Brooks Instrument 5851EM - Manual (Page 53)

4-15
Brooks ® Models 5965, 5851EM
Section 4 Maintenance
& Troubleshooting
Installation and Operation Manual
X-TMF-5965-5851EM-MFC-eng
Part Number: 541B122AAG
September, 2009
4-5 Restrictor Sizing
The restrictor assembly is a ranging device for the sensor portion of the
controller. It creates a pressure drop which is linear with flow rate. This
diverts a sample quantity of the process gas flow through the sensor. Each
restrictor maintains the ratio of sensor flow to restrictor flow, however, the
total flow through each restrictor is different. Different restrictors (active
area) have different pressure drops and produce controllers with different
full scale flow rates. For a discussion of the interaction of the various parts
of the controller, you are urged to review Section 3-3, Theory of Operation.
If the restrictor assembly has been contaminated with foreign matter, the
pressure drop versus flow characteristics will be altered and it must be
cleaned or replaced. It may also be necessary to replace the restrictor
assembly when the mass flow controller is to be calibrated to a new flow
rate.
Restrictor assembly replacement should be performed only by trained
personnel. See Section 4-3, Disassembly and Assembly, for restrictor
removal and installation.
Restrictors
The Model 5965/5851EM mass flow controller/device uses one type of
restrictor assembly depending on full scale flowrate and expected service
conditions.
1. Sintered element for nitrogen equivalent flow rates from 10 to 100
slpm
Sizing
All Model 5965/5851EM Series Restrictor Assemblies are factory adjusted
to provide a specific pressure drop for each flow rate. This corresponds to
the desired full scale flow rate. A list of restrictor assemblies used in the
Model 5965/5851EM mass flow controller/device is shown in Table 4-6.
Example:
The desired gas is Cyanogen (C
2
N
2
).
The desired full scale flow rate is 30 slpm.
Sensor conversion factor is 0.499 from Table 4-5.
Nitrogen equivalent flow = 30/0.499 = 60.1 slpm Nitrogen