Table of Contents
Advertisement
1-6 | 5830 Moisture Analyzer
Zero gas module
The zero gas module, shown inside of the dotted rectangle in Figure 1-2,
is used to provide a dry gas stream at the inlet of the analyzer. Function-
ally, the zero gas module is a very simple device, consisting of only three
diaphragm valves and a dryer. The diaphragm valves used in this device
are UHP grade components; all surfaces are electropolished, and the gas
connections are welded. Because of the high quality of the surfaces in
these components, residual moisture retention is minimized. Therefore,
the assembly is capable of rapidly achieving a dry condition once the ze-
roing cycle has been initiated.
Under normal operation of the analyzer, both DV1 and DV3 remain
closed to protect the dryer, while DV2 is open to allow the flow of sam-
ple gas into the analyzer. When performing the zeroing operation, DV2
is closed, while both DV1 and DV3 are open. Thus, the flow of sample
gas is diverted through the dryer, providing a dry reference (<10 PPBV)
for the analyzer. By providing this dry reference gas as an external chal-
lenge to the analyzer, the zero gas module enables the offsets present in
the system (e.g., electronic offsets, offsets caused by pressure differences
in the different flow paths, and offsets caused by low-level outgassing of
water vapor from upstream valving) to be measured. Normally, the mag-
nitude of these offsets is relatively small—less than the equivalent of 0.1
PPMV. However, as the intention of the Model 5830 analyzer is to have a
detection limit of only 0.02 PPMV, these minor effects must be accurately
determined and removed from the moisture measurement.
Zeroing the Analyzer
The zero process is used to correct any offsets in the sample system. By
comparing the two zero readings (reference and sample) with the inter-
nally generated "zero" gas, any offset can be zeroed out. This is done by
measuring the zero offset frequency and storing it in memory. This offset
is then subtracted from Δƒ before the sample moisture concentration is
calculated.
AMETEK recommends performing a zero every 24 hours for the first
month of operation. After this start-up period, the analyzer will have
dried down sufficiently to increase the zeroing interval to one week.
AMETEK recommends that the duration of the zero cycle be set to
NOTE
120 (i.e., 2 hours).
Advertisement
Table of Contents
