Basic Documentation

Laboratory Room Pressurization
Common pressure measurements are mostly rather high pressures such as steam,
compressed air, etc. We don’t need to include the term positive or negative because
it is usually obvious to us where the higher pressure is. So, in most everyday
references to pressure measurement, we tend to focus only on the numerical value
of the higher pressure and we do not need to think in terms of one side being the
positive part of a differential pressure measurement.
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However, with respect to room ventilation applications the differences in pressures
are very slight. Thus the higher or positive part of a differential pressure relationship
is not always obvious. Therefore, one must be more specific when referring to
ventilation related pressure measurements. For this reason, the terms positive and
negative are important with respect to room static air pressure measurements.
Otherwise, the potential direction of airflow (positive to negative) would not be
known.
Note that in the HVAC industry, the term static pressure is widely used in conjunction
with air pressure measurements. Adding the term static simply means that the
pressure difference being measured is not due to any motion of the air itself. Again,
this can be better understood with respect to the air pressure within a tire. The tire
pressure is not caused by the motion of the car, nor is it due to air movement within
the tire. Rather, the pressure is the result of the force that an air compressor has to
put on the air to get it to flow into the tire. Likewise, static pressure indicates that the
pressure relationship is not due to the movement of the air, but rather to the force
being applied by a ventilation system supply or exhaust fan on the air in the room. In
other words, a pressure difference between two rooms can exist even though there is
no airflow from one room to the other or air currents within the rooms.
The static pressures in ventilation system applications are tiny when compared with
more common pressure measurements of steam, water, or automobile tire pressure.
Therefore, a much smaller unit of pressure measurement is very desirable to express
ventilation system static pressures, so the unit inches of water column (in. WC) is
commonly used. With reference to room static pressure, 0.01 to 0.03 in. WC would
be typical values. (Note that if psi units were used instead of inches of water, 0.01 in.
WC would have to be expressed as 0.000361 psi, which is very cumbersome.)
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When referring to ventilation system static pressures, it is often necessary to use the
positive or negative qualifier, since it is not always obvious where the higher pressure
is. This is especially true if the measurement concerns the static pressure of rooms
or spaces. If we state that the static pressure of a laboratory is 0.02 in. WC negative
with reference to the adjacent corridor, it is equivalent to saying that the static air
pressure of the laboratory is 0.02 in. WC lower than the static pressure of the air in
the corridor. This can also be stated that the corridor is 0.02 in. WC positive with
respect to the laboratory. Regardless of the expression used, given the chance, air
will flow from the corridor into the laboratory room.
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Note that it is just as correct from a technical perspective, to refer to the lower side of a pressure measurement being
at a negative pressure. Although not commonly expressed in this manner, it would be just as correct technically to say
that the air outside of the tire was 35 psi negative with respect to the air inside of the tire.
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A pressure of 1 psi is equivalent to the force produced on a unit of area by a column of water 27.72 inches in height. A
pressure of 1 inch of water is therefore equivalent to the fraction 1/27.72 of a psi. This then becomes 0.0361 psi. A
pressure of only 0.01 inch of water then is 1/100 of 0.0361 which equates to 0.000361 psi.
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