Validation of Trace Moisture Measurements
SCS Overview Manual
4
–
3
The permeation device connects to a “T” assembly between port 6 and 3 of the
six-way valve, refer to Figure 4–2 on page 4–4. During normal operating
conditions, a portion of the process gas return from the sample cell flows
through one end of the “T” and carries the excess moisture released from the
permeation assembly back to vent. When the system is switched to validation
(refer to settings for
Mode 7
in the Firmware Operator’s Manual), the six-way
valve changes positions allowing the dry process gas (flowing at 3000 sccm)
through the “T” in the opposite direction, carrying the mixed gas into the
sample cell.
The concentration of pollutant obtained in ppm by volume may be computed
using the following formula:
where:
•
C = concentration of ppm in volume
•
F = carrier gas flow rate in ML/minute at 1 atm and 25
C (77
F)
•
P = permeation rate of the G-CAL assembly in nanograms/minute at
the temperature of the G-CAL (environment temperature)
The entire flow system is maintained at constant elevated temperature
(typically 50
C to 60
C [122
F to 140
F]).The constant temperature not only
minimizes species adsorption/desorption and prevents condensation, but in
combination with the regulated sample supply pressure and controlled flow
rates, ensures a constant mixture of C
p
in parts per million (ppm) by volume.
K (Water) = 1.358
Figure 4–1
Schematic of permeation tube
MEMBRANE
WATER VAPOR
NH
3
C
KxP
F
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=
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