What is used for sensing humidity?
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From: Jitt on 23 Mar 2010 12:01 In article <u6ihq51fct66e4reaeh6nodj2kicte43qi(a)4ax.com>, spam(a)spam.com says... > I've been goggling and all I've been finding is $50 to $100 plus, full > modules what is just the sensor used in for example dehumidifiers? > > I'm considering rigging a switch to control a bathroom exhaust fan in > my downstairs bathroom. > the classic is human hair- it changes length markedly with humidity --- news://freenews.netfront.net/ - complaints: news(a)netfront.net ---
From: Spehro Pefhany on 23 Mar 2010 11:26 On Tue, 23 Mar 2010 07:37:33 -0700, Jim Thompson <To-Email-Use-The-Envelope-Icon(a)On-My-Web-Site.com> wrote: >On Tue, 23 Mar 2010 15:17:14 GMT, zekfrivo(a)zekfrivolous.com (GregS) >wrote: > >>In article <hoai8d$5hu$1(a)usenet01.srv.cis.pitt.edu>, zekfrivo(a)zekfrivolous.com (GregS) wrote: >>>In article <u6ihq51fct66e4reaeh6nodj2kicte43qi(a)4ax.com>, Hammy <spam(a)spam.com> >>> wrote: >>>>I've been goggling and all I've been finding is $50 to $100 plus, full >>>>modules what is just the sensor used in for example dehumidifiers? >>>> >>>>I'm considering rigging a switch to control a bathroom exhaust fan in >>>>my downstairs bathroom. >>> >>>I installed a bathroom fan. The fan is in the attic for reduced noise. All you >>> hear is >>>sssshhhhh. I installed a hair humidstat in the old fan housing. I could not >>> really set it >>>properly. On humid days it would just stay on. I was also looking for another >>> wall >>>mounted unit that fits in a wall box. Currently I also have a timer controlling >>>the fan in parallel with the humidstat. The best solution is a condensation >>>sensor near the ceiling somewhere. I am also reconsidering how the fan exits >>>through the roof. i would like to keep that air in the house, and run it >>> downwstairs, >>>especially since I have high radon levels with negative pressure. >>> >> >>I may have found what I wanted.......... >> >>http://www.dewstop.com/dewstop_bathroom_fan_switches.html > >Just measure light reflection from a mirror ?:-) > > ...Jim Thompson Servo the mirror temperature to control the condensation and you can directly measure dew point.
From: D Yuniskis on 23 Mar 2010 11:56 Hammy wrote: > I've been goggling and all I've been finding is $50 to $100 plus, full > modules what is just the sensor used in for example dehumidifiers? The "cheapie" sensor is based on the fact that hair "grows" (stretches) up to 50% when wet (dry hair only stretches about 20%). Typically, animal hair is used (notably, horse). Note that these sorts of sensors are not calibrated, per se. You just set the control for "wetter" or "drier". "Modern, cheapie" sensors are typically involve a polymer. They operate based on changes in capacitance (physical size). They are more readily characterized but often can't handle environmental extremes (not a problem for a "bathroom"). "Doing it right", uses a chilled mirror hygrometer (dewpoint sensor). The controller chills a mirror against which a light is reflected. The temperature at which the reflected light becomes diffuse indicates when "dew" is forming on the mirror (i.e., when water vapor is falling out of the air). These are notoriously expensive as they are active devices (they *control* the temperature of the mirror to find this "dewpoint"), fragile and bulky. The mirror surface can also be scratched rendering them inaccurate -- and also limiting the environments in which they can be deployed (corrosive, etc). They are also slow (relatively speaking). In addition to the delay caused by the controller itself, there is often a delay associated with *sampling* the vapor stream (e.g., "plumbing" to the sensor). This can be *very* large (So, when used in a control loop, represent a big lag. *But*, they can be *calibrated* (instead of just "wetter" vs "drier") so are the sensor of choice for (most) process control applications where moisture content must be measured. There are limits on the range of RH at which a chilled mirror controller can reliably operate (e.g., maybe 5-90% RH). But, assuming the temperature sensor used in the controller is stable (typically a platinum RTD) and doesn't drift with age, then the dewpoint controller itself has excellent long term stability -- no need to be recalibrating it frequently! Since it is measuring the formation of *dew*, there are limits on the range of temperatures at which the controller can reliably operate. First, cooling (of the mirror) is usually done with a Peltier device. These typically only give you a ~40C depression meaning you can only measure dewpoints 40C below ambient. For greater ranges, you can use a two stage cooler (i.e., cool the hot side of the Peltier device with *another*!). Or more. :> Keeping the mirror *clean* is also a challenge. Any *thing* on the surface can serve as a nucleation site. Note that this gives you a *temperature*, not a "humidity". With an ambient temperature sensor, you can also compute "absolute" humidity (using psychrometric equation). If you really want to confuse yourself, ponder what happens when the "dewpoint" falls to -- or BELOW! -- 0C (google "frost point"). And, consider the consequences of having RH > 100% (!) ;-) There are other schemes, too! E.g., Google "gravimetric train" :> If you really enjoy a mental workout, this subject is right for you! > I'm considering rigging a switch to control a bathroom exhaust fan in > my downstairs bathroom. Aw, hell... you could probably just run the fan open loop or driven by temperature and get the sort of control you need. E.g., when you take a shower, the room tends to be humid. So, notice a "sudden increase" in temperature and use that to trigger the fan.
From: Jim Thompson on 23 Mar 2010 11:53 On Tue, 23 Mar 2010 11:26:38 -0400, Spehro Pefhany <speffSNIP(a)interlogDOTyou.knowwhat> wrote: >On Tue, 23 Mar 2010 07:37:33 -0700, Jim Thompson ><To-Email-Use-The-Envelope-Icon(a)On-My-Web-Site.com> wrote: > >>On Tue, 23 Mar 2010 15:17:14 GMT, zekfrivo(a)zekfrivolous.com (GregS) >>wrote: >> >>>In article <hoai8d$5hu$1(a)usenet01.srv.cis.pitt.edu>, zekfrivo(a)zekfrivolous.com (GregS) wrote: >>>>In article <u6ihq51fct66e4reaeh6nodj2kicte43qi(a)4ax.com>, Hammy <spam(a)spam.com> >>>> wrote: >>>>>I've been goggling and all I've been finding is $50 to $100 plus, full >>>>>modules what is just the sensor used in for example dehumidifiers? >>>>> >>>>>I'm considering rigging a switch to control a bathroom exhaust fan in >>>>>my downstairs bathroom. >>>> >>>>I installed a bathroom fan. The fan is in the attic for reduced noise. All you >>>> hear is >>>>sssshhhhh. I installed a hair humidstat in the old fan housing. I could not >>>> really set it >>>>properly. On humid days it would just stay on. I was also looking for another >>>> wall >>>>mounted unit that fits in a wall box. Currently I also have a timer controlling >>>>the fan in parallel with the humidstat. The best solution is a condensation >>>>sensor near the ceiling somewhere. I am also reconsidering how the fan exits >>>>through the roof. i would like to keep that air in the house, and run it >>>> downwstairs, >>>>especially since I have high radon levels with negative pressure. >>>> >>> >>>I may have found what I wanted.......... >>> >>>http://www.dewstop.com/dewstop_bathroom_fan_switches.html >> >>Just measure light reflection from a mirror ?:-) >> >> ...Jim Thompson > >Servo the mirror temperature to control the condensation and you can >directly measure dew point. Yep. As I posted _earlier_ today :-) ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | The only thing bipartisan in this country is hypocrisy
From: GregS on 23 Mar 2010 13:02
In article <hoao3s$lhp$1(a)speranza.aioe.org>, D Yuniskis <not.going.to.be(a)seen.com> wrote: >Hammy wrote: >> I've been goggling and all I've been finding is $50 to $100 plus, full >> modules what is just the sensor used in for example dehumidifiers? > >The "cheapie" sensor is based on the fact that hair "grows" >(stretches) up to 50% when wet (dry hair only stretches about 20%). >Typically, animal hair is used (notably, horse). You get good control around midpoint, but trying to set at 80% is hard. > >Aw, hell... you could probably just run the fan open loop >or driven by temperature and get the sort of control you >need. E.g., when you take a shower, the room tends to be >humid. So, notice a "sudden increase" in temperature and >use that to trigger the fan. Just get a ssssshhhh sensor. Water !! Or motion or sound, just turn the dam fan on when somebody is using the bathroom. greg |