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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

Where G is the specic gravity, Mw is the molecular weight of air, pf is

the owing pressure, Z is owing compressibility, Ro is the universal gas

constant, and T is the owing temperature.

The specic gravity, and Ro are known and are stored in a table used by the vortex

meter.

The hard coefcient to nd 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 nd the compressibility factor. The critical temperature and

pressure used in the Redlich-Kwong equation are stored in the uid data table with

the other coefcients.

Viscosity

The viscosity for real gases is calculated using the exponential equation for two

known viscosities. The equation is:



= aT

Where a and n are found from two known viscosities at

two temperatures.

n =

1n [(μ

)

/ (μ

)

]

1n (T

/ T

)

and

a =

(μ

)

1 2 ... 120 121 122 123 124 125 126 127 128 129 130 131

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