June, 15, 1942
ANALYTICAL EDITION
The load leads are connected to the primary of the transformer. and the secondary is connected directly to the coils of the bell. The bell and clapper are cut off. the interrupter contacts are removed. and a piere of smooth rubber is cemented to the nrmature, as shown in Figure 3. The pressure-adjustment tube and the manometer are of convenient size. and are made, mounted, and connected as shown in Figure 3. A dry-ice trap is used to collect vapors and prevent them from entering the vacuum pump.
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Determinations are carried out in the same manner as in the standard A. 8, T. M. distillation procedure, exceptthat the system is under reduced pressure. Uniform and reproducible pressures as low as 2 mm. can be obtained, as sho\Tn by Table 1. Readings were taken during the distillation of 1oo-ml. portions Of a large sample of methyl oleate at three different pressures.
A Study of the Electric Hygrometer R . S. EVAA-S A N D J . E. DkVENPORT Research Bureau, Consolidated Edison Company of New York. Inc., Brooklyn, 'I.Y.
I
N A previous paper (5) on the determination of water in
insulating oil, use was made of the electric hygrometer developed by Dunmore (3). This device appeared promising as a rapid field method for determining water in oil, and was likely to have other general applications wherever small amounts of water were a vital factor in industrial or laboratory operations. It seemed important, therefore, t o study the mechanism by which the electric hygrometer functioned. The effects of such factors as time of approach to equilibrium, temperature, inert gas pressure, mass of mater present, mass of hygroscopic salt on the unit, and resistance of the unit were studied. During the course of the study, a modified design of the hygrometer unit was developed and has been used throughout in the experimental results given below.
3Iodified Hygrometer The hygrometer unit described by Dunmore consisted of KO.38 AWG palladium wire wound on an aluminum cylinder coated with polystyrene and with a polyvinylacetate film containing different percentages of lithium chloride, A similar unit was constructed in this laboratory and tried out in the determination of water in oil, without further investigation of its characteristics. It was calibrated by employing oils of a known water content as determined by the combustion method (4). After several weeks of successful operation, the behavior of the coil became erratic. It was felt that the movement of the wire in the film due to the solvent action on the polyvinyl acetate by the oil vapors, especially by the chlorinated hydrocarbon type of insulating oil, may have contributed t o this phenomenon. Accordingly, a new hygrometer was constructed. A No. 20 S. T. Pyrex male ground joint with 5-em. tubulations on b o t h e n d s y a s selected. The closed tubulation from the smaller end of the ground surface was dipped in paraffin and threadlike bifilar markin s in the paraffin exposing the gfass were made in a lathe. The exposed glass was etched for 30 minutes in a sulfuric-hydrofluoric acid mixture. After removal of the paraffin, the entire glass cylinder with its spirally etched grooves was coated with commercial platinizing solution and heated to a dull red appearance. After cooling, the cylinder was sandpapered, whereupon the platinum was removed from the ridges, resulting in two spiral threads of platinum baked on the glass 0.062 em. apart and 90 cm. long. The finished coil had a diameter of 1.5 em. and was 2.5 em. long. A substitute for glass sealed FIGURE1. MODIlead-in wires was effected by paintFIED ELECTRIC HYGROMETER ing two stripes on the ground surface
of the male joint and extending them to the end of the other tubulation (Figure 1). At these points copper wires were soldered on with ordinary soft solder. The hygroscopic film was put on by merely immersing the coil in an alcoholic solution of the desired salt, allowing the alcohol to evaporate in the air. The removal of an impaired film could thus very simply be accomplished by immersion in water. Before application of the salt film, the coil was subjected to 100 volts direct current to reveal any possible electrical bridges or other imperfections. Measurements were carried out with 3 volts alternating current on the coil. Later it was found that almost all the conduction took place in the first quarter of the coil. Therefore, the recommended length of the coil is one fourth that given above.
Relation between Resistance and Water Vapor Pressure A knowledge of the relation between the water vapor pressure and resistance of the coil was obtained through a study of the behavior of the coil under three experimental conditions-namely, an approximate mass of water vapor, the pressure of which was varied by changing the volume with a mercury piston; the humidity of saturated salt solutions; and the formation of water from the explosion of a known volume of a hydrogen-oxygen mixture. I n Figure 2, a schematic diagram of the combined apparatus is shown. In buret B a prepared known mixture of hydrogen and oxygen is stored and introduced into the explosion pipet, A . The combination manometer-buret containing mercury had an inside diameter of 15 mm. in order to reduce to a minimum the sticking of the mercury column which was read with a cathetometer. The water vapor volume was chan ed by evacuating or applying pressure to the reservoir at the %ase. The entire apparatus was evacuated to a pressure of less than one micron at the start of an experiment and the hygrometer unit chamber at D was isolated by means of the three-way stopcock. The following data on a single quantity of water were obtained: (1) mass of water from the known hydrogen-oxygen mixture and a check of this value with that obtained from the product PV; (2) distribution of the water from (1) between the vapor space and the lithium chloride film; (3) partial pressure of water in equilibrium with the hygrometer unit; and (4)resistance of the hygrometer unit.
In the changing volume method, a suitable quantity of water vapor was introduced into the buret and, after the pressure and volume were read, was expanded into the previously evacuated hygrometer cell chamber. The difference between the calculated reduced pressure and the measured reduced pressure enabled one to estimate the micrograms of n-ater absorbed on the coil. The results are shown in Figure 3 for two hygroscopic salts, lithium chloride and potassium acetate. The observed mater vapor pressure where the sharp rise in the curve occurred corresponds to that of a saturated solution of the particular salt as given in International Critical Tables (6). Thus, in the vertical section of
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2r-4
a a e
al
I
a
d I
F
FIGURE2.
G
D~AGRAMMATIC SKETCHOF APPARATCS
A . Explosion pipet B. Hydrogen-oxygen mixture C. Buret and manometer D. Lithium chloride tube or electric hygrometer E. Dry ice trap F Degasifier and porous disk C . Dry ice trap and capillary
the curve there is present on the hygroscopic film a mixture of served on the 1 per cent lithium chloride coil as on the 0.5 salt hydrate and saturated solution. I n order that the water per cent lithium chloride coil. vapor pressure may remain constant (or nearly so) in this SATURATED SALT SOLUTIONS,I n these experiments a region on increasing the volume, water immediately passes small amount of solid salt was introduced into a side arm of from the coil into the surrounding space as is illustrated in the cell chamber, followed by the addition of 1ml. of saturated the lower curves-i. e., micrograms on coil os. pressure. The salt solution. The assembly was evacuated and placed in a attending increase in resistance may be caused by a reduction constant-temperature bath. I n Figure 4 the results are in the thickness of the saturated liquid film or by the formashown a t a temperature of 20 * 0.02" C. It was hoped that tion of isolated patches of liquid. this procedure would yield a rapid and accurate means of I n the more nearly horizontal section of the curve, no solid calibrating and checking the coil. However, in a closed system containing two liquids of different vapor pressure-i. e., phase is present and the film is an aqueous solution of electrolyte. The change in resistance in t,his section is caused by the film and the saturated salt solution-the transfer of water a change in the number of ions per unit volume. Support of through the intervening vapor space requires a long time this contention follows from a knowledge of the temperature before the resistance changes become inappreciable. The coefficient of conductance, which was found to be approximately that of TABLE I. RELATION BETWEEN PARTIAL PRESSURE AND WEIGHT OF WATER AND RESISTthe resistance readings became steady ANCE OF HYGROMETER in a relatively short time in this region, Volume of TemperaWater whereas there was always a drift obHz + 02 Hz ture Painitis, Pbiind Volume Hz + Oz PV Resistance .MI. % c. Mm. Mm. M1. Mg. Ohms. served in the constant-pressure section. Hygrometer 4 2.51 52.0 27.0 15.4 6.5 63.7 550 545 6460 Alcoholic solutions containing 0.1 2.42 52.5 27.8 14.8 6.7 63.6 925 900 5680 per cent lithium chloride gave resist2.58 52.5 28.0 15.3 7.1 63.6 990 935 5560 2.53 52.5 28.2 15.4 7.6 63.6 970 945 5365 ances above one megohm which were After 16 hours 27.8 7.8 5240 too high to be accurately measured on the alternating current bridge. There Hygrometer 12 2.51 52.0 26.5 15.2 6.1 63.7 965 935 4195 seemed to be no point in investigating 2.46 52.0 27.2 14.9 6.8 63.7 935 915 4100 solutions above 1 per cent, since it 27.0 5,9 4680 After 16 hours was desirable to maintain a minimum a Pinitis,= partial pressure of water before exposure t o lithium chloride of hygrometer unit. of water on the coil while operating in b PfinF, = partial pressure of water in equilibrium with lithium chloride corresponding t o the measthe variable-pressure zone. Approxiured resistance. mately twice as much water was ob-
ANALYTICAL EDITION
lune 15, 1942
509
oxygen should on explosion give a rapid and accurate method for obtaining small quantities of water. I n this manner a very small weight-for example, 10 micrograms of water-may be easily obtained, since a gas mixture may be accurately prepared containing a small percentage of hydrogen (1 ml. of a 1 per cent hydrogen in oxygen mixture yields 7.5 micrograms on combustion). While a t times there appeared to be some regularity between resistance and micrograms of water, erratic results were frequently obtained. I n all cases the partial pressure of water fell on a smooth curve when plotted against resistance, but this was not true when the micrograms of water were substituted for the partial pressure. The data given in Table I illustrate these conclusions, as well as the correlation between the Water measured by exploding a known mixture of
VAPOR
FIGURE3.
PRESSURE
OF
WATER
IN
MU,
RELATIONBETWEEN SALT COSCEXTRITIOX, PRESSURE, AND RESISTANCE OF COIL Temperature, 27 * 1' C .
WATER
supercooling of the saturated salt solution on evacuation, as well as the long time required for steady resistance readings, made it desirable to change to the third method of producing a known weight of water vapor. I n Figure 4 (different ordinate scale) as in Figure 3 the sharp rise in the ciirre was about t o take place a t a pressure corresponding to that of a saturated solution of lithium chloride. EXPLOSION OF HYDROGEN-OXYGEN MIXTURES.I n the determination of water in oil one is chiefly concerned with number of micrograms of water in a definite quantity of oil. -4measured volume of a known mixture of hydrogen and
VAPOR
2000
PERCENT RELATIVE HUMIDITY
FICrURE 5 . REL.4TIOK BETWEEN O F ROOM ME.4SURED HUMIDITY -4SD RESISTANCE O F COIL
VAPOR PRESSURE OF W4TER IN MM
FIGURE
4.
RESIBTANCE-m*ATER
T'.4POR PRESSURE
RELATION AT 20' C. Water vapor pressure controlled by saturated salt solutions
hydrogen and oxygen and measured by the product of pressure and volume. Although the differences between the two measurements were always in the same direction, the values are not known with sufficient accuracy to compute the quantity of water which is adsorbed on the glass surface. It was concluded that the hysteresis of the etched surface with its lithium chloride aqueous film was responsible for this anomalous effect. The past history of the film apparently determined the equilibrium between the water adsorbed and the partial pressure of water. Thus a different partial pressure would result on the addition of a fixed weight of water, depending on whether the previous experiment was one involving a larger or smaller weight of water Hysteresis phenomena involving water vapor have been recorded in the literature for many substances. The partial pressure of lvater vapor in equilibrium with cellulose is pro-
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nouncedly different on the adsorption curve than on the desorption curve. Recently, the hysteresis phenomenon connected with quartz and water vapor was described by Barrett, Birnie, and Cohen (@, and was shown t o consist of a rapid reversible adsorption and a slow non-reversible sorption. Unfortunately, the hygrometer could not be heated to a sufficient temperature to restore the surface each time to zero water content because of the probable dislocation of the conductors. However, if, as has been demonstrated, the resistance of the coil depended on the partial pressure of water, for a n unlimited supply of water vapor, disregarding sudden changes in humidity, the hygrometer unit should behave satisfactorily as a humidity indicator. I n Figure 5 is shown a comparison between the readings of a commercial spring hygrometer and a sling hygrometer and the resistance of the electric hygrometer a t room temperature over a 2-week period. I n some cases a discrepancy of as high as 6 per cent in relative humidity was observed between the sling hygrometer and the "Airguide" hygrometer.
Temperature and Inert Gas Pressure
-4 field instrument which is not greatly influenced by moderate changes in temperature would have many advantages over 2 temperature-compensated device. For this reason the major portion of the work has been carried out in the variable-pressure zone of the coil. I n this region, the temperature coefficient of resistance was relatively small (cf. Figure 6). I n the constant-pressure zone, the effect of temperature was often confused by the previous history of the coil and was very much greater in magnitude. 500,0001
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Electrical Resistance Bridges The laboratory resistance measurements were carried out on either a direct or alternating current bridge, depending on the magnitude of the resistance. An alternating current measurement is preferred, in order to reduce polarization effects to a minimum. The direct current bridge was a modified General Radio megohm bridge (1) with an external direct current galvanometer, l-mm. deflection of which was equivalent t o 0.01 microampere. An off-balance of 0.5 per cent can be detected in the range 1 to 100 megohms and progressively lower sensitivity at higher resistances. The alternating current bridge was a typical Jones and Joseph bridge (7) iyith a cathode ray tube used conveniently as a null detector with three stages of amplification. All measurements were made at 1000 cycles. An off-balance deflection of one part in 100,000 could be detected. A bridge suitable for use in the field should present no problem. Other Hygroscopic Compounds Unless otherwise stated, all the work reported in this paper was carried out using lithium chloride as the hygroscopic salt. It is apparent that for humidities lower than that corresponding to the vapor pressure of the saturated salt solution, a different and unreliable relation exists between resistance and water vapor pressure. The continuous drifts in resistance observed a t very low water vapor pressures were caused by the time required for the water to pass into the vapor space. I n addition to potassium acetate (Figure 3), some work mas carried out on 0.1 per cent aqueous phosphoric acid solution, but the hydrates of phosphoric acid are none too definite and i t was feared that any neutralization of the acid would cause a continued variation in resistance. A definite hydrate whose saturated solution has a very low vapor pressure was desired in attempting to estimate the water content of an oil sample by measuring the aqueous partial pressure above the oil sample. However, two additional factors-namely, the slowing up of the attainment of equilibrium by the dissolved air in the oil and the effect of small temperature changes on the oil-water vapor equilibrium-must be considered.
Conclusion lW000
I VI 0
5 Y
z 0
+
I
'O'OWI
I
I
I WI
MICROGRAMS Of WATER
FIGURE6.
EFFECTOF TEMPERATURE ON
RESISTAXEOF COIL
It has been shown experimentally that the resistance of the electric hygrometer, if without hysteresis, may be expected to follow the water vapor pressure in a regular manner except at a water vapor pressure below that of the saturated solution of the salt used as the electrolyte. By selecting the desired saturated solution, the electric hygrometer or a device of similar principle could be adapted to the control of humidity in industrial operations. Because of hysteresis, there is little possibility that the electric hygrometer can be used for water-in-oil determinations where one is interested in the mass of water in a closed space and not the vapor pressure of water. Acknowledgment
The curves a t the two different temperatures tend to become parallel to the abscissa as the water vapor pressure of the fdm approaches that of pure water. The curves also intersect, which indicates that there are two opposing effects at play. First, the increase of temperature brings about a decrease of resistance of electrolytic solutions, and secondly, at the higher temperature, the film possesses a higher vapor pressure and therefore water must pass from the coil to the surrounding space with attending increase in resistance. The effect of inert gas pressure appears to be one of delay of the attainment of equilibrium. Diffusion of water vapor to and from the coil is inversely proportional to the pre,ssure. At 1-micron inert gas pressure there is no measurable delay. At 2-cm. inert gas pressure, the time required for equilibrium is approximately 20 minutes
The electrical bridges employed in this work were constructed in this laboratory under the direction of TV. F. Davidson, director of research.
Literature Cited (1) Balsbaugh, IND. ENQ.CHEM., 31, 318 (1939). (2) Barrett, Birnie, and Cohen, J . Am, Chem. SOC.,62, 2839 (1940). (3) Dunmore, F. W., BUT.Standards J . Research, 23,701 (1939). (4) Evans, Davenport, and Revukas, IND.ENG.CHEM.,A N 4 L . ED., 11. --, 553 - - - 11939). . (5) Ibid., 13, 589 (1941). (6) Intern. Critical Tables, Vol 3. (7) Luder. J . Am. Chem. SOC.,62, 89 (1940). PREEENTED before t h e Conference on Electrical Insulation, Divislon of Engineering and Industrial Research, National Research Council. a t Wlliamsburg. V Y .
