INFICON Composer Gas Concentration Controller - Manual (Page 88)

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Composer Operating Manual
7 Start data gathering.
NOTE: Zeroing the instrument is not essential, but the concentration reading
should be steady.
8 Start data logging. Wait about half a minute.
9 Press the MANUAL button to bring forth a pop up window.
10 Set bubbler gas flow and adjust dilution flow if total flow is to be conserved.
11 Press the SET FLOW button on the pop up. Since, full scale output of the
bubbler MFC corresponds to 5 volts, the control voltage to the bubbler MFC
can be readily estimated as (5.0 V * bubbler flow /bubbler full scale). Let’s
call this value volts.
12 The concentration will start to increase and then settle down to a steady
value. After the concentration becomes steady, wait for about a minute and
then press the BYPASS button to terminate bubbler gas flow.
13 Stop data logging when the concentration is within 10% of its initial value
(near 0, if the instrument was zeroed).
14 Press STOP button to terminate all gas flows.
15 Now is the time for data analysis. When the logged data is plotted, it will be
similar to
Figure 3-11 on page 3-27 .
16 Use fifteen data points (about 15 sec) from the initial section to calculate
average and standard deviation. The average, Vo, is the initial
concentration baseline and twice the standard deviation will be called error
band, Err.
17 After the concentration has become steady (within Err) at the higher value,
calculate the average concentration over fifteen seconds. This average, Vf,
is the final concentration reading.
18 Compute the process gain, Kp = (Vf - Vo) / volts.
19 Now enumerate time, t, from the moment the bubbler flow is reset to zero.
Pick a number of data points at approximately uniform intervals, covering
the range from immediately after the concentration begins to drop off
(beyond Err) to slightly beyond halfway between Vo and Vf. We will call this
data set V(t). In this regime, as seen in
Figure 3-11 and Figure 3-12 on page
3-28 , the following relationship holds, where Td and Tc are process dead
time and time constant respectively;
V(t) = Vo + (Vf - Vo) exp-(t - Td)/Tc
20 Define a variable Y(t), such that,
Y(t) = In(Vf - Vo) - In (V(t) - Vo) = t/Tc - Td/Tc