An Improved Electrical Hygrometer and Its Applications* FRANCIS W. DUNMORE National Bureau of Standards, Washington, D. C.

ABSTRACT a hysteresis effect caused by the absorption of The electric hygrometer consists of an alu- by glass and other materials originally minum tube coated with polystyrene and used as bases. The polyvinyl acetate forms a wound with a bifilar winding of bare palla- porous binder for the lithium chloride which dium wire. The unit is then coated with a gives stability and uniformity to the units thin film of polyvinyl acetate with an amount without appreciably decreasing their rapid of lithium chloride in it depending upon the rate of response. range to be covered. The electrical With several units- of different sensitivity in resistance of the film between the two coils parallel, with resistors in series with each, is a function of humidity. If appreciable from 10 to 100% may be measured changes are involved, a correc- using a single scale of an indicating instru- tion is applied. ment. The thin aluminium tube enables the unit Electrical hygrometer units have been made to quickly assume the temperature of the air which have not varied more than 2 to 3% as it must if measuring relative humidity. over a period of several months. The palladium wire prevents an increase in Flight tests with two carried resistance caused by a film which forms on by the same balloon, one using the electric the surface of other types of wires. The hygrometer and the other the hair type hygro- polystyrene forms an excellent water-resistant meter, showed marked changes in humidity surface of high electrical resistance for the recorded by the electric hygrometer which the wire and water sensitive film, which prevents hair unit failed to register. I. INTRODUCTION HE PSYCHROMETER has long- been becomes progressively less at increas- a common means of determining" ing altitude. T the moisture content of air. There In a previous paper (*) an elec- are, however, many conditions for trical type of hygrometer was de- which this method is not well adapted, scribed for use with the , two particular cases being the meas- which, subject to certain precautions, urement of upper-air humidities by gave some promise of satisfying the means of the radiosonde and at the requirements for a remote humidity wing surface of aircraft. When re- indicating device without moving mote indication or recording of hu- parts or appreciable lag. midity is required, or when meas- Recent research has substantially urements must be made in small con- overcome the difficulties encountered fined spaces or at low , in the early work, resulting in a unit the psychrometer method is not prac- which now appears to be definitely tical. The hair-type hygrometer is superior to the hair-type hygrometer, now generally used, but its slow rate especially for radiosonde use. It also of response and other peculiarities has advantages over the psychrometer have made its use undesirable in method under many other conditions many instances. In radio-meteor- of measurement. The original fea- ography, the large time lag in re- tures of rapid rate of response and sponse of the hair hygrometer intro- inherent variation in resistance with duces serious errors, since sudden change of humidity have been re- humidity changs are often encoun- tained. tered by a rapidly ascending balloon. The method of using the electrical This lag increases with decreasing hygrometer in the radiosonde and an temperature with the result that the analysis of results obtained in routine accuracy of this type of hygrometer operation are described elsewhere. ( ) II. CONSTRUCTION DETAILS

(1.) Single-Element Unit: — The *Publication approved by the Director of electric hygrometer unit is constructed the National Bureau of Standards of the U. S. Department of Commerce. A preliminary on an aluminium tube about % inches paper appeared in the BULLETIN, for June 1938 ; part of the present paper has appeared in diameter, and 1 11/16 inches long in the Nat. Bur. Stand. Jn. Res., Dec. 1939, with a wall thickness of 0.01 inch. pp. 701-714, but new material is added here. —Ed.

Unauthenticated | Downloaded 10/06/21 09:17 AM UTC 250 BULLETIN AMERICAN METEOROLOGICAL SOCIETY [Vol. 21 The thin wall is necessary as the unit uniform film, units may be more easily must quickly attain the temperature duplicated in production. of the air surrounding it in order to The terminal end washers are made register the true relative humidity. of polystyrene resin to prevent leak- This is especially important in use age currents which might occur with with the radiosonde and on aircraft other materials due to adsorption of where sharp temperature changes water. are often encountered. The electrical resistance of the The outer surface of the tubing is polyvinyl acetate-lithium chloride film covered with a 0.01 inch coating of between the two palladium wire wind- polystyrene resin. This polystyrene ings is a function of the moisture con- coating forms a water-repellent sur- tained in the film. This moisture in face of high electrical resistance upon turn is proportional to the relative which is wound a bifilar coil of pal- humidity of the air in contact with ladium wire and a moisture-sensitive the film. The unit may, therefore, be film deposited. The water-resistant calibrated with relative humidity in properties of the polystyrene prevent terms of resistance. it from adversely influencing the functioning of the device and intro- (2) Multiple-Element Unit for ducing hysteresis effects. The bifilar Covering the Full Humidity Range coil of palladium wire is wound 20 without Switching Means:—If an at- bifilar turns per inch on the poly- tempt were made to cover the full styrene resin surface. It consists of humidity range with a single unit, 32 inches (about 25 bifilar turns) of the variation in resistance of the unit No. 38 AWG soft-drawn palladium would be so extreme that it would wire. This type of wire remains free extend outside the normal limits of a from any surface film or oxide which measuring device. would introduce errors in the indi- In the case of the radiosonde the cations of the unit. Resistance variation for a change in The moisture-sensitive film is ap- humidity from 10 to 100% from plied to the unit after it is wound + 30°C to —30°C should not vary with the palladium wire. This film outside the limits of 3 megohms to consists of partially hydrolized poly- 10,000 ohms. When used in an a-c vinyl acetate containing a small circuit such as with a Weston model- amount of lithium chloride, depending 764 microfarad-meter (calibrated to upon the range in the humidity scale read in ohms) the resistance varia- to be covered by the particular unit. tion for a change from 10 to 100% The lithium chloride is the water- humidity should be confined within sensitive element, the polyvinyl ace- the limits from 8 megohms to 75,000 tate forming a hygroscopic binder ohms. for it in the form of a thin film. This In order to make possible the above film readily passes with- condition for radiosonde use, a mul- out producing appreciable lag in the tiple-unit design with different per- response of the unit. The polyvinyl centages of LiCl coatings on each unit acetate containing the lithium chlo- was devised. By arranging three ride flows evenly on the smooth poly- units in parallel with 1%, 2%, and styrene resin surface thereby distrib- 3% lithium chloride coatings respec- uting the lithium chloride uniformly. tively, with the proper resistors in Lithium chloride and water alone series with each as shown in Fig. 1, would form small globules. With the it is possible to confine the resistance

Unauthenticated | Downloaded 10/06/21 09:17 AM UTC LEGENDS FIG. 1. Circuit arrangement and frequency- humidity characteristics for a three- element electric hygrometer as used in radiosonde. variation of the composite unit to the required limits.

For use in an a-c circuit a 5-ele- FIG. 2. Five element electric hygrometer circuit ment unit as shown in Fig. 2 has been arrangement with humidity-resistance characteristic (24°C). found necessary. In such an arrange- ment, unit A, Fig. 2, would have a On the other hand, at low humidities, polyvinyl-acetate coating only, B a A, B, C, and D are practically open- coating of 0.25% LiCl, C 0.5% LiCl, circuited while RI, R2, RS, R4, and R5 D 1.0% LiCl, E 2.2% LiCl. R5 is are not sufficiently high to prevent the greater than R4, R4 than R-$, etc. At change in resistance of unit E from the high humidities, units B, C, D, affecting the overall resistance at F. and E, although practically shorted, Thus E is the controlling unit at low are prevented from causing a short at humidities. In a like manner, units terminals F by resistors R2, Rs, R4, B, C, and D become the controlling and Rr,, yet the resultant resistance at unit in turn at intermediate humid- F will vary through limits below the ities. See Section IV-2-a, below. values of the parallel circuits of R2 R3 When mounting units in the 5-ele- R4 and R5 since unit A has resistor Ri ment arrangement, all materials used only, in series with it. Thus A is the should be highly water resistant. controlling unit at high humidities. III. OPERATING CHARACTERISTICS IN DIRECT-CURRENT RADIOSONDE CIRCUITS As the radiosonde developed by change the calibration of the unit ap- Diamond, Hinman, and Dunmore (3) preciably during the period of a flight. passes pulsating direct-current (1) Three-Element Unit:—For use through the electric hygrometer, the in the radiosonde circuit developed characteristics of the unit in such a for the Navy Department (2), a three- circuit differ somewhat from those element unit has been found satisfac- obtained when it is used in a-c cir- tory. It consists of three units con- cuits. Some polarization exists in d-c structed as outlined under II-2. Fig. circuits, but it is a fixed factor in a 1 shows this circuit arrangement, given type of circuit and does not while Fig. 3 shows the units mounted

Unauthenticated | Downloaded 10/06/21 09:17 AM UTC line graph in this figure is for the composite arrangement of the three units at 24°C connected as shown at A. (3) Temperature Correction:—The electric hygrometer functions like an electrolyte in that its resistance in- creases with a decrease in tempera- ture. In radiosondes where tempera- ture variations of 100 °C are encoun- tered it is of course necessary to apply a correction to obtain the true humid- ity. It has not been found possible, even with a composite 3-element unit, to cover the full 10-100% humidity scale over this extreme temperature range, but it has been feasible with the composite unit shown in Figs. 1 and 3 to cover a considerable portion of this range. The family of graphs determined empirically for different temperatures shown in Fig. 4 consti- tutes the standard. These are based FIG. 3. Three-element electric hygrometer in radiosonde mounting. on a panel which slides into the sun shield of the radiosonde. The units are coated with a 1%, 2% and 3% lithium chloride-polyvinyl acetate so- lution, respectively. The three units are connected in parallel with 32,500 ohms in series with the 2% and 3% units. (2) Frequency-Humidity Character- istics:—In the radiosonde circuit, the variation in resistance of the electri- FIG. 4. Graphs for evaluating true humidity cal hygrometer alters the audio-fre- from observed humidity (frequency) quency generated by a special audio- and temperature. oscillator which in turn modulates on the' average of many composite the carrier-frequency oscillator. This units taken at various low tempera- audio note is received on the ground tures and having individual units with where it is recorded on a graphical characteristic at 24°C as shown in frequency recorder. This record may Fig. 1. Knowing the temperature (as be interrupted in terms of humidity. is the case in most instances), the In Fig. 1 are shown in dotted lines the humidity is obtained from the fre- humidity-frequency calibration graphs quency-humidity graph for that tem- at 24°C for the individual 1, 2, and perature. 3% LiCl units when used separately If the method previously outlined in the radiosonde circuit. The full- for making and coating the units is

Unauthenticated | Downloaded 10/06/21 09:17 AM UTC carefully followed, it is possible to For use at varying temperatures obtain about 75% of the units which other than with the radiosonde, a will fall within ±3% of the standard multi-scale indicating instrument may graph. be used—see Section V-3.

IV. OPERATING CHARACTERISTICS IN A-0 MEASURING CIRCUITS (1) Circuit Arrangement:—For use quent readings are necessary, the hy- other than with the radiosonde, the grometer should be placed in the in- electrical hygrometer should be used put circuit of a suitable electron-tube in an alternating current as polariza- amplifier with the indicating or con- tion is thereby eliminated. In the case trol device in the output circuit. In of the radiosonde, this is not a serious such a circuit the current through the factor as it does not alter the opera- unit can be kept below 2 or 3 micro- tion of the hygrometer sufficiently to amperes which is probably within a change the calibration during1 jthe safe limit. period of time (1 hour maximum) in The capacity between the two wires which the unit is used. An a-c cir- running to the humidity unit should cuit arrangement consists of putting be kept as small as possible by separ- the unit across about 100 volts 60 ating them,. If appreciable, a circuit cycles alternating current with 0.75 arrangement described under section megohm and the input of a full-wave V-l should be used. copper-oxide rectifier in series with it. (2) Humidity-Resistance Character- The d-c output of the rectifier is con- istics. nected to a 0-125 microammeter. The (a) Single-Unit Characteristics: microammeter scale may be calibrated —Using such a circuit arrange- in terms of percent relative humidity. ment with the microfaradmeter cal- The voltage impressed on this cir- ibrated to read in ohms instead of cuit arrangement should of course microfarads, the humidity-resist- be kept at 100. The above circuit ar- ance characteristics shown in Fig. rangement is that obtained when 5 were obtained. Here graphs B C using the scale of a Weston 1000 Model-764 capacity meter, the elec- trical hygrometer taking the place of the condenser to be measured. This capacity meter may be used as an ohmmeter in this fashion since it is measuring only ohmic resistance in a non-reactive circuit. This circuit ar- rangement was adopted as it passes alternating current through the unit and this current is never greater than FIG. 5. Humidity-resistance graphs for five individual units (at 24°C) coated as 200 microamperes. The current must shown and used in the Weston Model- be kept under this value as heavy 764 capacity meter circuit. currents would alter the calibration. to E are for standard units with a The measuring current should not be 0.25%, 0.5%, 1%, and 2.2% lithium- allowed to flow through the unit con- chloride coating respectively. Graph tinuously, but only for the time ne- A is that obtained with the poly- cessary to take a reading. In the vinyl acetate film alone without case of control purposes or where fre- LiCl.

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The coatings on these units were gohm, R3 = 0.35 megohm, R4 = 1 so chosen that each unit covers a dif- megohm, R5 = 3 megohms, with A ferent range of humidity when vary- coated with polyvinyl acetate only, ing over the same resistance. B with 0.25% LiCl, C with 0.5% (b) Composite-Unit Arrange- LiCl, D with 1% LiCl, and E with ment :—While individual units as 2.2% LiCl. The resistances re- described above should find some quired will vary somewhat with the applications (See: V, Design for variation in the range of response Special Uses), it is evident from of different units with the same Fig. 5 that no one unit will cover percentage coating. The humidity the full humidity scale. However, resistance graph for the above com- by putting the units A to E (Fig. bination is shown in Fig. 2. With 5) in parallel each with its proper such an arrangement the microam- series resistor as shown in Fig. 2, meter scale may be calibrated to it is possible to obtain in effect a read in terms of relative humidity single unit which covers the hu- instead of ohms. Several compo- midity range from 10 to 100% at site units have not varied more room temperatures, with resistance than 2 or 3% from an original cal- variation confined within the limits ibration, over a period of 8 months. of 8 megohms to 75,000 ohms. A These units were kept indoors and satisfactory combination for Fig. 2 measured weekly in a humidity is: Ra = 75,000 ohms, R2 = 0.15 me- chamber.

V. DESIGNS FOR SPECIAL USES A hygrometer of the type described megohm resistor at the location of the is very flexible in its application since humidity unit and run the d-c output it is of the electrically operated type of the rectifier back to the location and may be modified to fit specific where the humidity is to be registered. needs. This method is necessary as the alter- If one is interested in a limited hu- nating current flowing across the midity range, as for example in the capacity of a long line to the humidity case of automatic control of humidity, unit would register on the indicating a single-element unit is desirable as meter. greater sensitivity may be obtained. (2). Battery-Operated Unit:—For See graphs in Fig. 5. It must be em- portable use and in places remote phasized, however, that the control from a 60-cycle a-c supply a measur- current cannot be allowed to flow ing circuit may be used in which the through the units continually. An alternating current for operating the electron-tube amplifier may be used unit is obtained from an electron-tube as described under IV-1. Replacement oscillator operating at 60 cycles. It units are inexpensive and may be may be powered from small dry cells easily plugged into circuit after a such as are used in portable radio given number of hours. sets. A dual triode may be used, one (1). Design for Use Where cu-c triode furnishing the 60 cycles and Power is Available:—In cases where the other forming an amplifier with it is necessary to run long lines from the hygrometer in the input circuit the humidity indicating device to the and indicating meter in the output humidity unit, it is necessary to locate circuit. the full-wave oxide rectifier and 0.75 (3). Multi-Scale Unit for Condi-

Unauthenticated | Downloaded 10/06/21 09:17 AM UTC tions:—For registering humidity un- rangement for use at room tempera- tions! For registering humidity un- tures may be used in which individual der a condition of wide variation in units with different sensitivities are temperature, such as on aircraft, a connected to the measuring instru- multi-scale indicator might be used. ment (Weston Model-764 capacity Knowing the temperature, the humid- meter set at minimum capacity set- ity scale for that temperature is used. ting) by means of a 5-point switch. A second type of multi-scale ar-

VI. RADIO SOUNDINGS USING THE ELECTRIC HYGROMETER electrolytic into the radiosonde transmitter circuit. Alter- nate traces of humidity and tempera- ture (as a function of ) are thereby recorded in terms of frequency, which are con- verted to humidity and temperature from suitable graphs. This flight rec- ord, typical of hundreds which have been obtained, shows the rapid rate of response of the electric hygrometer as well at its operation at tempera- tures down to at least —40 °C. Note that the humidity scale is much more open at high temperatures than at low temperatures. The dotted lines were added to the humidity record to facilitate interpretation. While no marked temperature in- versions are shown in this record many other electric hygrometer records have shown that such inver- sions are almost invariably accom- panied by abrupt humidity changes. Fig. 7 shows such a correlation. The deficiency in the operation of the hair-type hygrometer is strikingly illustrated in Fig. 7. This figure shows graphs plotted from the flight records obtained from two separate FIG. 6. Typical radiosonde flight record show- ing rapidity of response of the electric radiosondes, each carried by the same hygrometer. balloons, one having the hair-type In Fig. 6 is shown a typical radio- hygrometer and the other the electric sonde flight record obtained with the type. Below —13 °C the hair unit electric hygrometer. As the balloon merely indicates minor changes in ascends a calibrated switch moved by humidity while the electric hygrome- changes in atmospheric pressure oper- ter shows that large changes actually ates a relay which alternately con- occurred. This is typical of four nects the electric hygrometer and other dual records similarly obtained.

Unauthenticated | Downloaded 10/06/21 09:17 AM UTC Acknowledgement:—The author ex- P. Friez and Sons, Inc. for their co- presses his appreciation to the Julien operation in this work.

FIG. 7. COMPARISON OF THE RESPONSE OF THE ELECTRIC AND HAIR-TYPE HYGRO- METERS EACH IN A SEPARATE RADIOSONDE. BOTH RADIOSONDES CARRIED BY THE SAME BALLOON.

REFERENCES (3) H. Diamond, W. S. Hinman, Jr., and F. W. Dunmore: A method for the in- (1) Dunmore, F. W.: An electrical hygro- vestigation of upper-air phenomena and meter and its application to radio mete- its application to radio-meteorography. orography. National Bureau of Standards J. Research NBS, vol. 20, pp. 369-392, J. Research, vol. 20, pp. 723-744, 1938 1938, (RP1082). (RP1102) ; Bulletin Amer. Meteorol. Soc., Id, Development of a Radio-meteorograph June, 1938, pp. 225-243. System for the Navy Dept., Bulletin (.2) Diamond, H., Hinman, W. S., Jr., Dun- Amer. Met. Soc., March, 1937, pp. 73-99. more, F. W. and Lapham, E. G. An im- Improvements and experience in radio proved radio-sonde and its performance. soundings, by H. Diamond, W. S. Hinman, To appear in the National Bureau of Jr., A. H. Mears, and C. Harmantas. Standards Journal of Research, Aug. or Journ. of the Aeron. Sci., vol. 6, pp. 379- Sept., 1940. 383, July, 1939.

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