Yokogawa 415 Steam and Gas - Manual (Page 21)

Example 1
Flow is to be measured across an orifice in the range of 0 - 2000
scm/hr. Because flow needs to be measured over a 10:1 range, two
transmitters are spanned as follows:
Transmitter 2 0 - 600 scm/hr
Transmitter 1 0 - 2000 scm/hr
Hence, above 600 scm/hr, transmitter 2 is used and below 600
scm/hr, transmitter 1 is used. Since D. P. transmitters are accurate
over a 3:1 range, then the system will provide reliable readings
between 200 to 2000 scm/hr, which is a 10:1 turndown.
Both transmitters will be individually scaled to equations 5 & 6 or
7 & 8, as appropriate.
PROGRAMMING THE FLOW COMPUTER
For equations 4 to 8 to work correctly, a number of parameters need to
be programmed:
K
F
K-factor (for frequency producing flowmeters)
S
VB
(or S
M
) Span (for analog flowmeters)
T
B
Base temperature
P
B
Base pressure
G Specific Gravity of Gas
The flow computer will measure the flow input A (normalised between
0 and 1), the temperature, T
F
, and pressure, P
B
. Depending on the gas
equation selected, the compressibility factors and density are then
calculated. Other parameters must also be programmed and these are
fully detailed in section 5.
Operation 19