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Summary reference
on the nature of pressure, some common units of measure and
pressure references.
Pressure
Gas and liquid
molecules are in constant, random motion called "Brownian"
motion. The average speed of these molecules increase with increasing
temperature. When a gas or liquid molecule collides with a surface,
momentum is imparted into the surface. If the molecule is heavy
or moving fast, more momentum is imparted.
All of the collisions
that occur over a given area combine to result in a force. The
force per unit area defines the pressure of the gas or liquid.
If we add more gas or liquid to a constant volume, then the
number of collisions must increase, and therefore pressure must
increase. If the gas inside the chamber is heated, the gas molecules
will speed up, impact with more momentum and pressure increases.
Pressure and temperature therefore are related.
A Few fundamental
laws of gases:
Boyle's
Law: In
1662, Robert Boyle did the first quantitative experiments
with gases and pressure. He found that the volume of a gas
is inversely proportional to the pressure of the gas at
constant temperature.
PV = k
Charles' Law: Charles found out that the constant, k changes with temperature and that the volume of
a gas linearly with temperature.
Ideal Gas Law:Combine
Boyle’s law and Charles law, we get the ideal gas law:
PV=nrt
Where nr is constant for
a particular gas analogous to the number of
molecules and the relative size of the molecule. The ideal
gas law is very important in the study of gases.
The lowest pressure
possible in nature occurs when there are no molecules at all.
At this point, no collisions exist. This condition is know as
a pure vacuum, or the absence of all matter.
It is also possible
to cool a liquid or gas until all molecular motion ceases. This
extremely cold temperature is called "absolute zero",
which is -459.4° F. Temperature is measured in degrees, which
is rather meaningless. The Kelvin scale of temperature has a
zero point of absolute zero whereas the Celsius scale uses a
zero point equal to the freezing point of water. There is nothing
significant about 0°F in the Fahrenheit scale.
Pressure is
measured with the zero point of the scale as the "reference".
An absolute reference uses a pure vacuum as the zero point.
Therefore, all pressure measurements in the absolute scale are
positive.
Units of Measure
There are numerous
units of measurement for pressure. Since pressure is defined
as a force per area, the United States commonly uses units of
pounds per square inch (PSI), as well as pounds per square foot
(PSF). In Europe and Japan, the metric system uses Kilograms
per square centimeter (Kg/cm2 ).
Pressure
can also be stated in terms of the height of a liquid column.
If one pound of water were poured into a glass tube with an
area of one square inch, the weight of the water on that area
at the bottom of the glass tube is one pound, and the pressure
is therefore one PSI. At 39° F, the water column would be 27.68
inches tall. One inch of water column is annotated as 1"WC.
If
we replace the water with a heavier liquid, the pressure generated
increases. For example, it only takes 2.036 inches of mercury
to generate 1 PSI versus 27.68" water column because mercury
is so heavy. 1 PSI is equal to 2.036"Hg.
In
Europe the metric system is prevalent, so inches are replaced
with millimeters. 25.4 mmHg is equal to 1"Hg.
Evangelista
Torricelli did a lot of the early work in pressure measurement
and invented the barometer. 1 mmHg has been renamed Torr in
his honor. The Pascal is named after Blaise Pascal, another
early mathematician who discovered that air pressure decreases
with altitude and that fluid pressure is the same in all directions.
Other
pressure units of measure are the Atmosphere and the Bar, which
are both roughly equivalent to atmospheric pressure at sea level
on a "standard" day.
Here
is a list of the more common units of pressure measurement.
Units
of Pressure Measurement
Referred to 1 PSI
1
PSI=27.68"WC (inches of water column)
1 PSI=2.036"Hg (inches of mercury)
1 PSI=51.715 mmHg or Torr
1 PSI=0.068947 Bar
1 PSI=0.06804 Atmospheres (Note that 1 Bar is not
exactly 1 atmosphere)
1 PSI=6.8947 KiloPascals or KPa
1 PSI=0.0703 Kg/cm2
1 PSI=2.307 feet of water
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Pressure
Unit Conversion Constants
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| |
PSI(1) |
In.
H2O(2) |
In.
Hg(3) |
K
Pascal |
milli
bar |
cm
H2O(4) |
mm
Hg(4) |
| PSI(1) |
1.000
|
27.680
|
2.036 |
6.8947 |
68.947 |
70.308 |
51.715 |
| In.
H2O(2) |
3.6127
x 10-2 |
1.000 |
7.3554
x 10-2 |
0.2491 |
2.491 |
2.5400 |
1.8683 |
| In.
Hg(3) |
0.4912 |
13.596 |
1.000 |
3.3864 |
33.864 |
34.532 |
25.400 |
| K
Pascal |
0.14504 |
4.0147 |
0.2953 |
1.000 |
10.000 |
10.1973 |
7.5006 |
| milli
bar |
0.01450 |
0.40147 |
0.02953 |
0.100 |
1.000 |
1.01973 |
0.75006 |
| cm
H2O(4) |
1.4223
x 10-2 |
0.53525 |
3.9370
x 10-2 |
0.09806 |
0.9806 |
1.000 |
0.7355 |
| mm
Hg(4) |
1.9337
x 10-2 |
0.53525 |
3.9370
x 10-2 |
0.13332 |
1.3332 |
1.3595 |
1.000 |
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Notes:
(1) PSI - Pounds per square inch
(2) at 39°F
(3) at 32°F (4) at 4°F
(5) at 0°F
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References
Now
that the units of measure are defined, there are 5 commonly used
pressure references that need to be explained. Again, the reference
simply describes the zero point on the scale.
Absolute
As previously described, the zero point of an absolute reference
is the absence of all matter. There is no pressure at absolute
zero. All absolute pressure measurements made are therefore positive.
Absolute
pressure measurements are abbreviated with an "A". For
example, 0 PSIA o6 6"HgA.
Gage
To ignore the effects of changing weather, altitude or depth,
a "Gage" pressure reference is sometimes useful. This
reference measures pressure relative to the local atmosphere.
Changes in local atmospheric pressure occur due to weather, or
if the instrument is moving because of changes in altitude and/or
depth.
Gage
pressure measurement is abbreviated as "G". At sea level,
0 PSIG (Gage) is about 14.7 PSIA. In Denver, 0 PSIG is about 12.5
PSIA. It is possible to have a negative gage pressure. A vacuum
would be about -14.7 PSIG at sea level.
Transducers
with gage references are usually constructed by opening a hole
into the pressure sensor so that the local atmosphere can enter
the unit and counter the pressure being measured. This "back
side" of the sensor usually houses the electronics and other
apparatus that measures pressure. The reference hole (or "breather"
hole) will usually be specified as "dry". Only a clean,
dry gas should be allowed inside the hole. Since the test side
of most pressure transducers is rated for most gases or liquids,
this side of the sensor is called "wet". Gage pressure
sensors are usually "wet/dry" construction.
Sealed
Gage
By allowing local atmosphere to enter the dry side of a transducer,
there exists a pathway for water, water vapor, or corrosive chemical
mist to enter and cause damage. Some manufacturers will then seal
the back side of the sensor to prevent this contamination. The
zero point of the transducer is usually set at whatever the atmospheric
pressure was at the time and location of manufacture. Changes
in altitude and barometer will affect the reading, but if the
pressure range of the sensor is high (>1,000 PSI) these effects
may be considered negligible.
Sealed
Gage references are abbreviated as SG or S, example: PSIS, PSISG.
Vacuum
A vacuum reference can be thought of as the opposite of a gage
reference. The transducer’s zero point is local atmosphere, but
output increases as pressure is reduced. The sensor therefore
measures the amount of vacuum.
Vacuum
references are notated with "V", for example PSIV or
"HgV"
Differential
Differential pressure measurement is the difference between two
unknown pressures. Output is zero when the two pressures are the
same, regardless of magnitude.
Differential
Pressures are notated as "D" (PSID). The magnitude of
the common pressure is called "static" or "base"
pressure. Differential transducers are usually "wet/wet"
construction.
The
Relationship between Absolute, Gage & Vacuum Pressure References
Absolute
Absolute pressure transducers provide increasing output when
pressure is increased. Output is zero when the pressure
is absolute zero. There can not be a negative output because there
is no pressure less than absolute zero.
Examples of absolute pressure measurement include barometers and
altimeters.
Gage
Gage pressure transducers are referenced to local atmospheric
pressure, even if local pressure changes. Transducer output is
zero when pressure is at local atmosphere. Output increases with
pressure. If a vacuum is applied, output will fall below zero.
Examples of gage pressure measurement include tank level, air
compressors, and depth sensors.
Vacuum
Vacuum pressure transducers are referenced to local atmospheric
pressure, but measure the amount of vacuum instead of pressure.
Transducer output is zero when pressure is at local atmosphere.
Output increases with decreasing pressure (increasing vacuum).
Output is at full scale when pressure is at absolute zero.
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