Configuring the MKS 649 Compact Pressure Controller

In the 649 Pressure Controller, the Baratron® Pressure Transducer measures absolute pressure. Full Scale ranges of 10, 100, or 1000 Torr are available. Each 649 can control pressure from Full Scale to less than 2% of Full Scale. Prudent design suggests choosing the lowest possible Full Scale for the application, taking into consideration the overpressure to which the sensor may be exposed (both normal and accidental).

Please Note: This procedure is provided as a reference guide to sizing the orifice for most typical applications. To assure proper orifice size selection for the specific application conditions, particularly those where the procedure results in an orifice selection near the limit lines in the graph, please contact MKS Applications Engineers for assistance in selecting the proper valve orifice.

The flow through any orifice depends on the size of the orifice, the inlet and outlet pressures, and gas density. To simplify 649 Pressure Controller orifice selection, use the following procedure:

1. On the Index Number Table in Figure 1, choose your inlet pressure from the column of pressures on the left–the pressure that will be applied to the inlet of your 649. (Note that the values are absolute pressure.)
   
   

   Figure 1 - Index Number Table
   Next, from the row of pressures at the top of that table, select your differential (delta) pressure – this is the inlet pressure minus your outlet pressure. Locate the Index Number – where your selected row and column intersect.
2. If you are using N₂, skip to step #3. For other gases, calculate the Density Correction Factor (DCF) with this formula:
   
   
   
   Multiply this Density Correction Factor times the Index Number found in step 3, to determine your density-corrected Index Number.
3. Go to the Orifice Selection Graph (Figure 2) and locate your Index Number along the bottom axis.
   
   

   Figure 2 - Orifice Selection Graph
   Draw a vertical line at your Index Number. This line will intersect with the Max. Flow Rate lines for available valve orifices. Choose the orifice whose maximum flow rate exceeds your requirements.

The flow meter is simply sized.

1. Determine Full Scale flow rate and gas. If gas is N₂, skip to #3.
2. Divide the Full Scale flow rate by the thermal gas correction factor of the gas of choice relative to configuration.
3. Choose the flow rate whose flow just exceeds/equals the equivalent N₂ requirement.

Type 649 Control Valves should not be used for positive shutoff. Where positive shutoff is required, a separate valve should be installed.

When selecting the location of an external shutoff valve, consideration should be given to the maximum pressure rating of the internal transducer and to the possibility that leakage across the internal valve over time can build up and result in a sudden surge of gas.

This information subject to change without notice.