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Ratio Control Option Appendix F
The constant ratio-control option (“-RC” in the model code), provides the user with the ability to set and
maintain an exact ratio of gas flow between two (or more) mass flow controllers (MFCs). The control
box accomplishes this by scaling the setpoint to the slave controller(s) from the output of the master
controller. Thus, even though the ratio can be initially set up without flow, actual operation of the slaves
will not occur until a gas flow is established through the master. If the master inadvertently stops
flowing (the gas runs out), the slave(s) will close. Thus this feature provides a constant ratio control
when gas is flowing correctly, and a safety shut off for the slaves if the master gas runs out.
1. Channel one is typically set up as the master channel and should be cabled to the master
controller (see the label on the control box to confirm that channel one matches the range of the
master controller)
2. Channel two is typically set up as a slave channel (the channel that you want to follow the master)
and should be cabled to the slave controller. If more than one slave is being used, additional
channels may be set up at the factory. (It is convenient but not necessary to scale the slave
channel(s) in percentage rather than actual engineering units)
3. With the front-panel switch in the “Normal” position, each channel of the box will function
independently to supply command signals to the separate controllers. In the “Ratio” position,
channel one will be the master and the slave channel(s) will follow the lead of the master, using its
output signal to establish a setpoint(s).
4. If the constant-ratio feature is to be activated:
a. Begin with the front panel switch in the “Normal” position. (Flow is not needed at this point
though it can be done with or without it – the controllers don’t even need to be connected
to the cables)
b. Switch the channel dial to channel two (the slave channel). Calculate the percentage at
which you want this slave controller to follow the master. With the Set/Read switch in the
set position, dial in the flow that corresponds to this percentage.
Example #1: The master is scaled for a flow of 0-350, and the slave is also scaled for 0-350.
You desire to have the slave follow the master at a 100% flow ratio. In “Normal” mode, switch
the Set/Read to “Set” and dial in a setpoint of 350 to channel two (it makes no difference what
channel one is doing at this point). Since the two controllers are scaled the same, they will
always flow the same rate.
Example #2: If you desire channel two to follow the master at a ratio of 30%, set channel two
to 105 (30% of it’s full scale range).
Example #3: The master is scaled for a flow of 0-350, and channel two scaled for percentage
(0-100). If you desire channel two to follow the master at 65% of the slave’s capacity, set
channel two to 65.
Example #4: Master channel scaled for 0-350, slave scaled for 0-200. You want the slave to
follow at 40% of its own capacity. In “Normal” mode, set channel two to 0.4 X 200, or 80.
Now, when the master is flowing 350, the slave will flow 80, and when the master is reduced to
a flow of 175, the slave will reduce to a flow of 40.
c. Switch the Normal/Ratio switch to “Ratio”. Now, your ratio feature is scaled and the box is
ready to use for a constant ratio application.