User guide
2
Introduction
Figure 1: Model 235 plumbing schematic
The Model 235 has no valves to redirect the flows in a manner which would
provide a zero setting for this instrument. The dilution stream and carrier
streams pass through the mixing tee at all times.
Opportunities for setting and monitoring the flows for the diluent and carrier
MFC are provided in several screens on the touch panel controller. Instructions
for setting and monitoring these controls are covered in the “Initial Set-Up”
section of this manual, which begins on page 10.
Note: Excess span gas is wasted to the overflow vent to ensure that span gas is
delivered at near atmospheric pressure, for those analyzers which are sensitive to
sample feed pressure. The overflow vent is usually left open, but can be plumbed
to an external exhaust port. Any exhaust plumbing must be sized so there is no
increase in pressure into the analyzer under calibration.
CAUTION:
If the span gas is composed of a compound that is toxic,
hazardous, or in any other way a threat to life or safety at
the concentrations being generated, the overflow port must
be vented to an appropriate scrubber or abatement system
selected for the specific gas.
However, this can present challenges to proper operation of
this instrument; if the scrubber/abatement system is setup to
run with inputs at negative pressure, there can be conditions
in which sample can be “robbed” from the Span Out port.