Ametek 303B, User Manual

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Overview
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1-7
pressure. Volumetric flow measuring devices such as the bubble-o-meter
are not affected by different gas densities and will provide the correct
flow independent of density (at constant temperature and pressure).
Pressure
Since the flow controller maintains a constant flow, sample input pressure
changes do not affect instrument response. Pressure should be regulated
between 70 and 700 kPa (10 and 100 psi) gauge, but the instrument can ac-
commodate higher sample gas pressures if an external regulator is used to
reduce the pressure to the 70 to 700-kPa range. Use an external pump for
sample pressures below 70 kPa gauge.
Gas usually exits the system at atmospheric pressure. If the instrument is
operated with a back pressure, an appropriate correction must be made to
assure that flow through the instrument corresponds to 100 mL/min at 1
atmosphere and 25°C.
Temperature
A change in sample temperature does not affect the way the instrument
measures moisture. It does, however, affect gas density and moisture ab-
sorbed in sample system parts.
The unit is calibrated to operate at 25°C. If operated at a different tem-
perature, density change and the consequent change in moisture is based
on °K (= °C + 273).
Actual_Moisture • [(273 + 25)
Displayed_Moisture_Reading = ---------------------------------------------
(Act_temperature C + 273)]
The density change effect is about 0.33 percent/°C, but if the cell is cali-
brated with the bubble-o-meter with temperature compensation, all tem-
perature/density effects are again 0.
Other effects, a higher temperature may have is to release moisture held
in system components, even that trapped in stainless steel tubing. The
higher the tubing free carbon content, the higher is the moisture level it
can trap. Also, oils, grease, and other hydroscopic compounds coating
component walls can sorb and desorb moisture. As temperature rises,