Fluke RUSKA 2465A-754, User Manual

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RUSKA 2465A-754
Users Manual
2-6
English and the International System of units.
Corrections for local gravity can vary by as much as 0.5% thus it is very important to
have a reliable value for the local acceleration of gravity. A gravity survey with an
uncertainty better than 0.00001 m/s
2
is recommended.
Buoyant Effect of the Air
According to Archimedes's principle, the weight of a body in a fluid is diminished by an
amount equal to the weight of the fluid displaced. The weight of an object (in air) that has
had its mass corrected for the effects of local gravity is actually less than that corrected
value indicates. This reduction in weight is equal to the weight of the quantity of air
displaced by the object, or the volume of an object multiplied by the density of the air.
But the volume of an irregular shaped object is difficult to compute from direct
measurement. Buoyancy corrections are usually made by using the density of the material
from which the object is made. If the value of mass is reported in units of apparent mass
vs. brass standards rather than of true mass, the density of the brass standards must be
used. Apparent mass is described as the value the mass appears to have, as determined in
air having a density of 0.0012 g/cm³, against brass standards of a density of 8.4 g/cm³,
whose coefficient of cubical expansion is 5.4 x 10
-5
/ºC, and whose value is based on true
mass in value (see reference 4).
Although the trend is swinging toward the use of true mass in favor of apparent mass,
there is a small advantage in the use of the latter. When making calculations for air
buoyancy from values of apparent mass, it is unnecessary to know the density of the mass.
If objects of different densities are included in the calculation, it is not necessary to
distinguish the difference in the calculations. This advantage is obtained at a small
sacrifice in accuracy and is probably not justified when considering the confusion that is
likely to occur if it becomes necessary to alternate in the use of the two systems.
A satisfactory approximation of the force on a piston that is produced by the load is
given by:
g M F
brass
air
A
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
− =
ρ
ρ
1
Where:
F
is the force on the piston
M
A
is the mass of the load, reported as "apparent mass vs. brass
standards"
ρ
air
is the density of the air
ρ
brass
is the density of brass (8.4 g/cm³)
g
is the acceleration due to local gravity