Article 1038416 of alt.home.repair: Path: news.misty.com!not-for-mail From: nicksanspam@ece.villanova.edu Newsgroups: alt.home.repair Subject: Re: calibrating a hygrometer Date: 9 Aug 2008 18:17:10 -0400 Organization: Villanova University Lines: 12 Message-ID: References: NNTP-Posting-Host: acadia.ece.villanova.edu X-Trace: max.inside.misty.com 1218320232 3339 153.104.44.130 (9 Aug 2008 22:17:12 GMT) X-Complaints-To: abuse@misty.com NNTP-Posting-Date: Sat, 9 Aug 2008 22:17:12 +0000 (UTC) Xref: news.misty.com alt.home.repair:1038416 We can measure the dewpoint very accurately by stirring a glass with some water and an ice cube with a good thermometer and noting the water temp when condensation starts to appear on the outside of the glass. Then find RH = 100e^(-9621(Ta-Td)/Ta/Td), where Ta = Troom(F)+460 and Td = Tdp(F)+460. For instance, if Troom = 70 F and Tdp = 50.5, RH = 100e^(-9621(530-510.5)/530/510.5) = 50%. The exponential function e^() is the inverse natural log (shift ln) on an Casio fx-260 calculator ($9 at Walmart.) Nick