Pressure and Atmosphere

Obj 1 define pressure and force

  • pressure is force exerted over a unit of area

  • 3× 1lb cubes (1×1”) stacked on top of eachother makes 3lbs/in2 (or psi) force in this scenario is 3lbs.

  • If you stack the sideways, it would be 1psi but still 3lbs of force.

  • Pressure:

    • Force / unit area, osi, psi, psf, Pa, kPa, MPa.

    • A large force x small area = high pressure

    • Pascal (metric) is pressure produced when one newton is exerted over 1m2.

  • Force:

    • Push or pull of weight not restricted to area. Calculated over the entire area.

    • Pound force, newton, kilonewton.

    • Newton (metric) amount of force needed to accelerate 1kg at 1 meter/sec. To determine newtons, multiply object weight in kg by gravity (9.81).

Obj 2: 6 principles of hydrostatics

  1. Fluid pressure exerts in all directions

  2. Shape of container does not affect fluid pressure.

    • pascal’s vases: if the vases have different volumes, the height will stabilize between all vases at the same level.

    • Water seeks its own level.

  3. Fluid Pressure is Proportional to Depth

    • the height or depth of liquid affects the pressure exerted.

    • If a column height is halved, the pressure is also halved.

  4. Fluid Pressure is Proportional to Density

    • Pressure exerted relative to height depends on the density of that fluid.

  5. Horizontal runs do not increase pressure

    • Horizontal runs do not increase pressure.

  6. Volume of fluid does not affect pressure

    • if 2 tanks were joined at the same height and one tank had more volume, that tank would not affect the other tank

Obj 3: pressure constants for calculating pressures

  • Hydrostatics Pressure

    • Hydrostatic is study of fluids under pressure and at rest.

    • If water density is 62.4 lbs/ft3, convert it to psi:

      • 62.4lbs/ft3 / 12²= 0.433 psig

      • So 1 cubic foot of water is 0.433psig.

    • Know osi to psi conversion, inwc to ftwc, psf to psi conversion.

Obj 4 - Atmospheric Pressure

  • Imperial: STP is 60F, 29.92”Hg (mercury), air density of 0.076lbs/ft3.

  • Metric: STP is 0C, 101 kPa, air density of 1.29kg/m3.

  • Pressure can change boiling temperature of water.

  • Air is 78% Nitrogen, 21% oxygen, 1% is other gases (carbon, CO2, neon, methane)

  • Measuring Atmospheric Pressure

    • Torricelli experiment: mercury barometer.

    • 36” glass tube, closed at one end, filled with mercury. Placing thumb on open end of the tube, iverted it and placed in bowl of mercury. When removed thumb, mercury level dropped to a height of approx 29.92” above the mercury level in the bowl.

    • Atm press: 14.7 psia, 29.92”, 760mmHg. Pressures at sea level taken above a perfect vacuum.

    • Elevation vs pressure:

      • 2000ft increase, you lose 1 psig drop of atm press.

      • 1000’ increase you lose 1”Hg

      • 90m increase, you lose 1kPa

    • Mercury baroneters:

      • 36” long glass tube sealed on one end, filled with mercury.

      • Tube is inverted into a container of mercury. Level of mercury rises or falls based on atm press.

      • Moveable cup to adjust the zero

      • More accurate than aneroid barometers

    • Aneroid Barometers:

      • Convenient and not affected by position.

      • Evacuated chamber with flexible walls, partial vacuum in bellows and flexible walls are sensitive to pressure change.

    • Perfect vacuum is 0psia, compound gauges can read pressures below or above atmospheric pressure.

    • Maximum suction lift:

      • Atm pressure supports 760mm of mercury. How much meter of water pressure can be supported by atm pressure?

      • 13.6 SG x 760mm = 10336mm or 10.336m

      • A suction pump uses atmospheric pressure to lift water from shallow wells, this pump makes a vacuum then uses the atmospheric pressure to push it. This means the max theoretical depth of the well would have to be 10.34m, otherwise, the water would not reach the pump. 10.34 is too deep as no pump makes a perfect vacuum. Realistically it’s more like 7m.

    • Siphonage

      • necessary for plumbing systems like toilets, undesirable i others

      • Due to partial vacuum created as the liquid falls, atmospheric pressure on the water surface will push the water out the trap until water level is low enough for air to enter through the trap

      • Venting protects siphonage to create atmospheric pressure on both sides of a trap so siphoning cannot occur.

Obj 5 - Pressure and Force Calculations

  • pressure = height x density

  • 1 osi = 1.73 “Wc

  • 27.68”wc = 1 psi ( 1 foot of “wc makes 0.433psi)

  • 1 meter wc = 9.81 kPa

  • 27.68 inwc / 1 psi = 0.0361 lb/in2

  • Force = area * pressure

    • force : pounds force or newton or kilonewton

    • Area : in2 for lb/in2, or m2 for kPa

    • Pressure : psi, oz/in2, kPa

  • Pressure = height * density

Obj 6 - atm pressure measurement and conversion

  • partial vacuum is measured in absolute because below gauge press

  • Atmospheric pressure is always indicated as absolute pressure, indicated with an “a” after the resding (psia, kPa absolute)

  • Gauge pressure has atmospheric pressure factored in as the zero. 0psig is 14.7 psia or 101kPa absolute