ONICON F-2600 - Manual (Page 125)

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F-2600 & F-2700 Vortex Flow Meter Manual 02/15 - 0808-7 / 19204 Page B-4
Calculations for Gas (“Real Gas” and “Other Gas”)
Use this formula to determine the settings for “Real Gas; Gas” selections and “Other
Gas” selections entered in the Fluid Menu. The calculations for gas were taken from
Richard W. Miller, Flow Measurement Engineering Handbook (Third Edition, 1996).
Density
The density for real gases is calculated from the equation:
ρ =
GM w, Air p f
Z
f
R
0
T
f
Where G is the specific gravity, Mw is the molecular weight of air, pf is
the flowing pressure, Z is flowing compressibility, Ro is the universal gas
constant, and T is the flowing temperature.
The specific gravity, and Ro are known and are stored in a table used by the vortex
meter.
The hard coefficient to find is the compressibility, Z. Z is found using the Redlich-
Kwong Equation (Miller page 2-18).
The Redlich-Kwong Equation uses the reduced temperature and pressure to
calculate the compressibility factor. The equations are non linear and an iterative
solution is used. The vortex program uses Newton’s Method on the Redlich-Kwong
equations to iteratively find the compressibility factor. The critical temperature and
pressure used in the Redlich-Kwong equation are stored in the fluid data table with
the other coefficients.
Viscosity
The viscosity for real gases is calculated using the exponential equation for two
known viscosities. The equation is:
μ
cP
= aT n
k
Where a and n are found from two known viscosities at
two temperatures.
n =
1n [(μ
cP
)
2
/ (μ
cP
)
1
]
1n (T
K2
/ T
K1
)
and
a =
(μ
cP
)
1
T n
K1