Arnold Wexler and Saburo Hasegawa. .' T he r. elat ive hll m idity-te mpc:at llre relat ionships h. ave b~e n determ ined in air in equih b llum wit h sat urated ~al t solu t IOns of llt hlUm chlonde, L1C[·H20 ; magnesi um chloride M gCL·6HzO ; . ~odlU~n dl c!lromat~, Na2Cr207 ·2H~O ; magnesium n itrate, Mg (N 0 3!2 ·6H 20 ~ sodlU~ chlonde, NaCI; amm OJ1Jum sulfate, (NH 4),S04; potassium nitrate, K N O; ; and potas8 1um r~u lfate , ~( 2S 04' ~v~r a. te mpera:t ure ran ge of 0° to 50° C, using t he d cwp oint method . 1h.e rel.atlve hlllJ1J. dlty r~s a con t muous func tIOn of te mperat ure, and , excep t for sodIUm chlor!de, IS mO~l otO I1JC. 1he curve fo r sodium chloride in creases from 74. 9-p crccn t Tela tJ v, e hlllnlC lt ty at 0 C~? a maximum of 75. 6 l)er ce n t a t 30° C a nd t hen g radu all y d ecr ease8 to 7~. 7 per cent. 1he. ~ax lmum change 111 reJ~tlve humidi ty with temper at ure, al? ou t 1.5-petcent re lative. h lllTIldlty as t he te m pe rature 111 creases f rom 0° to 50° C occu rs wIth saturated sal t solu tIO ns of sod ium dichro mate a nd m ag nes ium ni trate.'

In 1971 Wexl er and Greenspan p ublished a formulation for the vapor pressure of water encompassing the temperatu re ran ge 0 t o 100°C. In this paper a revision is m ad e of tha t earli er formulati on t o m a ke it consist ent with the defin itive experimental va lue of the vapor pre sure of ,mter at it s trip le po int recently obtained b y Gu ildn er, J ohnson, and J ones . The two formula t ions a re essenti a ll y identical at t emperatures from 25 to 100°C. F or t emperatures below 25°C the new formul at ion predicts values that a re higher than the 1971 formul ati on. At the triple point, the vapor press ure given by the new formula ti on is 611.657 P a '\'hereas the value given by the 1971 formul a ti on is 611.196 P a . A t a ble is given of th e va por pressure as a functi on of t emperature at O.l-deg intervals over the range 0 to 100°C on th e I n t ernational Prac tica l T emperature Scale of 1968, t ogethe r with values of the temperature d erivati ve a t 1-deg interv als.K eywords : Clap eyron equati on; saturati on vapor pressu re over water ; st eam; vapor pressure ; vapor pressure of wa t er ; virial coe ffic ients.

A new fonn ulation is presenl ed for Ih e vapor press ure of ice from Ihe triple point to -100°C based on thermodynamic calculations. Use is made of Ih e d efinilive ex pe rim e nlal value of the vapor pressure of waler a t its triple poinl recentl y oblain ed by Cuildner , Johnson, and Jones. A tab le is given of Ihe vapor press ure as a function of le mpe rature at O.I-d egree inl e rv a ls ove r Ihe range 0 10 -100°C, logel he r wilh Ihe values of Ih e lemperature derivative at I-degree inl e rvals . T he formu laLion is compa red with published ex perime nlal meas urement s and vapor pressure equaLi ons. It is eslima led Ih at Ihi s formu lalion predi cls Ihe vapor pressure of ice wit h an ove rall unce rtainl y Ihal vari es from 0.016 pe rce nl al the Iripl e poinl 10 0.50 percenl a l -100°C.K ey wo rd s: C lausius-C lapey ron equation; sa turation vapor press ure over ice; th e rm a l properti es of ice; vapor pressure; vapor pressure at th e triple point; vapor pressure of ice; waler vapor. Introd uctionIn meteorology , air conditioning, a nd hygrometry, parti c ularly in the maintenance and use of standards and generators in calibrations and in prec is ion meas ure me nts, accurate values of the vapor pressure of the pure wa ter-s ubstan ce are essential. Because of this Wexle r and Greenspan [1]1 rece ntly published a ne w vapor pressure fo rmulation for the pure liquid phase, based on th e rm ody namic calc ulations, whic h is in excell e nt agreement from 25 to 100 DC with the prec ise measu reme nts of Stimson [2]. Wexler [3 ] s ubsequently re vised this formulation to ma ke it consis te nt with the defi nitive experi mental value of the vapor pressure of water at its triple point obtained by Guildner, Joh nson, and Jones . The purpose of this present paper is to apply a similar method of calculation to the pure ice phase and derive a ne w formulation for temperatures down to -100 DC. This new formulation for ice is constrained to yield th e id e ntical value of vapor pressure at the triple point as that given by the revised formulation for the liquid phase.A critical examination of the experimental vapor-pressure data of ice discloses the disconcerting fact that the dispersion among those values far exceeds modern accuracy re quirements. This dispersion arises, in part , from the inhe re nt difficulties ex p eri e nced by investigators in making precision measureme nts of these low pressures and from th e ambiguities in the te mpe rature scale used in th e early 19 00's when several major series of determinations were made. Thermodynamic calculations, based on acc urate thermal data, provide an alternate route to th e determination of vapor pressure. It is therefore not surprising that s uc h calculations have been made repeatedly for ice with varying degrees of success . It is interesting to note that these calculations have been preferred over the existent experimental vapor pressures, primarily because the calculations appear to yield less uncertainty than the measurements . . DerivationThe Clau...

So me prec ise meas ure me nts of the vapor pressure of liquid water a t se ve n te mpe ratures in th e range 25 to 100°C we re re ported recentl y by H. F. S tirilson of NBS. These meas ure me nts have a n es tim a ted s ta nd a rd deviation of 20 ppm or less, exce pt at 25°C where the es tim a ted s tanda rd de viation is 44 ppm. We have derived a formula whi c h yie ld s co mpute d va lu es of vapor press ure ag ree in g with Stim so n's meas urem e nts to within 7 ppm.We integrate d the C laus iu s-C lapeyro n e quation using th e acc urate c alo rim etri c da ta of O s born e, Stim so n, a nd Ginnings a nd th e Goff an d Gra tc h formulation s for th e vi ri a l coe ffi cie nts of water vapor to o btain an equation that has a rational basis. Th is e qu a ti on was th e n adj us ted to bring it into close r accord with Stim so n's press ure meas ure me nts . Two tabl es are give n of th e vapor press ure, e xpressed in pascals, as a fun ct ion of te mp era ture at O. l -deg inte rv a ls ove r th e ra nge 0 to 100°C, one on th e Int e rnationa l Pract ica l Tempe ra ture Scale of 1948 a nd th e oth e r on th e Inte rn atio nal Practical T e m· pe ratu re Scale of 1968.Key word s : C laus iu s-C lapeyro n equation; sa turati on vapor press ure over wate r; vapor press ure; vapor press ure of wale r; water vapor; s team: viri a l coe ffi c ie nt s.

Th~ random (~ne standard deviation) uncertainty in the mean values of B aw, arising from the scatter of prevIOusly obtamed NBS values of the enhancement factor, is estimated to range from 0.7 percent a[ 30 ·C to 1.4 percent of 50°C. The estimated systematic uncertainties range from 4 percent at 30 °C to 6 percent at 50°C, respectively.