7 Technical Description of the Device
134
The compressibility factor of the gas expresses the variation of the natural gas properties from the
properties of the ideal gas. By setting the parameters, a specific method according to a specific
standard (AGA NX-19 mod, AGA8-G1, AGA8-G2, SGERG-88, AGA8-92DETAIL, GOST 13390.2,
or GOST 13390.3) can be selected for compressibility factor calculation. For gases other than
natural gas, a constant compressibility value can be used. If the value of pressure or temperature
exceeds the limit of the standard selected to calculate the compressibility, the device calculates
the compressibility using a substitute compressibility value.
From the input pulse frequency, the device calculates the flow of gas in real-time by mathematical
filtration from the incoming signal.
Primary flow:
Q = ∆V/∆t [m
3
/h]
Where: ∆V ............................ increase in operating volume
∆t ............................. time between pulses with the accuracy of one hundredth of a
second
The instantaneous flow rate displayed on the corrector screen is updated every 10 seconds.
Flow rate at base conditions:
Q
b
= C * ∆V/∆t [m
3
/h]
Base conditions remark
The base conditions are the conditions on which the gas quantity measurement is recalculated.
These conditions are different in different countries.
E.g. for the Czech Republic:
absolute temperature Tb = 288.15 K (i.e. tb = 15 ° C)
absolute pressure pb = 101.325 kPa
7.9.2 Substitute volume values
For calculation under error conditions (i.e. because of a transmitter error, a value deviation from
the operating range or a device error), the substitute volume counters at measurement conditions
(V
s
) and at basic conditions (V
bs
) are installed in the device.
These counters are coupled with appropriate volume counters under normal (correct) conditions.
A detailed description of the behaviour of the device under normal (correct) and error conditions is
described in Section 8.4.1
7.9.3 Volume Correction under the Measurement Conditions
The device is equipped with the option of compensating the gas meter errors according to the
correction profile data specified in the gas meter test report. This possibility to use the corrected
volume V
c
is in accordance with CSN EN 12405-1 + A2. The function and the V
c
parameter can
be added by the manufacturer or at the ASC level. If this function is used, it must be ensured that
the error rate profile of the gas meter in relation to the flow rate Q corresponds to the actual
operating conditions.
The meter error is corrected using the f (Q) function. For the corrected volume, the following
applies:
V
c
= V
m
x f(Q)
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