is a type of stress which is exerted uniformly in all directions.
Force exerted per unit area
P = F/A
P = Pressure in Pascal (lb/inch)
F = Force in Newton (lb)
A = Area in square meter
Pressure
Increases as force increases or area decreases.
Atmospheric Pressure
is the pressure due to weight of the atmosphere above the point where it is measured.
Atmospheric Pressure
changes at different elevations because at higher elevation there is less weight of air than lower elevation areas.
Head
is the actual height of the column of liquid. A container or vessel can be any shape but head is only determined by the height of liquid.
Hydrostatic Pressure
is the pressure due to head of liquid column.
Hydrostatic Pressure
P = ph
P = pressure in (Pa or in psi)
p = density (kg/cu m or lb/cu in)
h = height (in m or feet)
Pascal's Law
is stating that the pressure applied to a confined static fluid is transmitted equal intensity throughout the fluid.
Example of Pascal's Law
A hydraulic press
Absolute Pressure
is pressure measured with a perfect vacuum as the zero point of the scale.
When measuring absolute pressure
the units increase as the pressure increases.
Absolute Pressure
cannot be less than zero and is unaffected by changes in atmospheric pressure.
Gauge Pressure
is pressure measured with atmospheric pressure as zero point of scale. Negative gauge pressure is gauge pressure that is less than atmospheric pressure.
Vacuum Pressure
is the pressure measured below atmospheric pressure, referenced to atmospheric pressure.
Differential Pressure
is the difference in pressure between two points of measurement.
Manometer
is a device for measuring pressure with a liquid filled tube.
Inclined-tube manometer
is a manometer with a reservoir serving as one end and the measuring column at an angle to the horizontal to reduce the vertical height.
Well-type manometer
is a manometer with a vertical glass tube connected to a metal well, with the measuring liquid in the well at the same level as the zero point on the tube scale
Well drop
is the ratio of the area of a well-type manometer tube to the area of the well.
Barometer
is a manometer used to measure atmospheric pressure.
Diaphragms
are a mechanical pressure sensor consisting of a thin flexible disc that flexes in response to a change in pressure.
Capsule
is a mechanical pressure sensor consisting of two convoluted metal diaphragms with their outer edges welded to provide an empty chamber between them.
Bourdon tubes
provide fairly large displacement motion that is useful in mechanical pressure gauges; the lesser motion of diaphragms is better in electromechanical sensors.
Pressure Spring
is a mechanical pressure sensor consisting of a hollow tube formed into a helical, spiral, or C shape.
Bellows
is a mechanical pressure sensor consisting of a one-piece, collapsible, seamless metallic unit with deep folds formed from thin-wall tubing with an enclosed spring to provide stability, or with an assembled unit of welded sections
Double-ended piston
is a mechanical pressure sensor consisting of a differential pressure gauge with a piston that admits pressurized fluid at each end.
Transducer
is a device that converts input energy into an output electrical energy.
Pressure Transmitter
is a pressure transducer with power supply and a device that conditions and converts the transducer output into a standard analog or digital output.
Pressure Switch
provides a discrete output when applied pressure reaches a preset level within the switch.
Resistance Pressure Transducer
is a diaphragm pressure sensor with a strain gauge as the electrical output element.
Capacitance pressure transmitter
is a diaphragm pressure sensor with a capacitor as the electrical element.
A differential pressure (d/p) cell
converts a differential pressure to an electrical (4 - 20mA) output signal.
Inductance pressure
transducer is a diaphragm or bellows pressure sensor with electrical coils and movable ferrite core as the electrical element.
LVDT
is an inductance transducer consisting two coils wound on a single nonconductive tube.
Stress
is the internal distribution of forces within a body that balance and react to the loads applied to it.
Strain
is a fractional change in the dimensions of an object as a result of mechanical stress.
Dry Leg
impulse line that is filled with a non-condensing gas
Wet Leg
an impulse line filled with fluid that is compatible with the pressure-measuring device
Dead weight tester
is a hydraulic pressure calibrating device that includes a manually operated screw press, a weight platform supported by a piston, a set of weights, and a fitting to connect the tester to a gauge.