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Dew Point Hygrometer with Constant Resistance Humidity Transducer

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Dew Point Hygrometer with Constant Resistance Humidity Transducer Utah State University DigitalCommons@USU All Graduate Theses and Dissertations Graduate Studies 5-1969 Dew Point Hygrometer With Constant Resistance Humidity Transducer Curtis B. Campbell Utah State University Follow this and additional works at: https://digitalcommons.usu.edu/etd Part of the Life Sciences Commons Recommended Citation Campbell, Curtis B., "Dew Point Hygrometer With Constant Resistance Humidity Transducer" (1969). All Graduate Theses and Dissertations. 2957. https://digitalcommons.usu.edu/etd/2957 This Thesis is brought to you for free and open access by the Graduate Studies at DigitalCommons@USU. It has been accepted for inclusion in All Graduate Theses and Dissertations by an authorized administrator of DigitalCommons@USU. For more information, please contact [email protected]. DEW POINT HYGROMETER WITH CONSTANT RESISTANCE HUMIDITY TRANSDUCER by Curtis B. Campbell A thesis submitted in partial fulfillme nt of the r e quirements for t he degree of MASTER OF SCIENCE in Meteorology UTAH STATE UNIVERSITY Logan, Utah 1969 ii TABLE OF CONTENTS Page LIST OF FIGURES iv A BSTRACT vi INTRODUCTION LITERATURE REVIEW 2 Historical note 2 State of the art 2 Limitations of current instruments 18 Desirable specifications for future work 21 Wate r in the atmosphere 23 Lithium chloride as a hygros copic a ge nt 27 Definition of the l ithium chl oride problem 31 ELECTRICAL AND HYGROSCOPIC PROPERTIES OF LITHIUM CHLORIDE 33 Some observations . 33 Electri cal r es istance of lithium chl oride as a f unction of solution concentration 33 Electrical resistance of lithium c hloride solution as a function of temperature 39 Hygroscopic charac t e ristics of dry lithium c hlorid e . 42 Summary of r esistance facto r s 47 A n a lysis and comparison 47 DEFINITION OF HYGROMETER PROBLEM 51 P e rformance characte ristics desired 51 Functional characteristics t o b e de velop ed 51 HYGROMETER DESIGN 55 Introduction 55 Discussion 55 iii TABLE OF CONTENTS (Continue d) Page Adjustine nt 60 Humidity transducer 62 CALIBRATION AND RESPONSE 65 Test method s and calibration 65 Hygrometer response .. 67 SUMMARY AND COMPARISON 76 FUTURE WORK 79 LITERATURE CITED . 80 APPENDIX 83 iv LIST OF FIGURES Figure Page l. Saturation vapor pressure of wat er over a p lane surface of water as a function of temperature 25 2. Plot of wate r vapor pressur e ove r aqueous lithium chloride as a function of temperature 28 3. Comparative plot of water vapor pressure over water and over aqueous lithium chloride as a function of temperature 30 4. Photograph of two drops of lithium chl oride solution 34 5. Schematic of test setup for measuring the e lectrical r esistance of lithium chloride as variation of water content takes place 36 6. Plot of variation of electrical resistance of lithium chloride as concentration of solution varies 38 7. Schematic of test setup for measuring the electrical resistance of a lithium chloride surface as a function of temperature when enclosed in a constant dew-point environment . 40 8 . Plot of variation of e lectrical resistance of lithium chloride film as a function of temperature 42 9. Photograph of dry lithium chlorid e from which wat er has been evaporated . 45 10. Plot of the wetting characteristics of lithium chl oride salt in a 0 C dew -point environment 46 11. Block diagram showing functional parts required in the hygrometer with arrows indicating the direction of power flow and control 54 12. C-R Hygrome ter schematic showing func tional d esign and parts list . 57 v LIST OF FIGURES (Continued ) Figu r e Page 13. Photograph of humidity transducer detai l s and C -R Hygrome t e r . 6 1 14. C-R Hygrome t e r characteristic curve showing relationship of hygromete r transducer t emper­ ature and the d ew point of the envi ronment in which it is operating 66 1 5 . Warm up r espons e curve for C -R Hygrometer w ith the humidity transducer in a 0 C d e w -point e nvironment . 68 1 6. Warm up r espons e curve for C - R Hygrometer wit h the humidity transducer in a 25 C d ew -point e nvironment . 69 17 A . Transie nt r esponse curve for C -R Hygrometer with humidity transduce r in a 0 C dew-point e n vironme nt 7 1 17B. Transient r esponse curve for C-R Hygrometer w ith the humidity transduce r in a 0 C d e w - point e nvironme nt 71 18 . Transie nt r espons e curve for C-R Hygrometer with the humidity transduce r in a 14 C d ew - point e n v ironment . 72 19. Transie nt r espons e fo llowed by oscillations in lithium chloride phas e transition hygrometer 73 20. R ecove ry from oscillations in lithium chloride phase transition hygrometer 75 Vl ABSTRACT Dew Point Hygrometer with Constant R e sistance Humidity Transducer by Curtis B . Campbell , Master of Science Utah State University, 1969 Major Professor: Dr. Gaylen L. Ashcroft De partment: Soils and M e teorology The hygroscopic and electrical resistance characteristics of lithium chloride are investigate d. Then an evaluation is made of the lithium c hloride characteristics that would be suitable in develop- ment of a dew point hygrometer with measurement accuracy of + 0. 3 c. The d eve lopment of a 12 volt battery-powered hygrometer is presented with circuit details and performance characteristics. Lithium chloride phase transition hygrometers tend to oscillate under certain conditions. A damping control is presented as part of the circuit details and recordings of the response characteristics and e ffectiveness of damping are presented showing effective control of the oscillations with a wide range of humidity transi e nts. (93 pages) INTRODUCTION While studying snow pack conditions with Dr. B e n L. Grove r in central Utah, an interesting moisture transport phenomena was noted. On many bright spring days snow was melting at lower mountain levels; whereas, the moisture content of the snow was actually increasing at higher elevations even though the skies were cloudless . A study was initiated t o elucidate the causes of this phenomena . To carry out the moisture transport study, it was necessary to measure the moisture status of the air as it moved up the mountain s lope . Attempts to measure this atmosphe r ic moisture, howeve r, were fraught with many difficulties . Dew-point measurements with we t and dry bulb psychrome ters we r e quite unsatisfactory because the temperatures were often below freezing. A dewce ll was tried but was not satisfactory because the necessary electrical power was not available and the accuracy was less than required. These experiences pointed out the n eed for a piece of e quipme nt that was accurate and had low electrical power r equirements so that it could be operated from batteries in remote areas. The d evelop­ ment of a dew point hygrometer for field use offered an interesting challenge and was selected as a thesis problem. 2 LITERATURE REVIEW Historical note The hair hygrome t e r is the first recorded instrume nt for measuring the amount of moisture in the air. About the year 1500 Leonardo da Vinci described a device whereby the quantity of moisture tn the atr was noted from the change in weight of a ball of wool or hair. In 1783 De Saussure d eve lo ped the hair hygrometer into its present style, i.e., measurement of changes in hair length. Since that time the hair hygrometer has probably been the most widely used instrument for humidity measurement and control. Numerous in- novations have b e en tried, but after over 1 75 years the hair hygro­ meter is essentially unchanged; more is known about it technically, but attempts to improve it have b een ineffective {Davey, 1 965 ). State of the art Mechanical !lygj:ometers. Hair and other mechanical hygro­ meters operate on the principle that absorption and d e sorption of wat e r from the air by an organic material causes a physical change in the material: change in weight, change in le ngth, change in volume, or a rotational effect. Through suitable simple lever systems these dimensional changes may b e made t o actuate a pointer or move a pen across a c hart. They can be calibrated in t erms of relative humidity 3 by a ppr opriat e t e c hnique s (B e ye r s, 1959, p. 7 0) . The main vi r tue s of t h e mechan ical h ygr omet e r are it s s impl ic­ it y of d e sign a nd c onstruction and its low cost. It indicat e s r e lati ve humidity di r e ctly ove r a mode rate range of t e mpe rature s . Its r e l i ­ abi l ity d ec r e as e s with d ecrea s ing t e mpe rature . Its chief defect s a r e a lack o f s t a bi lity u nder normal conditions of us e and an a p p r e c ia ble hyst e r esis. Some o f t he m ate r ials that ha ve b e en us e d a s the var iable humidit y e l ements a r e ha i r , c o tto n , wo o l, s ilk, wood , goldbeate r 's ski n , n ylo n , plasti c, pape r , a nd whale bone .
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