Units and scales of pressure measurements
Transcript of Units and scales of pressure measurements
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4.th week
Units and scales of pressure measurements
Pressure can be expressed with reference to any datum.
The usual datums are:
1. Absolute zero.
2. Local atmospheric pressure.
Absolute pressure: is the pressure when expressed as difference between its value and a complete vacuum.
Gage pressure: is the pressure when expressed as difference between its value and the local atmospheric pressure.
Negative, Suction, or Vacuum pressure: is the pressure below local atmospheric pressure with reference to gage
pressure.
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Types of pressure gages
1. The Bourdon gage: it is the device used for measuring gage
pressures. The red.pressure element is a hollow, curved, flat metallic
tube closed at one end; the other end is connected to the pressure to be
measured. the gage pressure measures pressure relative to the medium
surrounding the tube, which is the local atmosphere.
2. The mercury barometer: it is used to measure atmospheric pressure.
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MANOMETERS
In Fig(b). The meniscus can come to rest below A, as shown. Since the
pressure at the meniscus is zero gage and since pressure decreases with
elevation:
Fig(c) showed the manometer which is used for high negative or positive
gage pressure. a liquid with high specific weight is employed. It must be
immiscible in the first fluid. The pressure at A can be calculated as
follows:
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Where:PA= unknown pressure at A.
h1= height of liquid of the manometer.
h2= height of liquid want to measure its pressure in the
manometer.
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Differential Manometer:
Example 1:
EXAMPLE 2.
The classic use of a manometer is when two U-tube legs are of equal
length, as in Fig. E2.3, and the measurement involves a pressure
difference across two horizontal points. The typical application is to
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measure pressure change across a flow device, as shown. Derive a
formula for the pressure difference pa - pb in terms of the system
parameters in Fig.
Solution
Using our “up-down” concept as in Eq. (2.32), start at (a), evaluate
pressure changes around the U-tube, and end up at (b):
pa +ρ1gL +ρ1gh -ρ2gh -ρ1gL = pb
or pa - pb = (ρ2-ρ1)gh
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Fig. 2.6: a modern portable
barometer, with digital readout, uses
the resonating silicon element .
Solved problems
Ex1: A closed tank contains 1.5 m of SAE 30 oil, 1 m of water, 20 cm of
mercury, and an air space on top, all at 20C. If pbottom 60 kPa, what is
the pressure in the air space? γoil=8720N/m3, γwater=9790N/m3,
γmercury=133100N/m3.
Solution: Apply the hydrostatic formula down through the three layers of
fluid:
P bottom = Pair + γoil * hoil + γwater *hwater+ γ mercury *h mercury
60000 Pa= Pair + (8720 N/m )(1.5 m)+ (9790)(1.0 m) +(133100)(0.2 m)
Solve for the pressure in the air space: Pair 10500 Pa Ans.