Chapter 8: Pressure Measurement

1
Chapter 8 Pressure Measurement

• Agenda
• Mid-range Pressure Measurement (1.013-7000bar)
• Low-pressure measurement (lt1.013 bar)
• High-Pressure measurement (gt 7000 bar)

2
Pressure Measurement

• Definitions
• Absolute pressure pressure of the fluid
  absolute zero of pressure
• Gauge pressure pressure of the fluid
  atmospheric pressure
• Absolute pressure gauge pressure atmospheric
  pressure
• Differential pressure P1 P2
• Standard Atmospheric pressure 1.01320 x 105 Pa
  (1Pa 1N/m2) 101.32 kPa 1atm 14.696 lb/in2
  1.013 bar

3
Mid-range Pressure Measurement

• Device selection ? applications
• U-tube manometer
• Measure gauge or different pressures up to 2 bar
• P1 P2 (?m ?)h
• Where P1 unknown pressure P2 known pressure
  (atmospheric) ?m the fluid specific weight
  (kg/m3, lb/ft3) ? air specific weight
• Visual measurement not control systems
• Other type inclined manometer (draft gauge)

4
Example1

• an U-tube manometer is to be used to measure a
  gas pressure of nominal magnitude of 100N/m2
  relative to ambient (gauge pressure). The oil to
  be used with specific weight 9770 0.5 N/m3.
  Determine the deflection h in the level of the
  manometer liquid if ?gas 11.55 N/m3
• Solution
• We have P1 P2 (?m ?)h
• ? h (P1 P2)/(?m ?)
• 100/(9770 0.5 - 11.55 )
• 100/9758.5 ((0.0059770)2
  (0.00511.5)2)1/2
• 100/(9758.5 48.85) 100/(9758 0.5006)
• 0.010247 0.5006 m 10.247
  0.5006mm

5
Mid-range Pressure measurement
?
?
6
Mid-range Pressure Measurement

• Dead-weight gauge
• Null-reading type
• High accuracy
• Application calibration
• Diaphragm
• Measure gauge or different pressures up to 10 bar
• Resolution 0.1mm
• Applications convert diaphragm into a measurable
  signal

7
Diaphragm
8
Mid-range Pressure measurement

• Bellows
• Greater sensitivity than diaphragm, expensive
• Example2 an engineer finds a desirable
  potetiometric bellows gauge with a stateged range
  of 0-700kPa in a catalog. It uses a sliding
  contact potentiometer having a terminal
  resistance of 0O 10kO full scale. The following
  manufacturer information is available. A
  voltmeter having an input impedance of 100kO is
  available to measure output. For an applied
  transducer power voltage of 5V, estimate the
  expected instrument error in measuring a 350kPa
  pressure
• Other devices Bourdon tube Resonant-wire
  device Fiber-optic pressure sensors

9
Solution

• The output voltage without voltage meter
• Eo(p) RL(p)Ei/10
• ? Eo(0) 0V Eo(350) 2.5V Eo(700)5V
• The output voltage with voltage meter Rm at
  350KPa, RL(p) 5KO
• Eo(p) RL(p)RmEi/(RL(p)Rm)10
• 51005/(5100)10 2.381V
• Error Eo(p) - Eo(p)/Eo(p)
• (2.5-2.381)/2.5 0.0476 4.76

10
Bellows
11
Low Pressure Measurement

• Adaptation of Bourdon tubes 10 mbar
• Manometers and bellows 0.1mbar
• Diaphragm 0.001mbar
• Special pressure gauges for vacuum range
• thermocouple/thermistor gauge 0.001mbar -1 bar
• Mcleod and Pirani gaugeshigh accuracy,
  sensitivity
• Below 0.001mbarspecial ionization gauges
• thermocouple/thermistor gauge 10-4mbar
• Pirani gauges 10-5mbar
• ionization gauges 10-13mbar

12
High Pressure Measurement

• Measure the change of resistance of wires of
  special materials
• E.g. wire gold-chromium alloys and manganin
• Intelligent pressure transducers increase
  sensitivity, accuracy (?0.1 f.s.), measurement
  range. Compensate for hysteresis, non-linearities