Tests of Commercial Densimeters for LNG Service Page: 4
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3. THE COMMERCIAL DENSIMETERS
Three of the densimeter types tested and reported earlier [2,3] have
undergone some additional testing. The three are the displacement densimeter,
the vibrating cylinder densimeter and the vibrating plate densimeter. A second
vibrating cylinder type has been tested for the first time.
3.1 Vibrating Element Densimeters
A vibrating element in these densimeters is locked in oscillation at one of
its lower resonant frequencies. When this harmonically vibrating element is
immersed in a fluid of variable density, the resonant frequency shifts with the
fluid density such that, in the ideal case,
p = A + BT2
where p is the fluid density, r is the period of vibration and A and B are con-
stants. The constants A and B are derived from frequency measurements in fluids
of known density. Since temperature can affect the structural properties hence
the frequency of the vibrating element, such a densimeter should be calibrated
in the fluid or in fluids as near as possible in temperature and density to those
in which the instrument will be used.
3.2 Displacement or Archimedes Densimeter
This densimeter employs the Archimedes principle: an object immersed in a
quiescent fluid experiences an upward force proportional to the mass of fluid it
displaces. If the mass and volume of the object are known, the density of the
fluid is determined by the relation
p = po(M-Ma)/M . (2)
The density of the float po is given by M/V where M is the float mass in vacuum
and V is its volume. The quantity Ma is the apparent mass of the float when4
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Siegwarth, J. D. & LaBrecque, J. F. Tests of Commercial Densimeters for LNG Service, report, June 1982; Washington D.C. (https://digital.library.unt.edu/ark:/67531/metadc502562/m1/9/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.