Brooks Instrument 5861i - Manual (Page 33)

4-9
Installation and Operation Manual
X-TMF-5861i-MFC-eng
Part Number: 541B111AAG
November, 2008
Brooks ® Model 5861 i
Section 4 Maintenance
Where,
P1 = percentage (%) of gas 1 (by volume)
P2 = percentage (%) of gas 2 (by volume)
Pn = percentage (%) of gas n (by volume)
Example: The desired gas is 20% Helium (He) and 80% Chlorine
(Cl) by volume. The desired full scale flow rate of the mixture is 20
slpm.
Sensor conversion factor for the mixture is:
N
2
equivalent flow = 20/0.946 = 21.14 slpm
It is generally accepted that the mass flow rate derived from this equation
is only accurate to ±5%. The sensor conversion factors given in Table 4-3
are calculated based on a gas temperature of 21°C and a pressure of one
atmosphere. Gas conditions that vary widely from these reference
conditions may cause an additional error due to the change in specific heat
due to temperature and/or pressure.
4-6 Restrictor Sizing
The restrictor assembly is a ranging device for the flow sensor. 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 (micron porosity and active area)
have different pressure drops and produce meters with different full scale
flow rates. For a discussion of the interaction of the various parts of the
meter, you are urged to review Section 3-1 (Theory of Operation).
If the restrictor assembly has been contaminated with foreign matter, the
pressure drop vs. 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 flowmeter is to be calibrated to a new flow rate.
Restrictor assembly replacement should be performed only by trained
personnel. The tools required for the removal/replacement procedure are
as follows:
- Appropriate size wrench for the removal of the inlet process connection
and end block screws.
- Restrictor removal tool (Refer to Table 5-2)
- Restrictor O-ring, refer to the spare parts Section 5, for the correct part
number.