Week 8 Flashcards
Equation for pressure?
Pressure = Force / Area
P = F/A
Units: Nm^-2 or Pascal
What does pressure measure
Pressure measures the force the gas or liquid exerts on its container
What are the ways to measure pressure?
- Pressure Measurement
- Tyre pressure gauge
- Bourdon Gauge
-Piezometer - Diaphragm
-U-tube manometer
-Gauge
-Mercury Barometer
Aneroid Barometer
How to measure pressure using a pressure gauge? Derive the equation.
- Force exerted on the piston by the gas is given by F=pA (Force = Pressure x Area )
- Force exerted by the spring, F2, is given by F - k (x₀ - x₁) (Force = Spring constant ( Length of unstretched spring - stretched spring))
- These forces must be equal, pA = k (x₀ - x₁)
How does the tyre pressure gauge work?
- Inside the tube is a rubber bag which expands under pressure
- It cannot expand radially because of the tube, so it expands lengthwise.
How does the Bourdon Gauge work?
The Bourdon Gauge consists of a bent flattened tube. Whenvthis is pressurized, it tends to unroll or straighten out, and a
link mechanism transfers this motion to the indicator needle.
How does a diaphragm work?
The pressure transducer works by having a diaphragm which bends due to pressure.
What are piezometers?
Piezometers are used to measure
the hydraulic head of
groundwater or the pressure of a
fluid at a specific location in a
column
What are the holdbacks of the piezometer?
It is simple and reasonably
accurate, but is impractical for
very low or high pressures, or for
dangerous liquids – the top of
the tube must be open to
atmosphere. It does not work for
gases
How does the piezometer work?
The weight of a liquid is used to balance its own pressure.
Suppose a liquid in a pipe has a pressure p. If a hole is made in the top of the pipe, fix a vertical tube to it
The liquid will rise up the tube until its weight just balances the force due to pressure
How to calculate pressure using the piezometer?
p = pgh (Pressure = density x gravitational potential energy x height)
Units:
- Pressure = Pa or Nm^-2
- Density = Kgm^-3
- Gpe = ms^-2
- Height = m
What is Head in a piezometer and how to measure it?
Head is the height that a liquid would be pushed up a tube by the pressure
h = p/pg
Head (m) = Pressure (Pa) /Density (Kgm^-3 x GPE (N))
How does a u-tube manometer work?
A gas pressure is applied to one side of the
tube, and the other side is open to
atmosphere. The pressure difference is
calculated from the height difference of
the liquid surface
How to calculate pressure from a u-tube manometer?
Pressure difference: p – pa = ρgh
Pressure = density x GPE x height difference
What manometer liquid could be chosen?
-Mercury (136000 kgm^3) for high pressure
-Water, alcohol or oil (700 kgm^-3)
What is Hydrostatic pressure?
The pressure exerted by a fluid at equilibrium at any point of time due to the force of gravity
What is atmospheric pressure?
The force exerted on a surface by the air above it as gravity pulls it to Earth.
WHat is absolute pressure?
The Absolute Pressure is measured relative to absolute zero pressure, i.e. the pressure that would occur at absolute vacuum, where
there is no gas
What is a gauge?
A Gauge is often used to measure the difference between a system and
the surrounding atmosphere or the pressure measurement was referenced to atmosphere pressure (instead of vacuum).
This pressure is called the Relative or Gauge Pressure
What is a barometer?
A device used for measuring atmospheric pressure.
How does the mercury barometer work?
The mercury barometer is a vertical tube like the piezometer, but closed and evacuated at the top.
The barometer equation is:
pa = ρmgh
Pa: Atmospheric pressure (Usually 100kPa)
Pm: Density of mercury (Usually 13,600 kgm^-3)
What is an aneroid barometer?
The aneroid barometer is similar to the
pressure transducer above, but the inlet is
sealed and the space behind the diaphragm
is evacuated. Atmospheric pressure acts on
the other side and bends the diaphragm in
Often a metal capsule or a bellow is used instead of a single diaphragm
