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.
Humidistat.
Check out the Honeywell H8908B1002 carried by Home Depot ($26.97):
An applications sheet at:
The HIH-4000 series from Honeywell is pretty easy to use with a 5 V DC supply and a ratiometric output of relative humidity that's pretty linear.
The GE HS-15P is about half the cost but it wants an AC drive and so is a bit more effort.
-- Rich Webb Norfolk, VA
They don't use "sensors" in cheap humidifiers. Try searching for "humidistat". They're electromechanical devices. In the old days, horsehair was used, nowadays a hygroscopic polymer (eg. Nylon aka polyamide) is used.
Just measure light reflection from a mirror ?:-) ...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
Years and years ago ;-) an associate and I were dabbling in improving the efficiency of evaporative coolers.
In the process of our research we found instruments that used a Peltier cooler, cooling a mirror, to locate dew point.
It would seem that an angled light beam reflecting off a mirrored surface would do the trick... you don't need cooling.
I'd detect loss of reflection, then run for a timed period ?? ...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
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.
greg
I may have found what I wanted..........
controlling
Servo the mirror temperature to control the condensation and you can directly measure dew point.
I bought another controler for the bathroom, but it did not have the reversed function to turn on when humidity goes up. Most are designed for humidifiers. I can't tell if the honneywell has the reversed contacts available, but does not mount in a standard wall box, and might not have enough sensing on the outside.
greg
Thats sounds simple. Every camcorder has something inside. Well, only tape machines.
greg
you
controlling
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
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!
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.
the classic is human hair- it changes length markedly with humidity
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Ha! Yes! ;-)
Many homes have fan wired to light fixture *or* in addition to light fixture. But, that means you have to remember to turn it off, later. (timer? -- hence my suggestion "open loop")
Probably the most reliable... just make it a "pulse stretcher"... motion equals 20-30 minutes of fan time... fart clearance ;-) ...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
You get good control around midpoint, but trying to set at 80% is hard.
Just get a ssssshhhh sensor. Water !! Or motion or sound, just turn the dam fan on when somebody is using the bathroom.
greg
Yea that's the ideal solution, are you coming over to rip out and replace all the drywall and then patch and repaint? It wouldn't be so bad but there is also a boxed in duct that blocks the routing for the logical placement of the switch;by the door and the light switch.
I thought of just running it off the light switch but I don't really want the fan on every time the lights on.
I can run a wire relatively easily parallel to a joist about six feet to a crawl space which has a light switch. That's where I can get power to the fan with minimum damage. I was just looking for a way to activate an SSR or relay to switch the fan on without having to go into the crawl space. I figured a humidity sensor could be used to trip a relay or SSR.
I see digikey has these.
Flow sensor-- it will run from the time the toilet is flushed 'till the tank refills, without that annoying racket from the fan when you're in there trying to read EDN. ;-)
How about wireless? I am using wireless keychain remotes and RF receivers to run my hot water circulation pump. I will also install RF receivers to the fans later. I have not been able to decode the RF encoding, but at least can detect any button press. There are false positive and negative actions, but no big deal for me.
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