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Pressure Measurement Chapter 5, Wheatstone Bridge - Coggle Diagram
Pressure Measurement Chapter 5
Pressure is (P = F/A) SI unit :
Pascal (Pa) = N/m²
Absolute pressure:
fluid pressure above the reference value of a perfect vacuum or the absolute zero pressure.
Gauge pressure: pressure above the reference value of atmospheric
pressure.
Atmospheric pressure:
at sea level is nearly 1.013x10⁵ N/m² or 760 mm of Hg. (Pressure in
the surrounding air)
Medium pressure measurement
Manometer
Definition:an instrument for measuring the pressure acting on a column of fluid, consisting of a U-shaped tube of liquid.
Formula:
h = (p1-p2)
/pg
Characteristic of a manometer fluid:
non-corrosive and not have any chemical reaction with the
fluid which pressure is being measured.
low viscosity to ensure quick respond to pressure changes.
should have negligible surface tension and capillary effects.
Bourdon Tube
Definition: an elastic-element type of pressure transducer. It is commonly used for measuring the gauge pressure of both gaseous and liquid fluids.
When pressure applied the elastic elements will get deformed.
Based on this deformation, the pressure can be measured.
High Pressure Measurement
Special techniques have to be used for measure the pressure
above 1000 atm.
technique
the electrical resistance change of a manganin (alloy of Cu, Ni, Mn) or gold chrome wire, with hydrostatic pressure to accommodate a bulk compression effect.
Pressure Transducer
Definition : pressure transmitter, is a transducer that converts pressure into an analog electrical signal. One of the most common is the strain-gage base transducer.
Pressure applied to the pressure transducer produces a deflection of the diaphragm which introduces strain to the gages.
The voltage at point
D = V × RX / (R3 + RX)
The voltage at point C = V × R2 / (R1 + R2)
The voltage (V) across galvanometer or between C and D is,
VCD = V × RX / (R3 + RX) − V R2 / (R1 + R2)
For finding the unknown value of resistance
RX = R3 × (R2 / R1)
Low Pressure (Vacuum) Measurement
Vacuum measurement unit are Torr and micron.One Toor is a pressure equivalent of 1 mm of Hg while 1 micron is 10¯³ Torr.
McLeod gauge
Equation for applied pressure using McLeod gauge .The minimum pressure normally measurable is 10^-4 bar
Pu = ah^2/Vu
Pirani gauge
Pirani gauge is similar to a thermocouple gauge but has a heated element that consists of four coiled tungsten wires connected in parallel.
Such gauges cover the pressure range 10^-5 mbar to 1mbar.
Ionisation gauge
The ionization gauge is a special type of instrument used for measuring very low pressures in the range 10^-13 to 10^-3 bar.
Knudsen Gauge
An instrument for measuring very low gas pressures (1 μPa to 1 Pa) based upon the momentum transfer by gas molecules between two closely-spaced plates immersed in the gas
Linear Variable Differential Transducer(LVDT)
Definition : an electromechanical sensor used to convert mechanical motion or vibrations, specifically rectilinear motion, into a variable electrical current, voltage or electric signals, and the reverse.
Formula
Open Wiring LVDT
D = MV
out
Ratiometric Wiring LVDT
D= M(Va-Vb /Va+Vb)
Wheatstone Bridge
