Brooks Instrument 5850EM - Manual (Page 65)

4-15
Brooks ® Models 5964, 5850EM
Section 4 Maintenance Installation and Operation Manual
X-TMF-5964-5850EM-MFC-eng
Part Number: 541B121AHG
November, 2008
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 5964/5850EM mass flow controller/device uses two types of
restrictor assemblies depending on full scale flowrate and expected service
conditions.
1. Wire mesh for Nitrogen equivalent flow rates above 3.4 slpm. These
restrictor assemblies are made from a cylinder of wire mesh and are
easily cleaned if they become contaminated in service.
2. Anti-Clog Laminar Flow Element (A.C.L.F.E.) - This type of restrictor
assembly is used for Nitrogen equivalent flow rates less than 3.4
slpm.
Sizing
All Model 5964/5850EM 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 5850EM mass flow controller/device is shown in Table 4-4.
Example:
The desired gas is Silane (SiH4).
The desired full scale flow rate is 200 sccm.
Sensor conversion factor is 0.625 from Table 4-3.
Nitrogen equivalent flow = 200/0.625 = 320 sccm Nitrogen