This article describes the most commonly used pressure measurement methods and equipment in the process industry.

1. Electrical & Electronic Measurement
Part – VIII
Pressure Measurement
ER. FARUK BIN POYEN
faruk.poyen@gmail.com
AEIE, UIT, BU

2. Contents:
 Definition
 Measurement Methods
 Manometer Method
 Bourdon Tube
 Diaphragm
 McLeod Gauge
 Pirani Gauge
 Ionization Gauge
 Dead Weight Piston Gauge
 Capacitive Pressure Gauge
 Piezoelectric Pressure Gauge
2

3. Pressure - Definition:
Defined as the amount of force applied to a surface or distributed over it and is measured as
force per unit area. (P = F/A)
Units of Pressure: The basic unit of pressure in SI units is the Pascal (Pa).
It is defined as force of 1 Newton (N) per square meter (m2). That is: 1 Pa = 1 N/m2.
High Pressure Unit
1 N/m2 = 1 Pa;
1 atm = 14.696 psi = 101.325 kPa
Low Pressure Unit
1 millibar = 100 dyne/cm2 = 14.5 * 10-3 psi
1 micron = 10-6 Hg = 19.34 * 10-6 psi
1 torr = 1 mm Hg = 19.34 * 10-3 psi
3

4. Relationship between Units
 Relationship between Units
4
Pressure units
 V
 T
 E
Pascal Bar
Technical
atmosphere
Standard
atmosphere
Torr
Pounds per
square inch
(Pa) (bar) (at) (atm) (Torr) (psi)
1 Pa ≡ 1 N/m2
10−5
1.0197×10−5
9.8692×10−6
7.5006×10−3
1.450377×10−4
1
bar
105
≡ 100 kPa
≡ 106
dyn/cm2
1.0197 0.98692 750.06 14.50377
1 at 0.980665×105
0.980665 ≡ 1 kp/cm2
0.9678411 735.5592 14.22334
1
atm
1.01325×105
1.01325 1.0332 1 ≡ 760 14.69595
1
Torr
133.3224 1.333224×10−3
1.359551×10−3
1.315789×10−3
≡ 1/760 atm
≈ 1 mm Hg
1.933678×10−2
1 psi 6.8948×103
6.8948×10−2
7.03069×10−2
6.8046×10−2
51.71493 ≡ 1 lbF /in2

5. Pressure Measurement Methods
 Pressure Measurement Methods
1. Manometer Method
2. Elastic Pressure Transducer
3. Measuring Vacuum Method
4. Force Balancing Method
5. Electrical Pressure Transducer
5

6. Pressure Measurement Methods
Elastic Pressure Transducer:
1. C – type Bourdon Tube
2. Diaphragm Pressure Transducers
3. Bellows
6

7. Pressure Measurement Methods
 Measuring Vacuum Method:
1. Capsule Gauge
2. Mc Leod Gauge
3. Thermal Conductivity Gauge
1. Pirani Gauge
2. Thermocouple gauge
4. Ionization Gauge
5. Knudsen Gauge
7

8. Pressure Measurement Methods
 Force Balancing Pressure Gauge
1. Dead Weight Piston Gauge
2. Ring Balance Gauge
3. Bell Type Pressure Gauge
8

9. Pressure Measurement Methods
 Electrical Pressure Transducers
1. Strain Gauge Pressure Transducer
2. Potentiometric Pressure Transducer
3. Capacitive Pressure Transducer
4. Reluctance Pressure Transducer
1. Linear Variable Differential Transformer
2. Servo Pressure Transducer
3. Piezoelectric Pressure Transducer
9

10. Manometer Method
 Types
1. U-Tube
2. Well Type
3. Barometer
4. Incline
5. Micro
6. Ring Balance
 Range: 0.2 MPa or 2 kg/cm2
 𝑃1 − 𝑃2 = 𝜌 − 𝜌1 ℎ1 − ℎ2 𝑔 = 𝜌 − 𝜌1 ℎ𝑔
10

11. Bourdon Tube
 Elastic type Transducer
 Cross-sectional tubing when deformed in any way will tend to regain its circular form
under the action of pressure.
 Commonly used materials: phosphor-bronze, silicon-bronze, beryllium-copper, inconel,
and other C-Cr-Ni-Mo alloys
 Range: 100,000 psi (700 MPa)
 C – type Bourdon tube: 27 ° .
 C – type, Helix type or spiral type.
11

12. Bourdon Tube
 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.
 Types of Bourdon Tube
1. Spiral –Low range 10 –100 Kpa
2. C type –Medium range 100 – 5000 Kpa
3. Helical – High range 5000 – 20000 Kpa
12

13. Bourdon Tube – Pros & Cons
 Advantages of Bourdon Tube
1. Low cost
2. Simple construction
3. Wide variety of ranges
4. High accuracy
 Disadvantages of Bourdon Tube
1. Low spring gradient
2. Susceptible to shock and vibration
3. Susceptible to hysteresis
13

14. Diaphragm
 Low pressure measurement.
 Materials used: phosphor-bronze, silicon-bronze, beryllium-copper, inconel, and other
C-Cr-Ni-Mo alloys.
 Non – metallic (slack diaphragm) has no elastic characteristics.
 Make: Polythene, neoprene, silk, synthetic material.
 Non – metallic, metallic.
 Metallic has good spring characteristics.
 Range: 50 Pa – 0.1 MPa
14

15. Diaphragm
 Depends on following FACTORS:
1. Number and depth of corrugation
2. Number of capsules
3. Capsule diameter
4. Shell thickness
5. Material characteristics
15

16. Diaphragm - 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.
16

17. Mc Leod 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.
17

18. Mc Leod Gauge
 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.
18

19. Pirani Gauge
 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.
19

20. 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.
20

21. 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.
21

22. Ionization Gauge – Hot Cathode Type
 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: Vacuum Range: 10-8 to 10-3 Torr.
 Output current varying between 10-9 and 10-4 A.
22

23. Ionization Gauge – Hot Cathode Type
Schematic of Hot Cathode Tube
 Pressure of Gas is proportional to
 𝑃 =
1
𝑆
∗
𝐼 𝑃
𝐼 𝐺
• 𝐼 𝑃 = Plate Current
• 𝐼 𝑃 = Grid Current
• S = Sensitivity of gauge
23
P.C: Supervacoil

24. External Type Hot Ionisation Gauge
 Hot cathode type ionization gauge consists of a basic vacuum triode.
 The grid is at a large +ve potential with respect to the cathode and the plate.
 The plate is at a -ve potential with respect to 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.
24

25. 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.
 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.
 Works at extreme high temperature and low pressure.
25

26. 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.
26

27. Ionization Gauge – Cold Cathode Type
 Collimating magnetic field increases the path length for the electrons, enabling
discharges possible at pressures down to about 10-5 Torr.
 Non – linear.
27

28. Dead Weight Piston Gauge
 It is a force balancing pressure gauge.
 Pressure easily converted to force with introduction of surface area.
 It is used in calibration purposes for bellows and diaphragms.
 It is used in higher steady pressure measurement.
 It is a continuous balancing system.
 The units of measurement are force and area.
 It is mostly linear.
 Accuracy < 0.1 %
 Range up to 300 psig.
28

29. Dead Weight Piston Gauge - Principle
 It consists of a very accurately machined, bored and finished piston which is inserted
into a close-fitting cylinder.
 The area of cross section of both the piston and cylinder are known.
 A platform is provided at the top of the piston where standard and accurate weights are
placed.
 An oil reservoir with check valve is provided at the bottom.
 The oil can be sucked by displacement pumps on its upward stroke.
29

30. Dead Weight Piston Gauge - Principle
 For calibration, a known weight is first placed on the platform and fluid pressure is
applied on the other end of the piston until enough force is developed to lift the piston
weight combination and the piston floats freely within the cylinder between limit stops.
30

31. Capacitive Pressure Transducer
 Consists of a parallel plate capacitors coupled with a diaphragm, usually metal and
exposed to the process pressure on one side and the reference pressure on the other side.
 Electrodes are attached to the diaphragm and are charged by a high frequency oscillator.
 The electrodes sense any movement of the diaphragm and this changes the capacitance.
 The change of the capacitance is detected by an attached circuit which then outputs a
voltage according to the pressure change.
 This type of sensor can be operated in the range of 2.5 Pa - 70MPa with a sensitivity of
0.07 MPa.
31

32. Capacitive Pressure Transducer
 The principle of operation of capacitive pressure transducers is based upon the
familiar capacitance equation of the parallel plate capacitor
𝐶 =
∈0∈ 𝑟 𝐴
𝑑
C = capacitance in Farad; A= area of each plate (m2); d= distance between plates
(m); ɛ0 = 8.854 * 10-12 Farad/m2; ɛr = dielectric constant
 The capacitance of the parallel plate varies inversely with the distance between
them.
 With increase in pressure, the distance d (of the diaphragm) becomes less and
therefore C is increased and vice versa.
 Hence the bridge is unbalanced and a current flows which gives a measure of
the change of pressure.
32

33. Capacitive Pressure Transducer
 Advantages:
1. Gives out rapid response to changes in pressure
2. It can withstand a lot of vibration and shock
3. It is extremely sensitive
4. Offers a good frequency response
 Disadvantages:
1. Metallic parts need insulation from each other
2. Affected by dirt and other contaminants
3. Temperature causes error.
33

34. Piezo Electric Pressure Transducer
 Piezoelectric characteristics of certain crystalline materials are used.
 Electricity is generated when pressure is applied.
 Some of these materials are Barium Titanate Sintered powder, quartz, tourmaline,
Rochelle salt.
 The main advantages of these crystals are that they have high mechanical and thermal
state capability, capability of withstanding high order of strain, low leakage, and good
frequency response.
 Each crystal has three sets of axes – Optical axes, three electrical axes OX1, OX2, and
OX3 with 120 degree with each other, and three mechanical axes OY1, OY2 and OY3
also at 120 degree with each other.
 The mechanical axes will be at right angles to the electrical axes.
34

35. Piezo Electric Pressure Transducer
 Some of the parameters that decide the nature of the crystal for the application are
1. Angle at which the wafer is cut from natural quartz crystal
2. Plate thickness
3. Dimension of the plate
4. Means of mounting
35

36. Piezo Electric Pressure Transducer
 Advantages of Piezo electric Pressure Transducer
1. Very high frequency response.
2. Self-generating, so no need of external source.
3. Simple to use as they have small dimensions and large measuring range.
4. It has a large dielectric constant. The crystal axis is selectable by orienting the
direction of orientation.
 Disadvantages of Piezo electric Pressure Transducer
1. It is not suitable for measurement in static condition.
2. Since the device operates with the small electric charge, they need high impedance
cable for electrical interface.
3. The output may vary according to the temperature variation of the crystal.
4. The relative humidity rises above 85% or falls below 35%, its output will be
affected. If so, it has to be coated with wax or polymer material.
36

37. Comparison of Methods
 Comparison of Methods
37