
 Shell thickness
 Material characteristics
 Materials used for the metal diaphragms are the same as those used
for Bourdon Tube.
 Non-metallic or slack diaphragms are used for measuring very small
pressures. The commonly used materials for making the diaphragm are
polythene, neoprene, animal membrane, silk, and synthetic materials. Due to
their non-elastic characteristics, the device will have to be opposed with
external springs for calibration and precise operation. The common range for
pressure measurement varies between 50 Pa to 0.1 MPa.
 The best example for a slack diaphragm is the draft gauge. They are used in
boilers for indication of the boiler draft. The device can control both
combustion and flue. With the draft, usually of pressure less than the
atmosphere, connected, the power diaphragm moves to the left and its motion
is transmitted through the sealing diaphragm, sealed link and pointer drive to
the pointer.
 The power diaphragm is balanced with the help of a calibrated leaf spring.
The effective length of the spring and hence the range is determined by the
range adjusting screw. By adjusting the zero adjustment screw, the right hand
end of the power diaphragm support link as also the free end of the leaf
spring, is adjusted for zero adjustment through the cradle. 18