Pressure Units
In the United States, the units of pressure measurement depend greatly upon what is
being measured. For example, tire pressure is measured in pounds per square inch
(psi) where zero is atmospheric pressure. From an engineering viewpoint, that is called
psig and the “g” stands for gauge or gauge pressure. There is another common psi
scale known as psia, where the “a” stands for absolute and zero on the scale is a perfect
vacuum. If only psi appears in the nomenclature rather than psia or psig, then it must
be determined from context which is meant.
Physically, pressure is force acting on an area.
Pressure = Force / Area
Force is mass x acceleration = mass x distance / time2 = kg∙m/s2. Because this unit
combination, kg∙m/s2 is used so frequently, it is given the special name Newton and the
symbol N. Thus pressure has units N/m2. Following this logic, N/m2 is also very
common, and it is given the name Pascal, in honor of Blaise Pascal a Frenchman who
first stated that gas pressure is transmitted equally in all directions.
m
kg 
Pascal
Newton
2
m
s
2
2
m
Using a barometer and a ruler, an Italian, Evangelista Torricelli, discovered that the
pressure of the atmosphere would support a column of mercury to a height of 760 mm.
That unit, 1 mm of Hg, came to called a torr, and is still in widespread use.
Torr = 1 mm of Mercury
In lyophilization the pressure is so low that it is convenient to use a milli-torr as a
standard unit. Note that a milli-torr is 1/1000 times 1 mm of Mercury – that is, it is a
micrometer of mercury. Thus came the name “micron” which means a milli-Torr, which
is, of course, a micrometer (micron) of mercury.
Micron = mTorr = _Hg
The bar is another pressure unit, widely used by meteorologists. It is nothing more than
105 Pascals. In the United States, we often see milli-bars (mbars) used in reference to
hurricane pressures. The unit acknowledges the Pascal, which is a rather derived units,
insofar as the kg is fixed and unrelated to atmospheric pressure. Conveniently, or
perhaps not, 1 atmosphere is 1.013 bars, 1013 mbars and 101325 Pascals. For
practical (not calculation) purposes, one can think of bars and atmospheres as being
about the same.
American refrigeration engineers use the worst of all units, referring to “vacuum” in
inches of mercury. As is turns out, 1 atm is equal to 29.921 in_Hg at 0C. Thus if a
“vacuum” gauge is reading 18 in_Hg, its meaning is 29.921-18 = 11.921 psia. This unit
is typically found on air conditioning manifolds. They compound the issue by measuring
pressure above atmospheric in psig. Thus, a perfect vacuum is 29.921 in_Hg and 1 atm
is “zero” and 2 atm is 14.696 psig.
Conversion between units is easily done at scores of internet sites, or you can use the
formula below.
Pascal
Bar
mbar
Atm
Torr
mTorr
PSIA
In_Hg
Pascal
1 N/m2
100,000
100
101,325
133.322
0.13322
6894.757
3386.38
Bar
10-5
1 dyne/cm2
.001
1.01325
0.00133322
1.333 x 10-6
0.068948
0.033864
mBar
0.010000
1000
1
1013.25
1.333
1.333 x 10-6
68.948
33.864
Atm
9.869 x 10-6
0.986923
986.9 x 10-6
1 atm
0.00131579
1.315 x 10-6
0.068046
0.033421
Torr
0.00750062
750.061683
0.75006
760
1 mm Hg
.001
51.714933
25.399939
mTorr
7.500617
750061.683
750.062
760000.
1000
1
51714.933
25399.939
PSIA
145.03774 x 10-6
14.503774
0.014504
14.695949
0.01933677
19.337 x 10-6
1 lb/in2
0.491153
In_Hg
295.3006 x 10-6
29.530059
0.029530
29.921332
0.03937017
39.37 x 10-6
2.036026
1 in Hg
To use this conversion table:
1. Find the “from” unit in Column 1.
2. Find the “to” unit in Row 1 (across the top)
3. Use the crossing value as a multiplier. “From Units” x Multiplier = “To Units”
FYI: The units for each multiplier are (Row Name) / (Column Name)
Example: Convert 150 mTorr to PSIA.
Step 1: 150mTorr is 0.15 Torr
Step 2: Finding Torr on the left column and PSIA on top, the multiplier is
0.01933677.
Step 3: 0.15 Torr x 0.01933677 PSIA / Torr = 0.0029 psia.
In summary, Pascals and Bar are derived units from the kilogram. PSIA is a derived unit
based on the pound. Torr and In_Hg are units based on the height of a mercury
column in a barometer. Atmospheres is a fraction of the pressure exerted (on
average at zero deg C) at sea level by the air, using 1 atm as that pressure.