1. Pressure measurements
Bachelor Degree in Chemical Engineering
Course: Process Instrumentation and Control
(Strumentazione e Controllo dei Processi Chimici)
Measuring devices of the main process variables
Pressure measurements
Rev. 2.4 – April 3, 2019

2. PRELIMINARY DEFINITIONS
ABSOLUTE PRESSURE:
Pressure referenced to the perfect vacuum.
BAROMETRIC PRESSURE:
Pressure exerted from the atmosphere on the Earth’s surface (it changes with the altitude and the weather conditions).
DIFFERENTIAL PRESSURE:
Pressure difference between two different points.
GAUGE (GAGE in USA) PRESSURE:
Difference between the absolute and the barometric pressure.
RESIDUAL PRESSURE:
Absolute pressure that is referenced to the perfect vacuum and is below the barometric pressure.
VACUUM:
Pressure measured below the barometric pressure after taking this latter as a reference.
From “Fundamentals of Instrumentation & Process Control”, 2006 Control Station
absolute pressure = gauge pressure + barometric pressure
Process Instrumentation and Control - Prof M. Miccio

3. STANDARD ATMOSPHERIC PRESSURE
The standard atmospheric pressure, referred to as P atm (n), corresponds to the value the barometric pressure exerted by the atmosphere
at the sea level
at 0 ºC
with average weather conditions.
It is defined as being equal to:
P atm (n)= Pa
= bar
= atm
= kg/cm2
= psi
= mm (Hg)
= m (H2O)
Process Instrumentation and Control - Prof M. Miccio

4. TYPES OF PRESSURE MEASUREMENTS
Perfect vacuum
pressure under measurement
Atmospheric pressure
Variable reference pressure
Constant reference pressure
Absolute pressure
Gage pressure
Differential pressure with a variable reference
Differential pressure with a constant reference
Manometer
Process Instrumentation and Control - Prof M. Miccio

5. DEFINITIONS Pressure sensors
Quite often, all pressure sensors have been improperly called manometers even though it is actually necessary to distinguish between pressure sensors:
Manometer is the instrument which measures positive gage pressures and with pressures zero-referenced to the barometric pressure.
Deprimometer is the instrument that measures absolute pressures lower than the barometric pressure and has zero at barometric pressure. Measure the vacuum.
Vacuum gauge is the instrument that measures the absolute pressures between the absolute vacuum and the barometric pressure, the vacuum meter has therefore the zero of the scale that corresponds to the absolute vacuum. Measure the residual pressure.
Barometer is the instrument that measures only the absolute value of the atmospheric pressure (barometric pressure). 
Process Instrumentation and Control - Prof M. Miccio

6. CLASSIFICATION Pressure Sensors
In the most common sensors, the pressure measurement can be performed by:
1. a variation of the liquid level;
2. a deformation of an elastic element;
3. a change of an electromagnetic quantity.
1. In the first category there are instruments based on the measurement of the level of a liquid column:
Ex.: “U” manometers
2. In the second category, there are instruments based in the deformation of an elastic element because of the action of pressure forces. They are generally based on macro-deformations. e.g.:
Bourdon pressure gauges;
Diaphragm manometers
Bellows manometers
3. In the third category there are the transducers in which a variation in pressure, which may involve a micro-deformation of an elastic element (diaphragm, wire, etc.) that produces an output signal generated by the change of a electrical property (capacity, resistance, inductance, frequency). Ex.: strain-gauge 
Process Instrumentation and Control - Prof M. Miccio

7. Process Instrumentation and Control - Prof M. Miccio
PRESSURE MEASUREMENT SENSORS
1. a variation in height of liquid 2. a deformation of an elastic element 3. a variation of an electromagnetic quantity
Process Instrumentation and Control - Prof M. Miccio

8. LIQUID COLUMN MANOMETER or “U” MANOMETER
STEVIN’S LAW
Instruments based on the measurement of a level of a liquid column
Measuring ranges: mm Hg
bar
mm H2O
NOTE: they are used:
in the range of low differential pressures
Almost exclusively for gases and vapors
as local indicator and not in automatic control 
WaterColumn.swf PI
Process Instrumentation and Control - Prof M. Miccio

9. Process Instrumentation and Control - Prof M. Miccio
PRESSURE MEASUREMENT SENSORS
1. a variation in height of liquid 2. a deformation of an elastic element 3. a variation of an electromagnetic quantity
Process Instrumentation and Control - Prof M. Miccio

10. BOURDON PRESSURE GAUGE
it consists of a closed-end elastic tube with an elliptic section, shaped as an arc, with a fixed terminal;
the free terminal changes its position because of the deformation stress exerted by the pressure of internal fluid;
the measurement of the terminal displacement provides the pressure measure.
NOTE:
They are used as local indicators in Piping & Instrumentation Diagram  PI
Process Instrumentation and Control - Prof M. Miccio

11. BOURDON PRESSURE GAUGE and ASSEMBLING ACCESSORIES
Standard manometer with quadrant Ø 40 mm
Quadrant with double graduation:
in psi and kg/cm2
Manometer with quadrant Ø 50 mm
in bar and in psi
Connection
Central threaded male 1/8" BSP
Process Instrumentation and Control - Prof M. Miccio

12. Process Instrumentation and Control - Prof M. Miccio
PRESSURE MEASUREMENT SENSORS
1. a variation in height of liquid 2. a deformation of an elastic element 3. a variation of an electromagnetic quantity
Process Instrumentation and Control - Prof M. Miccio

13. CAPACITIVE PRESSURE TRANSDUCERS
BEFORE the deformation
F = (P1 - P2)* Area
with P1>P2
AFTER the deformation
Measurement principle:
change of the CAPACITY C
er relative dielectric constant
e0 dielectric constant of the vacuum
Process Instrumentation and Control - Prof M. Miccio

14. CAPACITIVE PRESSURE TRANSDUCERS
CAPACITIVE CELLS
with CERAMIC MEMBRANES
CELL
with METALLIC DIAPHRAGM
ATMOSPHERIC PRESSURE
VACUUM
pressione
PRESSURE
PRESSURE
PRESSURE
CELL with DOUPLE COMPENSATOR for DIFFERENTIAL PRESSURE
NOTE:
The dielectric fluid can move from a side to the other one. In this way, it automatically compensates the change of the dielectric fluid density.
Process Instrumentation and Control - Prof M. Miccio

15. TRANSDUCTION OF A CAPACITY VARIATION IN AN ELECTRIC SIGNAL
Transduction deformation - capacity
An a.c. Wheatstone bridge is used as rectifier in order to produce a direct signal.
The sensitivity is redoubled if there are two capacities and they have opposite variations, inserting them on two different legs of the bridge.
Process Instrumentation and Control - Prof M. Miccio

16. ELECTRICAL RESISTANCE-BASED PRESSURE TRANDUCERS
Strain-gauge
It operates on the variation of an electrical resistance due to the change of the length and the cross sectional diameter (2°Ohm’s law) when a conductor is submitted to a tensile stress.
thin (few microns) metallic plate;
very high resistance (hundreds ohms) which changes with the deformation;
extension 0.2-5%
bonded with insulating material
NOTE:
- Cheaper sensors
PROTECTION
Substrate
Wires
Termination
Strain gauge
Adhesive
Tratto da Dr. Chi-fu Wu, ME 4903-Special Problem in ME, GTREP, Savannah (USA), Oct. 21, 2004
Piezoelectric transducers
They can be alternative to the strain-gauges.
In this case the electrical resistance and the voltage across the sensible element, which is a crystal of a certain solid material (silicon or another semiconductor), changes directly because of the deformation.
Process Instrumentation and Control - Prof M. Miccio

17. TRANSDUCTION OF A RESISTANCE VARIATION IN AN ELECTRIC SIGNAL
Transduction deformation - resistance
(with Wheatstone bridge)
4 extensometers
(2 for tensile and 2 for compression)
on 4 legs of a bridge
Valim E + e -e - +e
 ADVANTAGES
Accuracy 0.1% on measured value
Rangeability (75 100):1
Measuring range: “The pressure range is 0 – bar” [see the University of Michigan Chemical Engineering Process Dynamics and Controls Open Textbook at ]
Process Instrumentation and Control - Prof M. Miccio

18. INDUCTIVE DIFFERENTIAL PRESSURE TRANSDUCER
Measure principle:
Variation of the INDUCTANCE L P1 P2
Transduction deformation - inductance
The deformation of the metallic diaphragm produces a translation of two units of ferrite respect to 2 fixed coils placed on the two sides of diaphragm.
The deformation occurring as a consequence of the pressure difference is transduced in a inductance variation (of opposed direction in the 2 coils).
This is further transduced in an electric signal by an a.c. Wheatstone bridge with rectifier or
including inductances as tuning elements of oscillators (LC circuits).
Process Instrumentation and Control - Prof M. Miccio

19. RESONATORS ON SILICON  ADVANTAGES Measurement principle:
Variation of the RESONANT FREQUENCY
double diapason of a “resonant” silicon mono-crystal immersed in a permanent magnetic field
resonant frequency depends on the transverse deformation produced by the measuring pressure
MEMS = Micro ElectroMechanical Systems
 ADVANTAGES
No attrition
No AC/DC converter
Better performances and greater stability with time (little calibration requirement)
Process Instrumentation and Control - Prof M. Miccio