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1-6 | 3050-AMS Moisture Analyzer
Dew/Frost Point Measurements
Dew point values are calculated from the moisture concentration mea-
surements (PPMV) and the sample or process pressure. For temperatures
below 0 °C, the 3050-AMS uses equations to determine the water vapor
pressure over ice, rather than over super-cooled water. Thus, the 3050-
AMS reports a frost-point temperature consistent with the physical form
of the condensed phase in a real process stream.
Water in super-cooled liquid state will only exist as a transient species,
as the first bit of condensate starts to form on a smooth-chilled surface
(i.e., the true method of measuring a dew point).
Given more time, the dew layer will turn to ice if the temperature is
below the freezing point.
While the frost-point temperature will be higher than the dew point
temperature, the exact magnitude of the difference will be a function
of both the pressure and dew/frost point temperature.
Another consideration, when using the 3050-AMS analyzer to calculate
dew/frost point temperatures, is the sample or process pressure. The
range of process pressures allowed by the Model 3050-AMS is restricted
for many sample gases. Many gases liquefy at relatively low pressures,
so calculating a dew/frost point temperature for these compounds has no
value. Therefore, the 3050-AMS is designed to report an error condition,
the process pressure alarm, to alert the user to conditions that result in
ambiguous dew/frost point values.
Calculation of Dewpoint Values for Hydrocarbon
Streams
The calculation of dewpoint values for hydrocarbon streams is covered by
several standards. Because there are differences between the dewpoint
values calculated with the older North American standard (ASTM D1142,
which is based on the work published in IGT Research Bulletin #8) and
the new European standard (ISO 18453, which is based on the work pre-
sented in GERG-Water Correlation: GERG Technical Monograph TM14,
2001), AMETEK has provided a means of using either method in the
3050AM product. Specifically, in the gas list we have cited two separate
entries for natural gas. The first entry is labeled "Natural Gas", and will
provide dewpoint/frostpoint values consistent with ASTM D1142. The
second entry is labeled "Natural Gas GERG", and will provide dewpoint/
frostpoint values consistent with ISO 18453. All other aspects of analyzer
performance are identical for both of the two natural gas listings.
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