Automatic Frost Point Hygrometer STAKLET 11. .JURI' A N D WILL1 431 LICEIT, JR. Z-nioersity of Cincinnati, Cincinnati, Ohio An automatic instrument is described, which has been found satisfactory for the deterrhination of small amounts of moisture in air. It is applicable in air conditioning, food processing, control of noncondensable inert atmospheres in metallurgical processes, and meteorological investigations.
T
HE determination of small amounts of moisture in air is a familiar problem (1, 4 ) . Chemical methods, dew point h? grometers, infrared absorption instruments, and conductivity irl-truments are available Czepek ( 5 ) has reviewed dew point iwthods: ( A ) freezing out moisture and weighing, ( B ) closing an clwtric circuit by the formation of a deposit (limited to range of -40" to 200" C.), (C) change in radiation on a photocell at den point, (D) passing air through a tube or over a rod having a temperature gradient along the length, (E) determining dew point from the break in cooling rurve m hich appears after deposition, :ind ( F ) cooling a mirror to ohseive dew visually. ilt dew points down t o -90" F. and in a rapid succession of measurements it becomes futile t o consider chemical, infrared, and Czepek's methods -4,B, E, and F. Conductivity instruments, besides being eypensive, have still other limitations. During the course of drying experiments, methods C and D s e r e investigated. An automatic frost point hygrometer was developed and is the subject of the present report. GENERAL DESCRIPTION OF HYGROMETER
I block diagram of the automatic frost point hygrometer is qhown in Figure 1
-
Photocell
hiicioammeter
Pulse gcnerator
.I
Light
----f
.t
i
+-
Amplifier
I
Potentiometer
(copper bolt) Heater
Bridge f
Test
I &Id surface nnd thermocouple
t
Ammeter
-
I
4 Electronic
formed and very little diffuse light reached the photocell, which was so arranged that it could see the gold surface a t all times. If the frost point were a t some higher temperature, say -70" F.. frost formed slowly until some predetermined thickness (determined by presetting the electronic rheostat) was reached. While the frost built up, the photocell received more and more light arid the pulse generator produced electrical pulses at a correspondingly greater rate. At the same time the bridge converted greater amounts of energy, and the microammeter so indicated. The amplifier simplv magnified the pulses and impressed them on the electronic rheostat, which was insensitive to these pulses until the predetermined setting had been reached. At this instant the rheostat passed current into the heater, the more the greater the frost deposit. The heat supplied by the heater incrclascd the temperature of the gold surface. As this temperature tended to exceed -70' F. (in the present case) the frost began to evaporate and the reverse cycle set in. The net result was that the instrument tended to regulate itself, so that the predetermined thickness of frost H B S always maintained on the surface and the frost point was indicated by the potentiometer. This qualitatively was the case, regardless of whether the frost point lav a t -70" F. or any other within the range of the instrument. T*hisis why it is claimed that the true frost point can be measured. At the true frost point the "frost-test air supply," as a system, must be in thermodynamic equilibrium. Frost must (macroscopically) neither form nor disappear. That this was so was confirmed by the constancy of the microammeter reading for a given test air supply. Quantitatively the thirkness of frost varied borne\\ hat over the range of the instrument and the extent of variation could be controlled by presetting the electronic rheostat. This variation does not affect any given frost point. Once the instrument had been set for automatic operation, the ammeter and microammeter were unnecessary For hand operation a manual rheostat iias substituted foi the amplifier and electronic rheostat. Under these circumstanc-es the microammeter and ammeter a e r e used t o determine equilibrium conditions necessary a t the frost point. A microammeter reading of about 100 at the frost point appeared to be most satisfactory
rheostat FROST POmT HYGROMETER
( c o p p r bolt)
Coolant Figure 1.
Simplified Diagram of Frost Point Hygrometer
A copper bolt with gold hrad as provided with a thermocouple and heater as sho\\n nt the loner left. The lower end of the bolt n a q immersed in a coolant ahich tended t o reduce the bolt temperature to about - 110" F The heater tended to rompensatc foi the coolant effects; thus the gold surface could he regulated to some intermediate temperature, depending upon the rate of heat qupplied. The temperature of the gold surface could be determined continuouslv by means of the potentiometer and the thermocouple which was attached to the lower side of the gold surface. I n order to determine the frost point of a test air stream, thc stream was blown over the gold surface, which was lighted as shown in Figure 1. Xormally a t the start of this operation the heater supplied no heat and in practice the equilibrium temperature of the gold surface was approximately -90" to -95" F. If the frost point of the test air supply n a s below -95" F. no frost 1 Present address, Department of Chemical Engineenng, Unlverslty of Tennewee. Knoxville. Tenn.
The frost point hygrometer was built in two separate unit
