Pressure Measurements

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Pressure Measurements
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• Pressure is force distributed over an area.
• Force units
• Pounds or Newtons
• Pressure
• Pound (force) per square inch (PSI)
• Atmosphere (14.696 PSI) (non-SI but widely
  recognized)
• Bar ( nearly the same as the Atm. use is
  discouraged
• mmHg (used primarily in scientific research and
  medicine)
• inHg (Torr) (used primarily in meteorology)
• inWater (too small for use in fluid power)
• Pascal (N/m) (too small for use in fluid power)

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Standard Atmospheric pressure depends on
convention.

• Common definition
• Altitude Average sea level
• Temperature 68 degrees Fahrenheit
• Humidity 32
• Other definitions exist

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The Barometer (Evangelista Torricelli Inventor)
Simplifed Version
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Simple Barometer
Near perfect vacuum
Atmospheric Pressure assumed to be 14.7 PSI
Mercury
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Barometer
The level automatically adjusts to the current
atmospheric pressure.
The height of the column of mercury determines
the pressure exerted below.
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Simple Barometer
14.7 PSI
14.7 PSI
The force (weight) of the column of mercury
balances the force of the atmospheric pressure on
the reservoir.
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Head Pressure
h is the distance from the level in the reservoir
to the top of the mercury. D is the weight
density of the liquid (mercury)
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Weight Density

• Mercury is 13.593 heavier than water.
• A cubic foot of mercury weighs 62.4lbs times
  13.593 or 848 pounds.
• One way of stating the weight density is 848
  pounds per cubic foot.
• The multiplier 13.593 is called the specific
  gravity. The SG of water is defined as 1.

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The height of a mercury column that produces 14.7
PSI

• From the formula for head pressure

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760 mmHg balances 14.7 PSI
0 PSI
Assuming that the pressure on the reservoir is
atmospheric pressure.
760 mm Hg
14.7 PSI
14.7 PSI
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Converting between mmHg and PSI
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Converting between inHg and PSI
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Atmospheres and Bars
For our purposes the atmosphere unit and the bar
unit are both equal to 14.7 PSI
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Vacuum Pressure
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Gage Pressure