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A fast responding electric hygrometer

1955; US government; Volume: 55; Issue: 2 Linguagem: Inglês

10.6028/jres.055.008

2376-5305

Arnold Wexler, Samuel B. Garfinkel, Frank E. Jones, Saburo Hasegawa, Albert Krinsky,

Sensor Technology and Measurement Systems

The performance characteristics of a fas t resp onding elec tric h yg rom ete r of high sen it ivity are presented in som e detail.The hygrom et er is made by evaporat in g, under vac uum, potassium m etaph osphate t o obtain a thin film of materi al (3 microinches thick) on a specially treated glass blank.The logarithm of the r es istance is approximately linear with relative humidity at constant t emperature.The resistan ces vary from 4x10' t o 1013 ohms.For the temperature range of -40 0 t o + 40 0 C, a group of nearly para llel calibration curves is obtained.The manufacturing process is s ufficiently well co ntrolled so tha t h ygromete r elem ents, made in different batches, closely conform t o a m ean calibration curve.The average h yst eresis for the hy grom eter element over the t emperature range 0 0 to 40 0 C is 2.7 per cent relati ve humidity.The p olari zatio n effect for reversed direct curre nt loading (1 minute cycles) is of t he order of 0.3 perce nt relative humidity at the end of two hours; it is 1. 4 to 2.2 perce nt for in te rmi tte nt direct curre nt loading (15 seconds on, 45 seconds off) ; a nd it is 3. 7 to 6. 8 perce nt for co ntinuous direct curre nt loading.Elem e nt s aged by humidi ty cyclin g appear to be less subj ect t o polarization.F ollowing a di screte chan ge in r elati ve humidity, t he average response time of the clem e nt for a 63 perce nt change in indica tion a t -20 0 C, wi th a n a ir veloc ity of 800 feet per minute para llel to t he long edge of t he eleme nt, is 1. 9 seco nds for a relative humidity cha nge fr om 41. 1 t o 82.3 perce nt (with respect to wa ter) ; it is 3.4 seco nds for t he sam e relat ive humid ity change in the reverse directi on.Times for 90 percen t cha nge in indi cation a re 5.0 a nd 16.4 seco nds, res pecti vely .At room temperature, t he a verage resp onse time, for a 63 perce nt chan ge in indication, with a veloc ity of 768 feet per minu te, is 0.10 seco nd for a di scret e change in rela ti ve humidity from 33 to 80 percent, and 0.42 second for the same chan ge in the reverse directi on.Corresp onding t imes for 90 perce nt change arc 0.74 a nd 11. 2 eco nds.In co mparison wi t h lithium chloride elem ents currently produced for radiosonde usc, t he res po nse t ime of t he elem ent was found to be less by a t least an order of mag nitude.