I am using a dry bulb/wet bulb humidity controller on an environmental chamber now. I find it to be quite reliable, due to it's simplicity. But I have highly accurate thermocouple temperature measureing equipment available, I have deionized water available to keep the little wet bulb sock clean, and a permanent fan blowing on the wet bulb. If these things are not handy, then dry/wet bulb becomes a messy hassle. Go with a commercial sensor if you have any money available at all. -- Lawrence ----- Original Message ----- From: "Vasile Surducan" To: Sent: Friday, November 02, 2001 1:51 AM Subject: Re: [PIC]:HUMIDITY SENSOR. > Dry/wet temperature measurement methode have the disadvantage of > great errors. > The name of the tools using this principle is psychometer, or absorption > psychometer. And the error cames from response time of the wet temperature > sensor and the problem of keeping clean the lithyum-clorure sock on this > sensor. Also if you don't have a fan which continuously absorbe the > unknown air on the sensors your humidity meter will show a false > measurement. > > Vasile > > On Thu, 1 Nov 2001, Thomas McGahee wrote: > > > Humidity is probably best measured using a technique that measures > > what is called the "relative humidity". > > > > In this method you compare the temperature of a "dry" temperature > > sensor and a "wet" temperature sensor. > > > > The "dry" temperature sensor is simply exposed to the air, which > > will have a humidity content that we wish to measure. The "wet" > > temperature sensor has a piece of gauze around it, and the bottom > > end of the gauze resides in a container that supplies water. The > > water will wick up the gauze and will provide the equivalent of > > 100% humidity. The two sensors will report the exact same temperature > > only at 100% relative humidity. The lower the relative humidity, the > > greater the difference between the two readings. > > > > There are tables available that will tell you what the relative > > humidity is across various temperature ranges. A look up table > > or a mathematical algorithm can be used to convert the two > > temperature readings into a relative humidity reading. > > > > ************ > > If you want to use a sensor that changes capacitance with humidity, > > then my capacitance meter project may be useful: > > http://www.redrival.com/mcgahee/ > > You would not need all the switching ranges in your application. > > > > Another useful method is to make a little RF oscillator whose > > frequency is a function of the capacitance of the humidity > > sensor. You then convert frequency or period into humidity > > by means of a lookup table or mathematical algorithm. You can > > often help to stabilize such a setup by including a small > > fixed capacitance in parallel with the capacitance to be > > measured. > > > > Fr. Thomas McGahee > > > > ----- Original Message ----- > > From: moshe > > To: > > Sent: Thursday, November 01, 2001 5:40 AM > > Subject: [PIC]:HUMIDITY SENSOR. > > > > > > > Hi everyone. > > > trying to implement humidity sensor type:H1-PHILIPS, to my pic 16F84. > > > I build a capacitor to frequency converter ( arround the 555 > > multivibrator), > > > but the results are poor. > > > does anyone have any good/new idea about humidity measuring, application > > > note etc.?? > > > > > > thank you for advanced. > > > moses. > > > > > > -- > > > http://www.piclist.com#nomail Going offline? Don't AutoReply us! > > > email listserv@mitvma.mit.edu with SET PICList DIGEST in the body > > > > > > > > > > -- > > http://www.piclist.com#nomail Going offline? Don't AutoReply us! > > email listserv@mitvma.mit.edu with SET PICList DIGEST in the body > > > > > > -- > http://www.piclist.com hint: To leave the PICList > mailto:piclist-unsubscribe-request@mitvma.mit.edu > > -- http://www.piclist.com hint: To leave the PICList mailto:piclist-unsubscribe-request@mitvma.mit.edu