4-8
Brooks
®
Model SLA5840
Section 4 Maintenance
& Troubleshooting
Installation and Operation Manual
X-PR-SLA5840-Series-RevB-RT-eng
Part Number: 541B201AAG
October, 2017
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 a 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.
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. Consult factory/Service Center.
Sizing
All SLA5840 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 SLA5840
mass flow controllers/meters are shown in Table 4-2
Example:
Select a restrictor for a SLA5840
The desired gas is Silane (SiH4).
The desired full scale flow rate is 200 sccm.
Sensor conversion factor is 0.625 from Table 4-1.
Sensor Nitrogen equivalent flow = 200 x1.000 = 320 sccm Nitrogen
0.625
In the example above a Size P restrictor would be selected from Table 4-2.
Note: If the calculated flow rate is such that two different size restrictors
could be used, always select the smaller size.
If a mixture of two or more gases is being used, the restrictor selection
must be based on a Nitrogen equivalent flow rate of the mixture.
Example:
Select a restrictor for a SLA5840
The desired gas is 20% Helium (He) (Sensor factor = 1.386 and 80%
Chlorine (Cl
2
) (Sensor factor = 0.876) by volume. The desired full scale
flow rate of the mixture is 20 slpm. Sensor conversion factor for the
mixture is:
Sensor Nitrogen equivalent flow = 20 x 1.000 = 21.16 slpm Nitrogen.
0.945
In this example a Size 4 Wire Mesh Assembly would be selected.
