Low frost-point humidity generator

A low frost point humidity ge ne rator has been developed at NBS to provid e a ca pability for ca libration , tes ting a nd resea rch a t ver y low le vel s of wate r vapor co ntent in s uch gases as atmospheric ai r, carbon dioxide a nd nitroge n. Th e ge nerato r prod uces fros t poin ts from -30 to -100 C a t ambient pressu res f rom 500 to 200 ,000 pas cals (0.005 to 2 a tm.). T hi s is eq uivale nt to mixi ng ra ti os of 4 X 10 -6 to 51 gra m s of wa ter vapor pe r kil ogram of d ry air and to vapor press ures of 1.4 x 10 -3 to 38 pascals. Th e ge nerated test gas can be fed to a test c hamber with ind epende nt te mperatu re co ntrol bet wee n + 25 and -100 C. The un certa inty of th e frost point in the test cham be r is estim a ted not to exceed 0.05 deg C. Inte rco mparisons with a frost-point hygrometer as well as two grav im etri c c hec ks ga ve resu lts wh ic h in aU cas es agreed to within 0.2 deg C in fro st-poin t te mpe rature.