Ametek PK II, Operating Instructions Manual, Page: 7 / 28

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2.0 CORRECTION FACTORS
An error in pressure determinations may result from any uncertainty of the mass of the
loading weights and from the measurement of the effective area of the ball and nozzle.
Other sources of error, however, may not be easily recognized. Other sources of error
include the air buoyancy of the weights, gravity, thermal expansion and elastic deformation
of the ball and nozzle, and the air heads involved. All of these corrections, with the
exception of local gravity (except when specified), thermal expansion, and air heads have
been corrected for by AMETEK prior to the tester being shipped.
The following technique is suggested to compute the corrected tester output pressure
readings:
Gravity
Local gravity values differ depending upon geographic locations. Pressure is defined as
“ force per unit area ” , so that mass values must be converted to force values. To accomplish
this, the acceleration of gravity must be used. The acceleration of local gravity may be
determined by having a gravitational survey made of the local area with a gravimeter or by
contacting various governing bodies such as the National Oceanic and Atmospheric Admin-
istration, U.S. Department of Commerce. Once the local value of gravity is known, the
pressure may be corrected using the following formula:
Where:
P
G
= tester output pressure corrected only for gravity
G = local gravity
G
W
= gravity value for which the tester is calibrated
P
N
= pressure value of weights applied
P
G
=
G
W
x P
N
G