MKS 246C, Instruction Manual

Page: 71 / 92

Chapter Four: Operation Manual Flow Control
61
How To Manually Control Ratioed Gas Flows
This type of operation enables you to control the flow rates of multiple 246 units as a fraction
(ratio) of the flow a master 246 unit, using an internal set point signal (refer to Figure 4, page
27). The master unit uses the in5V set point signal; the set point signals for the slave
units are controlled by the output of the master unit.
Note
The MFC that is set to control at the highest percentage of its rated flow
should be connected to the master 246 unit.
In the following example, we will assume a system with four 246 units, each with a 100 sccm
MFC flowing nitrogen (N
2
) gas. Units (Channels) 2 to 4 will be set to a ratio of 75, 50, and 25%
of the flow in unit (Channel) 1.
1. Set the SET POINT SOURCE SWITCHES for Channels 1 to 4 to the FLOW position.
This connects an in5 V reference to the top of the SET POINT CONTROL.
Since the +5 V corresponds to full rated flow, each Set Point Control can be adjusted up
to a maximum of 100% for the attached MFCs rated flow.
2. Ensure that the SET POINT CONTROL is properly calculated and adjusted for each
MFC in use.
Refer to step 8, page 57, for an example calculation. In this example, the SET POINT
CONTROLS for Channels 1 to 4 should be set to 100, 75, 50, and 25 respectively.
3. Leave the SET POINT SOURCE SWITCH for the master 246 unit (Channel 1) in the
FLOW position and move the switches for Channels 2 to 4 to the RATIO position.
The 246 unit is now adjusted to control the MFCs connected to Channels 2 through 4 to
75, 50, and 25% of the flow rate through Channel 1.
4. Turn the flow ON by placing the FLOW CONTROL SWITCH for all channels to the
ON position.
The set point signal is applied to the MFC, and flow begins after a slight delay. The
correct flow is realized within approximately 1.5 seconds, depending on the type of MFC
being used.
5. Turn the flow OFF by placing the FLOW CONTROL SWITCH for all channels to the
OFF position.