Delta Class SLA6900 UHP & SLA6800 Series Metal Sealed MFC's/MFM's
Installation and Operation Manual
X-TMF-SLA6900-SLA6800-MFC-eng
Part Number: 541B100AAG
August, 2009
4-7
Section 4 Maintenance
& Troubleshooting
P
n
= percentage (%) of gas n (by volume)
Example: The desired gas is 20% Helium (He) and 80% Chlorine (Cl
2
) by
volume. The desired full scale flow rate of the mixture is 20 slpm. Sensor
conversion factor for the mixture is:
Nitrogen equivalent flow = 20/.945 = 21.16 slpm Nitrogen
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 4-5 are
calculated based on a gas temperature of 21°C and a pressure of one
atmosphere. The specific heat of most gases is not strongly pressure, and/or
temperature, dependent. However, gas conditions that vary widely from these
reference conditions may cause an additional error due to the change in
specific heat caused by pressure and/or temperature.
4-4 Orifice Sizing
The Orifice Sizing Nomograph, Figure 4-2, is used to calculate the control
valve's orifice size when changing any or all of the following factors from the
original factory calibration:
gas
operating pressure (inlet and outlet)
flow range
The flow controller's orifice is factory-sized to a preselected gas, operating
pressure and flow range. Note that the orifice is marked with its size in
thousandths of an inch. When changing the aforementioned factors, calculate
the new orifice size by following the procedure and example outlined in the
following paragraphs.
Example: Determine the orifice size for the following conditions:
Gas:
Hydrogen
Flow Rate: 2,000 sccm
Outlet Pressure:
30 psig
Inlet Pressure:
50 psig
1. Determine Nitrogen equivalent flow rate (refer to Table 4-4).
OR
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