This ppt discusses pressure measurement techniques involved in low pressure measurement methods.

1. PRESSURE
MEASUREMENT
PART – II of III
ER. FARUK BIN POYEN, Asst. Professor
DEPT. OF AEIE, UIT, BU, BURDWAN, WB, INDIA
faruk.poyen@gmail.com

2. Contents:
1. Elastic Pressure Transducer:
 C – type Bourdon Tube
 Diaphragm Pressure Transducers
 Bellows
2. Measuring Vacuum Method:
 Capsule Gauge
 Mc Leod Gauge
 Thermal Conductivity Gauge
 Pirani Gauge
 Thermocouple gauge
 Ionization Gauge
 Knudsen Gauge
2

3. Bourdon Tube
Bourdon Tube
Elastic type Transducer
Range: 100,000 psi (700 MPa)
Cross-sectional tubing when
deformed in any way will tend to
regain its circular form under the
action of pressure.
C – type Bourdon tube: 27 ° .
Commonly used materials:
phosphor-bronze, silicon-bronze,
beryllium-copper, inconel, and
other C-Cr-Ni-Mo alloys
C – type, Helix type or spiral type.
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4. Principle:
As the fluid pressure enters the bourdon tube, it tries to be reformed and
because of a free tip available, this action causes the tip to travel in free
space and the tube unwinds. The simultaneous actions of bending and
tension due to the internal pressure make a non-linear movement of the free
tip. This travel is suitable guided and amplified for the measurement of the
internal pressure.
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5. Spiral –Low range 10 –100kpa
C type –Medium range 100 – 5000kpa
Helical – High range 5000 – 20000kpa
Advantages of Bourdon Tube
Low cost
Simple construction
Wide variety of ranges
High accuracy
Disadvantages of Bourdon Tube
Low spring gradient
Susceptible to shock and vibration
Susceptible to hysteresis
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6. Diaphragm
Low pressure measurement.
Non – metallic, metallic.
Non – metallic (slack diaphragm) has no
elastic characteristics.
Make: Polythene, neoprene, silk, synthetic
material.
Metallic has good spring characteristics.
Materials used: phosphor-bronze, silicon-
bronze, beryllium-copper, inconel, and
other C-Cr-Ni-Mo alloys.
Range: 50 Pa – 0.1 MPa
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Diaphragm Pressure Transducer

7. Diaphragm Type Transducer
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8. Working Principle:
When a force acts against a thin stretched diaphragm, it causes a deflection
of the diaphragm with its centre deflecting the most.
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.
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.
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9. Corrugated Design and amount of deflection
Depends on following FACTORS:
*Number and depth of corrugation
*Number of capsules
*Capsule diameter
*Shell thickness
*Material characteristics
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10. Advantages
 Moderate cost
 High over range character istics
 Adaptable to absolute and differential
pressure measurement
 Good linearity
 Available in materials which are good
corrosion resistive
 Small in size
 Adaptable to slurry services
Disadvantages
 Affected by vibration and shock
 Maintenance is difficult
 Limited to relatively low pressure
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11. Bellows – Same principle as Diaphragm
Made of cascaded capsules.
Multiple individual diaphragms are
fastened together.
One piece expansible, collapsible, axially
flexible.
Range: 0.2 to 1 kg/sq cm.
Carbon steel. Phosper bronze, Silicon
bronze, Beryllium copper, Trumpet brass.
Many convolutions or folds.
Thin metal into
• Turned into from solid stock of metal.
• Soldered or welded stamped annular rings.
• Rolled tube
• By hydraulically forming a drawn tubing.
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Bellow Type Pressure Gauge

12. Advantages
 Moderate cost
 Able to deliver high force
 Adaptable for absolute and
differential pressure
 Good in low to moderate
pressure range
Disadvantages
 Ambient temperature
compensation required
 Unsuitable for high pressure
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13. Measuring Vacuum Method
MEASUREMENT BELOW ATMOSPHERIC PRESSURE. 10-3 - 10-9 TORR .
MECHANICAL TYPE – MC LEAD GAUGE,
THERMAL TYPE – PIRANI GAUGE & THERMOCOUPLE,
IONIZATION TYPE – HOT CATHODE & COLD CATHODE,
RADIATION VACUUM GAUGE – ALPHATRON, QUARTZ REFERENCE

14. Mc Lead Gauge
Vacuum Gauge with same principle as
manometer.
Range: 10-4 Torr
Multiple compression technique.
𝑉
𝑑𝑝2
𝑑𝑡
= 𝐾(𝑝1 − 𝑝2)
V- Volume of the bulb
dp2 /dt – Pressure Gradient in time
between the two elements
K – Flow conductance in the capillary.
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McLeod Gauge

15. Working Principle
The gauge is used to compress a small quantity of low pressure gas to produce a
readable large pressure.
The mcleod gauge is independent of gas composition.
Bulb b of the gauge is attached to capillary aa’.
The mercury level in the gauge is lowered up to l1 by lowering the reservoir, thereby
allowing a little process fluid to enter b.
By raising the reservoir, the gas is now compressed in the capillary aa’ till mercury
rises to the zero mark in the side tube and capillary bb’. The capillary bb’ is required
to avoid any error due to capillary.

16. Pirani Gauge
Fine wire of tungsten or platinum
0.02 cm in diameter.
Temperature range: (7-400) ° Celsius
Heating Current: 10 – 100 mA.
Range: 10-3 Torr to 1 Torr.
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Pirani Gauge

17. Working Principle
When the pressure changes, there will be a change in current. For this,
the voltage V has to be kept constant.
The resistance R2 of the gauge is measured, by keeping the gauge
current constant.
The null balance of the bridge circuit is maintained by adjusting the
voltage or current.
An additional reference gauge can also be used in the adjacent arm of
another pirani gauge, in the bridge circuit.

18. Thermocouple Vacuum Gauge
Similar to pirani gauge.
Hot wire temperature measured
by thermocouple.
The sensitivity of such an
instrument depends on the
pressure and the wire current.
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Thermocouple Vacuum Gauge

19. Working Principle
Two sets of thermocouples are used to measure temperatures of heater wires
in the two chambers and oppose each other.
When there is a difference in pressures, there occurs an unbalance which is
measured by a potentiometer circuit.
Instead of a single thermocouple per wire, a thermopile is often chosen to
increase sensitivity.

20. Ionization Gauge – Hot Cathode Type
 A column of gas is introduced into which, a potential difference V is applied with free
electron in the space. This causes the electron with a charge e to acquire a kinetic energy Ve.
If the pressure range of the gas in the column goes below a certain limit, called the critical
pressure, then corresponding to a voltage larger than the critical voltage Vc, the energy Ve
may be high enough to initiate ionization, and positive ions will be produced when the
electrons collide with the gas molecules.
 The value of Vc is smallest for Cesium (3.88V) and largest for helium (24.58V), among
monoatomic gases or vapours. For diatomic gases like N2, H2 and so on, it is roughly about
15V. This is known as the ionization potential and at this potential the pressure is also
important.
 At very low pressures, during the intervals of time for transit from the cathode to the plate
in a vacuum chamber, more than one collision is unlikely for an electron. Then for a fixed
accelerating potential V>Vc, the number of positive ions formed would vary linearly with
the value of pressure. Thus, a determination of the rate of production of positive ions for a
given electron current should give a measure of the pressure.
 Range: 10-8 to 10-3 Torr. Output current varying between 10-9 and 10-4 A.
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21. External Type Hot Ionisation Gauge
Hot cathode type ionization gauge consists of a
basic vacuum triode.
The grid is at a large +ve potential wrt the
cathode and the plate.
The plate is at a -ve potential wrt the cathode.
This method is known as the external control
type ionization gauge as the +ve ion collector is
external to the electron collector grid with
reference to the cathode.
The +ve ions available between the grid and the
cathode will be drawn by the cathode, and those
between the grid and the plate will be collected
by the plate.
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External Type Ionisation Gauge

22. Internal Type Hot Ionisation Gauge
Here the grid is the positive ion collector and the plate is the electron
collector.
It consists of a helical grid with a potential of +150 volts.
This huge potential attracts the electrons and thus causes gas ionization.
At -30 volts, the gas ions are attracted to the central ion collector, thus producing an ion
current of 100 mA/Torr.
At extreme high temperature and low pressure,
High current is passed through electrodes to
stop increase of pressure.
Internal control type is a better option to measure
pressure as low as 10-9 Torr.
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Internal Type Ionisation Gauge

23. Ionization Gauge – Cold
Cathode Type
Device consists of two cathodes and a hollow
anode in between.
Input voltage greater than 2 Kilovolt is applied
between them.
A strong magnetic field is produced due to the
applied voltage and thus the electrons are ejected.
At pressures below 10-2 Torr, the mean free path of
the gas is so large that a collision may not occur at
all so that discharge is not sustained or ionization
may not be initiated.
Collimating magnetic field increases the path
length for the electrons, enabling discharges
possible at pressures down to about 10-5 Torr.
Non – linear.
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Ionization Gauge – Cold Cathode

24. Alphatron Vacuum Gauge:
radiation gauge
Cold Cathode Ionzation Gauge.
Uses alpha particle to ionize the gas.
Number of ions directly proportional to gas
pressure.
Range: 103 to 10-3 Torr.
Current flow: 10-13 and 10-9 Amperes
Ions produced by the alpha particles are
collected by the collector electrode
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Alphatron Vacuum Gauge

25. Quartz Reference Vacuum Gauge
Principle similar to Bourdon tube.
2 bourdon tubes are used and a
formed into a helix.
When a pressure difference between
the two occurs, the setup begins to
rotate.
This rotational deflection is picked up
using an optical circuit .
Quartz is that it has good spring
characteristics.
Range: 1 milliTorr for 100 milliTorr .
Gets eroded by fluorine content
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Reference Quartz Vacuum Gauge

26.  Chapter 12: Pressure Measurement, “Industrial Instrumentation and Control” by S K
Singh. Tata McGraw Hill, 3rd Edition. 2009, New Delhi. ISBN-13: 978-0-07-026222-
5.
 Chapter 10: Pressure Measurement, “Instrumentation, Measurement and Analysis”.
2nd Edition, B C Nakra, K K Chaudhry, Tata McGraw-Hill, New Delhi, 2005. ISBN:
0-07-048296-9.
 Chapter 6: Pressure Sensors, “Fundamentals of Industrial Instrumentation”, 1st
Edition, Alok Barua, Wiley India Pvt. Ltd. New Delhi, 2011. ISBN: 978-81-265-
2882-0.
 Chapter 3: Pressure Measurement, “Principles of Industrial Instrumentation”, 2nd
Edition. D. Patranabis, Tata McGaw-Hill, New Delhi, 2004. ISBN: 0-07-462334-6.
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