Brooks Instrument 5861E - Manual (Page 30)

4-10
Model 5861E
Section 4 Maintenance Installation and Operation Manual
X-TMF-5861E-MFM-eng
Part Number: 541B107AAG
November, 2008
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. The specific heat of most gases are not strongly pressure and
temperature dependent, however 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 sensor portion of the
flowmeter. 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 full scale 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.
Restrictor removal tool (contained in service tool kit P/N S-778-D-017-AAA)
Restrictor O-ring, refer to the spare parts Section 5, for the correct part
number.
The Model 5861E Mass Flowmeter utilizes porous metal restrictor
assemblies for all flow rates.Restrictor elements with porosities of 40 to 60
microns are used in different combinations. Up to three restrictors
elements can be placed in one assembly. These restrictors are assembled
by pressing the porous metal elements into the header plate.
When sizing a restrictor assembly for gases other than nitrogen, the
sensor conversion factor must be used to calculate a nitrogen equivalent
flow rate. Use the following equation to obtain the desired flow in nitrogen
equivalent units. Refer to Table 4-3 for the conversion factor list.
N
2
equivalent = desired gas flow
sensor conversion factor