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## LO1-UD3- PRESSION MEASUREMENT

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Auteur Author: dodopace
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UNIVERSITÁ DEGLI STUDI DI SALERNO

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. 2.7 of March 25, 2021
PRELIMINARY DEFINITIONS
ABSOLUTE PRESSURE:
Pressure referenced to the perfect vacuum.

BAROMETRIC (or ATMOSPHERIC) 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: From “Fundamentals of
Pressure measured below the barometric pressure after taking this latter Instrumentation & Process
as a reference. Control”, 2006 Control Station

absolute pressure = gauge pressure + barometric pressure

15.09.2021 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)= 101325 Pa
= 1.01325 bar
= 1 atm
= 1.03323 kg/cm2
= 14.6959 psi
= 760 mm (Hg)
= 10.3323 m (H2O)

15.09.2021 Process Instrumentation and Control - Prof M. Miccio 4
TYPES OF PRESSURE MEASUREMENTS

pressure
a) Absolute pressure Perfect
under
vacuum
measurement

b) Gage pressure Atmospheric pressure
pressure under Manometer
measurement

Variable pressure
c) Differential pressure with a variable reference under
reference pressure measurement

pressure
Constant under
d) Differential pressure with a constant reference measurement
pressure
reference

15.09.2021 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, 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.
It measures 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.
It measures the residual pressure.

Barometer is the instrument that measures only the absolute value of the
atmospheric pressure (barometric pressure). 
15.09.2021 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 an electrical property (capacity,
resistance, inductance, frequency).
Ex.: strain-gauge
15.09.2021 Process Instrumentation and Control - Prof M. Miccio 7
PRESSURE MEASUREMENT
SENSORS
1. A VARIATION IN HEIGHT OF LIQUID
2. A DEFORMATION OF AN ELASTIC ELEMENT
3. A VARIATION OF AN ELECTROMAGNETIC
QUANTITY

15.09.2021 Process Instrumentation and Control - Prof M. Miccio 8
LIQUID COLUMN MANOMETER
or “U” MANOMETER
Measurement principle:
measurement of a level consequent
to the displacement of a liquid column
STEVIN’S LAW:

Measuring ranges: 0.01 - 2 bar
10 -1500 mm Hg
10 - 2500 mm H2O
Applications: in the range of low differential pressures
almost exclusively for gases and vapors
as local indicator and not in automatic control  PI

gage P differential P absolute P
15.09.2021 Process Instrumentation and Control - Prof M. Miccio 9
PRESSURE MEASUREMENT
SENSORS
1. A VARIATION IN HEIGHT OF LIQUID
2. A DEFORMATION OF AN ELASTIC ELEMENT
3. A VARIATION OF AN ELECTROMAGNETIC
QUANTITY

15.09.2021 Process Instrumentation and Control - Prof M. Miccio 10
BOURDON PRESSURE GAUGE
and ASSEMBLING ACCESSORIES
Measurement principle
deformation of an elastic element because of the action of
forces induced by the fluid pressure

mm
in bar and in psi

cutaway drawing Connection

15.09.2021 Process Instrumentation and Control - Prof M. Miccio 11
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
15.09.2021 Process Instrumentation and Control - Prof M. Miccio 12
PRESSURE MEASUREMENT
SENSORS
1. A VARIATION IN HEIGHT OF LIQUID
2. A DEFORMATION OF AN ELASTIC ELEMENT
3. A VARIATION OF AN ELECTROMAGNETIC
QUANTITY

15.09.2021 Process Instrumentation and Control - Prof M. Miccio 13
CAPACITIVE PRESSURE TRANSDUCERS

Measurement principle:
variation of the CAPACITY C

er relative dielectric constant
e0 dielectric constant of the vacuum

BEFORE the deformation

F = (P1 - P2)* Area
with P1>P2

AFTER the deformation

15.09.2021 Process Instrumentation and Control - Prof M. Miccio 14
CAPACITIVE PRESSURE TRANSDUCERS
CAPACITIVE CELLS
with CERAMIC MEMBRANES CELL
ATMOSPHERIC PRESSURE
...

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