5 computing the system’s time constant, 6 calculation example, 2 how to precisely tune a control loop – INFICON Composer Gas Concentration Controller User Manual
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Composer Operating Manual
3.2.1.5 Computing the System’s Time Constant
This Instrument System operates at 1 Hz in the Amplitude Track Mode; that is,
it provides a new measurement of concentration every second. Now, how fast
does the Reactor respond to a change? It is determined by the gas flow rate
through the Reactor’s Delivery Line and the volume of the Reactor’s gas
Delivery System between where the change is induced and its entry into the
Reactor’s chamber. The volume within the Reactor’s Chamber will also extend
this basic time constant but is complexly affected by the reactor’s flow design.
Keep this basic time response in mind as you set the system’s time constant
and remember that nothing will happen faster than this basic response time;
see
for a quick calculation.
3.2.1.6 Calculation Example
A TMI delivery line has a flow rate of 150 sccm. The volume of the Delivery
System is defined by 2 meters of 1/4" (6.35 mm) tubing and 100 cc of empty
“head space” above the Bubbler. The 2 meters of tubing has an internal volume
of about 40 cc, combined with the head space gives a total volume of 140 cc.
In addition, the Transducer’s Swept Volume is about 18 cc for a total volume of
158 cc. The basic time constant is then calculated: 158 cc/ 150 sccm = 1.053
minute = 63.2 seconds.
3.2.2 How to Precisely Tune a Control Loop
Here is a method to precisely determine process parameters for the
Composer’s control loop. This method is robust in systems with fairly wide
range of dead time and time constants and is quite accurate. The method
requires data logging and off-line calculations. The Monitor software, with its
data logging and manual gas control capabilities, would be a very convenient
interface. A simplified but less accurate method is later described in
3.2.3, How to Determine Process Parameters, on page 3-28
.
1
Start Monitor software in Amp Track mode.
2
Select Show Advanced Data in Setup >> Data Display menu.
3
From the Setup >> Process Parameters menu option, set appropriate
MFC parameters, gas parameters and total gas flow.
4
If total flow is to be conserved, the DIP SW#5 on CFGR2 must be set.
Connect MFC control cables to the back of the instrument.
5
Evacuate the system.
6
Press BYPASS button to start dilution gas flow at predetermined rate and
wait until pressure stabilizes.