Drying an unheated space: how do you measure moisture?

A dew point sensor is a lot simpler than an etalon, trust me. (I've spent a fair amount of time in the past year building etalon-stabilized diode laser systems.)

The coating and alignment of an air-spaced etalon are no joke, for one thing. Interrogating the etalon requires a stable tunable laser, which is also nontrivial. It also requires the spacer to have a much lower temperature coefficient than the air density, which means fused quartz or Invar (or perhaps a better glass such as ULE or Zerodur).

On the other hand, a dew point sensor needs a cheap micro, a bit of window glass or bathroom mirror, a small Peltier cooler, plus a LED and a photodiode, arranged any-old-how. It's also easy to run at AC--if you run the Peltier's temperature up and down a few times while watching the optical signal rise and fall, you get an excellent dew point measurement.

Besides the measurement difficulty, there's the selectivity to consider. The etalon responds far more to temperature and air pressure than to humidity. Acoustic thermometers are also, well, primarily thermometers. IC temperature sensors are the absolute pits--slow and inaccurate compared to almost anything else. If you were going to all the trouble of making fancy measurement gizmos like etalons, you'd definitely want a better temperature sensor than that.

Cheers

Phil Hobbs

The Honeywell HIH-4000-series sensors aren't terribly expensive. The part numbers that include calibration data (HIH-4000-3 or -4) are only about US$25 qty one (Joerg just had a stroke) and has a linear output voltage with RH, roughly from 1 to 4 V.

Or ... a home-made dew point sensor with a peltier cooler, a mirror, laser pointer, and photodiode, sensing the light scatter as the condensation forms on the mirror face?

The sensors in the typical Oregon Scientific modules do seem to lose sensitivity as they age. I recently replaced an outdoor sending unit and the RH now (AFAICT) is right on. Just (?) decode the data packet and you're there.

Aim a light bulb at them to keep them above the dew point.

Did that around 1980... works exceptionally accurate for finding dew point... until the mirror cruds up. Maybe add a windshield washer and wiper ?:-)

...Jim Thompson

What are you trying to solve: trying to keep them from

Dew point sensors (chilled mirror) require maintenance -- especially in that sort of {basement, outbuilding, etc.} environment. And, they are expensive.

You can "compute" moisture content of the air (cf. "Psychrometric Calculations") probably to a good enough degree of accuracy to tell you when your tools might be "at risk". But, what are you going to *do* in that case? Run a dehumidification process? Can you be sure to be able to extract enough moisture to bring the environment to a friendlier level? What happens when your "basement floods" (figuratively)?

Would you consider, instead, a thin film of oil?

Or, encasing in an enclosure with a dessicant? (how often are the tools accessed?)

Or, ensuring the enclosure always remains "hot enough"?

Or, moving the tools to a more hospitable location?

Personally, I keep my tools in the household living space. They're just worth more to me than something *else* that I could move out to less hospitable environs. YMMV.

I was thinking more along the lines of a Newton's rings kind of element, maybe with the photomosaic bits from an optical mouse to read out a fringe shift. It just has to have significantly different response from a speed-of-sound sensor, to the barometric pressure, in order to resolve the humidity. Dew point will take awkward amounts of power, I suspect.

Consider the photovoltaic-powered ventilation fan. Dead simple to install and it only draws air during the day when humidity is at it's lowest. It also runs just fine during power outages.

I installed three of them and I am very happy with the results.

--Winston

Got only one crazy idea.

Ramp up the voltage to an arc gap. Arc gap depends on humidity. While playing around with some of my HV experiments I had to breath on a gap to start the arc.

The sensitivity of an etalon to humidity is very low compared with its sensitivity to temperature and barometric pressure. Measuring three things when you care about only one is usually a losing proposition IME.

If you're planning to run a fan big enough to dry out a crawl space, a few dozen joules per measurement won't be a huge issue, I shouldn't think--you'd only need to do it every half hour at most.

Cheers

Phil Hobbs

Oh, yes, I'm aware the light-path part mainly measures barometric pressure. The thing is, sensors for RELATIVE humidity are no good whatever in determining ABSOLUTE humidity, unless you know the barometric pressure. The proposed air-exchange may be between two different relative humidities at two different temperatures, but presumably only one barometric pressure reading is required.

Speed of sound changes about 3 parts per thousand with humidity, at 20C, which is large compared to lightspeed changes.

So, measuring three things CAN give you absolute humidity, no two will, as I see it, suffice.

I think you only need the temperature and RH. Dalton's law of partial pressures says that to leading order, the equilibrium partial pressure of water is independent of the partial pressures of other gases.

Cheers

Phil Hobbs

On a sunny day (Sun, 16 Oct 2011 13:57:39 -0700 (PDT)) it happened whit3rd wrote in :

I dunno, but last week I bought an inside-outside temperature meter on ebay from China for less than 5 Euro inclusive shipping (has been shipped already). Just want it in the living room to see the outside temp. In spite of all the computer sensors I already have. But looking for that on ebay also showed more advanced weather sensor stations for a low price. How to interface to it? Maybe use a webcam and decode the LCD segments... LOL I have a mechanical hygrometer, calibrated it long time ago with salt and water. Could also be read with a webcam, or mechanical contact :-)

The pressure is irrelevant to this task.

The humidity and temperature are significant. You don't want to vent if the internal temperature is below the dew point for the exterior air - otherwise some air circulation is usually considered a good thing.

If you are serious about this then your best bet is to measure the absorption of water vapour in the relatively strong 940nm line - a wavelength for which cheap IR LEDs are reasonably available.

You might need a bandpass filter on the detector though...

Papers online suggest it should be a workable option over a suitable path length - might be tricky to make it work well enough:

water.

Basic low tech hygrometers may not be all that accurate in absolute terms, but they should be more than good enough for this application.

Regards, Martin Brown

What about if you use an accoustic resonator to measure the speed of sound in air.

(OK you still have pressure and temperautre to correct for....) maybe two accoustic resonators one with outside air and the other 'dry' air. I'm not sure how to dry the air and get it back to the outside temperature.

George H.

ld

SHT15 and friends are about $13. Two-wire digital output, calibrated RH and temperature sensors integrated into a chip. About 2% RH accuracy IIRC. It includes a heater which can be enabled. I think it's for dew-point sensing or some such.

Uncalibrated capacitive RH sensors and RTDs are about $2 each.

You seem to have conflicting requirements: > But, how do you know when the outside air has fewer > grams/M**3 of water than the inside air? > The vent control ought to respond to ABSOLUTE humidity.

For outside vs inside on the cheap - and for fun - use hair. One sensor outside, one inside. Feed a comparator, and you're done. The devil is in the details - getting enough delta L to move a slide pot (for example) to get enough delta V to trigger the comparator. After initial "calibration" - setting both sensors to identical output in the same environment - the delta L should track with two identical length hairs from the same source. You could window the thing if you need a range.

For ABSOLUTE, do you mean outside ABSOLUTE vs inside ABSOLUTE? Or are you considering running a dehumidifier or heater based on a single measurement?

Simplest of all was stated in another post - shine a light on the tools continuously, to keep them a little warmer than ambient.

Ed

It seems to me that knowing inside/outside parameters won't really help. I think you really need to know the temperature of the _tools_ and the dew point temperature of the ambient air. If the tools themselves are below the dew point, dew will form on them. It also seems to me that preventing dew formation is easier than measuring the conditions under which it may occur.

its=20

IME.

Somehow i suspect that a wet bulb / dry bulb thermometer and a video camera and some software will work better. Add a barometer and it gets real good.

?-)

its=20

three=20

IME.

gets

hair-under-tension