Buck 1011C, Руководство по эксплуатации

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Since proper operation of a condensation hygrometer depends strongly on the condition of
the mirror surface, the following discussion of the effects of various types of contaminants
may be helpful.
Particulate Contaminants. Particulate matter which is insoluble in water may accumulate
on the mirror surface, but usually will not affect the instrument accuracy until the mirror
reflectance is reduced substantially, and in many cases, will improve instrument response
by providing condensation sites.
Water Soluble Materials. Materials such as naturally occurring salts, which readily
dissolve in water, are detrimental to accurate vapor concentration measurement by any
condensation method. These materials readily go into solution with the water condensate
on the mirror surface, then reduce the vapor pressure in accordance with Raoult’s Law. As
the concentration increases with time, the saturation vapor pressure of the liquid solution
decreases, and the control loop responds by elevating the mirror temperature to maintain a
vapor pressure that is in equilibrium with the partial pressure of the water vapor in the
atmosphere. The displayed dew point, therefore, drifts upward with respect to the true dew
point. Because the measurement error increases gradually, it often goes undetected. This
condition is most easily detected by noting the indicated dew point, cleaning the mirror,
balancing the detector, then measuring the dew point again. If the new reading is lower
than the first reading, there is probably soluble material present in sufficient quantity to
cause a measurement error.
Gaseous Contaminants. If a gaseous material were present which had a higher
condensation temperature than that of water, even if present in very low concentrations,
the system control loop will eventually control on that material rather than on water. The
system then displays the condensation temperature of that material, not water. Such
material would accumulate on the mirror only while chilled. Fortunately, while such a
situation could arise in an industrial environment, it is normally not a problem in the
natural atmosphere. An exception might be when flying through emissions from a power
plant or industrial smokestacks.
Minimizing Effects of Contaminants. In marine environments, or other situations where
contaminant levels are unusually high, one may reduce the rate at which contaminants
accumulate on the mirror by reducing flow and/or by increasing condensation thickness.
4.2.3 Supercooled Dew Points
When making measurements in the frost point region of 0 to –40
°
C, there is the possibility
that the mirror is actually controlling on the supercooled dew point, which is 0.1 to 4
°
C
below the actual frost point. (See Appendix 1 for frost point/dew point conversions.) This
is a relatively improbable situation, since dew cannot exist indefinitely below 0
°
C, and will
sooner or later transform to ice.
To guarantee that the mirror is controlling on frost rather than supercooled water, switch S-
1 to MAX COOL briefly to force cool the mirror several degrees lower than its previous
reading. Upon release of the MAX COOL switch, the mirror may attempt to overshoot