A.SZUDER

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MECHANICAL MEASUREMENTS
Prof. Dr. Ing. Andrei Szuder Tel.
40.2.1.4112604 Fax. [phone] www.labsmn.pub.
ro szuder_at_labsmn.pub.ro
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PRESSURE MEASUREMENT
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Pressure measurement

• Pressure driven equipment (IC engines, turbines,
  etc.)
• Pneumatic or Hydraulic mechanical elements
• Biomedical applications (Blood Pressure,
  Barometric Chambers)
• Losses in pipes and ducts energy efficiency
• Atmospheric conditions (weather forecast,
  altitude)
• Indirect measurement of flow rate or velocity
• Scuba diving
• Many, many more ...

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PRESSURE MEASUREMENT

• Absolut, Differential
• Barometer
• Manometer

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Absolute pressure
Pabs 0
Presiune referinta
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Barometer
0
P0
A
h
h
Patm
Patm A
Well-type manometer
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Differentialpressure
P1
P2
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Types of pressures
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Static And Dynamic Pressure
Dynamic pressure Stagnation pressure (A) -
Static pressure (B)
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Static And Dynamic Pressure
Dynamic pressure Stagnation pressure (A) -
Static pressure (B)
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Types of pressure transducers

• Liquid Column manometers
• Elastic tubes, diaphragms, membranes (equipped
  with displacement or strain sensors)
• Semiconductor elements (with implanted stress
  elements)
• Piezoelectic elements (directly convert crystal
  lattice stress into voltage)

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Liquid Column Manometers
P2
P1
P1 A
A
h
h
U tube manometer
P2 A
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Liquid Column Manometers
P1
P1 A
A
P2
h
h
P2 A
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InclinedManometer
P1
P1 A
hr
P2
h
g
A
P2 A
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Pressure transducers
Patm
P2
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Pressure transducers

• Elastic elements
• Changing pressure change the shape of the elastic
  element
• Shape changing is detected by a resistive or
  position transducer

Tub
Tip C
Spirala
Tub rasucit
Elicoidal
Tuburi Bourdon
P Diferentiala
Plata
evacuat
P Absoluta
Capsula
Ondulata
Diafragme
Diferential sau absolut
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Pressure transducers

• Elastic elements
• Changing pressure change the shape of the elastic
  element
• Shape changing is detected by a resistive or
  position transducer

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Pressure Sensor range
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Elastic Type Manometers
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More Elastic types...
Two dummy gagesmounted elsewhere
Why do we not put 4 active gages?
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Dial-type Manometer
Dial-type Manometer as a mini measurement system
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Diaphragm type manometers
To be able to detect pressure, we need to detect
the diaphragm deflection
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Strain gauges used with Diaphragm
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Strain gage based pressure cell

• When a strain gage, is used to measure the
  deflection of an elastic diphragme or Bourdon
  tube it becomes a comonent in apressure
  transducer
• Strain-gage transducers are used for narrow-span
  pressure and for differential pressure
  measurements.
• Essentially, the strain gage is used to measure
  the displacement of an elastic diaphragm due to a
  difference in pressure across the diaphragme
• If the low pressure side is a sealed vacuum
  reference, the transmitter will act as an
  absolute pressure transmitter.
• Strain gage transducers are availablefor pressure
  ranges as low as 1300 MPa

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Capacitance based pressure cell

• Capacitance pressure transducerswere originally
  developed for use in low vacuum research. This
  capacitance change results from the movement of a
  diaphragm element
• (The diaphragm is usually metal or metal-coated
  quartz and is exposed to the process pressure on
  one side and to the reference pressure on the
  other. Depending on the type
• Differential pressure transducers in a variety of
  ranges and outputs of pressure, the capacitive
  transducer can be either an absolute, gauge, or
  differential pressure transducer.
• Capacitance pressure transducers have a wide
  rangeability, from high vacuums in the micron
  range to 70 MPa.

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• The potentiometric pressure sensor provides a
  simple method for obtaining an electronic output
  from a mechanical pressure gauge.
• The device consists of a precision potentiometer,
  whose wiper arm is mechanically linked to a
  Bourdon or bellows element.
• This type of transducer can be used for low
  differential pressure applications as well as to
  detect absolute and gauge pressures.

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The resonant wire pressure transducer

• The resonant-wire pressure transducer
• was introduced in the late 1970s.
• a wire is gripped by a static member at one end,
  and by the sensing diaphragm at the other. An
  oscillator circuit causes the wire to oscillate
  at its resonant frequency.
• A change in process pressure changes the wire
  tension, which in turn changes the resonant
  frequency of the wire. A digital counter circuit
  detects the shift. Because this change in
  frequency can be detected quite precisely,
• This type of transducer can be used for low
  differential pressure applications as well as to
  detect absolute and gauge pressures.
• Resonant wire transducers can detect absolute
  pressures from 10 mm Hg, differential pressures
  and gauge pressures up to 42 MPa. Typical
  accuracy is 0.1 of calibrated span, with
  six-month drift of 0.1

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Piezoelectric sensors

• Piezoresistive pressure sensors are sensitive to
  changes in temperature and must be temperature
  compensated.
• Piezoresistive pressure sensors can be used from
  about 21 KPa
• to 100 MPa.
• Resonant piezoelectric pressure sensors measure
  the variation in
• resonant frequency of quartz crystals under an
  applied force. The
• sensor can consist of a suspended beam that
  oscillates while isolated from all other forces.
  The beam is maintained in oscillation at its
  resonant frequency. Changes in the applied force
  result in resonant frequency changes. The
  relationship between the applied pressure P and
  the oscillation frequency is
• P A(1-TO/T) - B(1-TO/T2)
• where TO is the period of oscillation when the
  applied pressure is zero, T is the period of
  oscillation when the applied pressure is P, and A
  and B are calibration constants for the
  transducer.
• These transducers can be used for absolute
  pressure measurements with spans from 0-100 kPa
  to 0-6 MPa or for differential pressure
  measurements with spans from 0-40 kPa to 0-275
  kPa .

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Magnetic pressure transducers

• These included the use of inductance, reluctance,
  and eddy currents. Inductance is that property of
  an electric circuit that expresses the amount of
  electromotive force (emf) induced by a given rate
  of change of current flow in the circuit.
• Reluctance is resistance to magnetic flow, the
  opposition offered by magnetic substance to
  magnetic flux.
• In these sensors, a change in pressure produces a
  movement, which in turn changes the inductance or
  reluctance of an electric circuit.

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Optical pressure transducers

• Optical pressure transducers detect the effects
  of minute motions due to changes in process
  pressure and generate a corresponding electronic
  output signal.
• A light emitting diode (LED) is used as the light
  source, and a vane blocks some of the light as it
  is moved by the diaphragm. As the process
  pressure moves the vane between the source diode
  and the measuring diode, the amount of infrared
  light received changes.
• Optical pressure transducers do not require much
  maintenance.
• They have excellent stability and are designed
  for long-duration measurements.
• They are available with ranges from 35 kPa to 413
  MPa and with 0.1 full scale accuracy.

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Sensor/Cavity System Response (Helmholz Resonator)
The fundamental natural frequency of the
tube/cavity system may be expressed as
where C is the sound velocity, L and a are the
length and area of the connecting tube and V is
the cavity volume. In this second order system
air acts as mass, the pressure acts as a spring
and the connecting tube as a damping element.
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Bourdon tube over pressure protection

• Most pressure instruments are provided with
  overpressure protection of 50 to 200 of range
  These protectors satisfy the majority of
  applications. Where higher overpressures are
  expected and their nature is temporary (pressure
  spikes of short durationseconds or less),
  snubbers can be installed.
• If excessive overpressure is expected to be of
  longer duration, one can protect the sensor by
  installing a pressure relief valve. However, this
  will result in a loss of measurement when the
  relief valve is open.

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Mechanical High pressure sensors

• In the case of the button repeater ( figA), the
  diaphragm can detect extruder pressures up to
  10,000 psig and can operate at temperatures up to
  4300C because of its selfcooling design. It
  operates on direct force balance between the
  process pressure (P1) acting on the sensing
  diaphragm and the pressure of the output air
  signal (P2) acting on the balancing diaphragm.
  The pressure of the output air signal follows the
  process pressure in inverse ratio to the areas of
  the two diaphragms. If the diaphragm area ratio
  is 2001, a 1,000-psig increase in process
  pressure will raise the air output signal by 5
  psig.
• Another mechanical high pressure sensor uses a
  helical Bourdon element (Figure B). This device
  may include as many as twenty coils and can
  measure pressures well in excess of 10,000 psig.
  The standard element material is heavy-duty
  stainless steel, and the measurement error is
  around 1 of span. Helical Bourdon tube sensors
  provide high overrange protection and are
  suitable for fluctuating pressure service, but
  must be protected from plugging. An improvement
  on the design shown in Figure B detects tip
  motion optically, without requiring any
  mechanical linkage.

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Vacuum mesurement

• Vacuum gauges in use today fall into three main
  categories
• mechanical,
• thermal,
• ionization.

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Vacuum mesurement
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Semiconductor-type Sensors
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Static Calibration
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Pressure transducers
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Pressure transducers
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Pressure transducers
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Pressure transducers
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Pressure transducers
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Pressure transducers
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Pressure servo-transducer
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Piezoelectric pressure transducer
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Preso-sensitive switch
B