Pressure Measurement
• Learning Outcomes:
– State pressure and units for pressure, gauge, absolute and differential pressures
– Describe pressure measuring methods:
Bellows type,
Bourdon-tube,
mercury barometer,
manometers and rotameters
– Describe pressure detection circuitry
Pressure and units
• Pressure is force per unit area (fluid, gas)
• Units: Pascal (Pa), atmosphere, bar, psi
Pascal: SI unit for pressure – the pressure or stress that arises when a force of one
newton (N) is applied uniformly over an area of one square metre (m2). Pa = N/m2
Atmosphere (atm): 1 atm = 101.325 Pa
Bar: 1 bar = 105Pa
Psi (pounds-force per square inch absolute): 1 Pa = 0.000145 Psi, 1000 Pa = 0.145 Psi
Gauge, Absolute and Differential Pressures
• All pressures must be measured relative to some reference.
• Absolute: p2 = 0
• Atmospheric (gauge): p2 = atmospheric pressure
• Differential: p2 = certain pressure
Pressure Measuring Methods
• General principle: Pressure can be converted to force by letting it act on a know area:
diaphragm, Bourdon tube, bellows, flapper-nozzle, piezo-electric
• High vacuum pressure measurement is not related to force measurement: column of
liquid as in manometers
• Differential pressure transmitter: converts pressure to other types of signal, e.g.
electrical signals (4 mA – 20 mA)
Manometers
• U-tube Manometer
• Well-typed Manometer
• Inclined Manometer
Summary of Manometers
• Use: for static pressure measurement, differential pressure measurement
• U-tube & Well-type: 1 mm Hg or water to 1 m of Hg or water, approximately 1 mbar to
100 mbar or 1.5 bar
• Inclined: 1 mm to 300 mm of water, approximate 1 mbar to 30 mbar
• Advantages: not used in control systems
Pressure Detection Circuitry
Transducer – D/P Transmitters
• Converts pressure to other forms of signal, electrical signal for indication and control
• Output: volt (0-5V, 0-10V) or current (4mA – 20mA)
• Commonly used types:
– Bellows/Bourdon resistance (potentiometer)
– Inductive
– Linear variable differential transformer (lvdt)
– Capacitive
– Diaphragm & inductive or piezo-electric (crystal material)
Functional Uses of P. Transducers
• Three basic functions: indication, alarm and control
• Environmental concerns:
– Atmospheric pressure: Pressure instrument is sensitive to variations in the
atmospheric pressure surrounding the detector – cause the indicated pressure to
change
– Ambient temperature: resistance of components, then reduce accuracy and
reliability
– Humidity: high moisture affects electric/electronic equipment: short circuits,
grounds, corrosion
Summary
• Pressure measurement: units for pressure, absolute, gauge and differential
pressures
• Pressure measuring methods: mercury barometer, Bourdon tube, bellows type,
manometers (U-tube, Well-type and Inclined)
• Pressure detection circuitry (transducers)
• D/P transmitters