8-48 Qualitek mR Owner’s Guide
implies that the depth of an air column bounded by two constant-pressure surfaces
will increase as the temperatures in the column decreases.
Density varies mostly with pressure over large vertical distances; at constant
height, pressure variation with temperature becomes important. In the low
atmosphere, air is heavy, with a stable mass of roughly one kilogram per cubic
meter (1 oz/cubic foot). A room of 500 cubic meters (650 cubic yards) thus
contains 0.05 metric ton of air. At an altitude of 3 km (2 miles), however, density is
30% less than at sea level.
This difference in air density can cause variations in flow readings from one
location to another when elevations are quite different and no corrections are
made.
Qualitek mR measures true air flow in units standardized to 29.92 inHg (14.68 psi)
at 20 degrees C. Although the Qualitek mR’s measurements are accurate and
traceable to NIST standards, flow readings could be different if the tester is used at
drastically different altitudes. This is not a tester fault; rather, conditions have
changed and Qualitek mR measures this difference. At a high altitude, air is less
dense than air at a lower elevation. When compressed to the same relative
pressure, the air will exhibit a density shift when compared with air compressed to
the same relative pressure at a lower level.
Difference in air density is the primary reason users can observe slightly different
results when temperature and gauge pressures are identical. For this reason, it is
important to establish all test parameters at the same altitude and under the same
conditions in which the tester will be used.
Compensation may be necessary to reconcile air density variations between the
two locations if measurements are made at different altitudes.
Compensation %
A high compensation % may indicate a high comp value and may not be a valid
test. The Compensation % = (comp / (comp + cal)) * 100.
Fluids vs. Solids
The distinguishing feature of a fluid (gas or liquid), in contrast to a solid, is how
easily the fluid can be deformed. If a shearing force, even a very small force, is
applied to a fluid, the fluid will move and continue to move as long as the shear
acts on it. For example, the force of gravity causes water poured from a cup to
flow. Water continues to flow as long as the cup is tilted. If the cup is turned back
up, the flow stops because the gravitational force is then exactly balanced by force
of the cup wall.
Gas vs. Liquid
Unlike liquids, gases cannot be poured from one open container into another, but
they deform under shear stress just the same. Because shear stresses result from
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