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Appendix F Calculations and theory
Page F.1
Appendix F Calculations and theory
F.1 The VOS effect on density measurements
This sub-section shows how the 795x gas flow computer software works out the velocity of sound factors that are
used for correcting line density.
Two methods are provided:-
1. Pressure method
This method is preferred and is for applications where live pressure measurement is available to the 795x.
2. Specific gravity method
This is also known as the “User Gas Equation Method”. It is for applications where pressure measurement is
not available. Inputs of Specific Gravity and Temperature are required.
Equation F.1#1
: Density with the correction for the effect of V.O.S.
Using:
¸
¸
¸
¸
¸
¸
¸
¹
·
¨
¨
¨
¨
¨
¨
¨
©
§
¸
¸
¹
·
¨
¨
©
§
τ
+
¸
¸
¹
·
¨
¨
©
§
τ
+
ρ=ρ
2
g
2
c
1
C*
K
1
C*
K
1
*
Where:
ρ
= True line density (V.O.S. corrected)………..……..….{Menu Data:< ““Meter dens A”>}
1
ρ = Line density (uncorrected)
c
C = V.O.S. of calibration gas (m/s)………………………...…….…….{See Equation F.1#2}
g
C = V.O.S. of measured gas (m/s) ………………………………………………..……………
…………..{See either
Equation F.1#3a (pressure method) or F.13#b (SG method)}
K = Density transducer VOS constant
= 2.10 * 10
4
for a 7812 Gas density transducer
= (1.35 * 10
4
for a 7810 Gas density transducer)
= (2.62 * 10
4
for a 7811 Gas density transducer)
τ = Periodic time of density transducer output signal (in μs)