RE: Dry Cameras

["Bob Dubner" <rdubner@compuserve.com>]

I can't stand it any more.

The gadget that Tom remembers is a sling psychrometer, it works sort of like
a wet-bulb thermometer, except that the forced air flow from waving it in
the air gives a quicker result.

A wet-bulb thermometer gives you an indication of how fast the water is
evaporating from the wick; the faster the evaporation, the drier the air,
and the greater the temperature difference between the wet-bulb and dry-bulb
thermometers the lower the relative humidity.  The actual dew point and
relative humidity have to be determined from tables.  The wet-bulb
temperature is *not* the dew point.  You don't need to move air over a
wet-bulb/dry-bulb thermometer setup; diffusion will do the job if it is not
sealed up.

But Tom?  Given everything else that's going on in your setup, and given all
the Peltier coolers you have flying around, you are in a position to measure
the dew-point simply and directly, assuming you are too far from an airport
to trust the aviation weather reports.

Attach a polished piece of metal to the cold side of a Peltier cooler.
Attach a thermometer to it.  Bounce a light source off the metal into a
sensor.  Ramp the TEC current up until frost begins to form on the polished
metal, interfering with the reflected light. Read the dew point off of the
thermometer.


> -----Original Message-----
> From: owner-tass@listserv.wwa.com [mailto:owner-tass@listserv.wwa.com]On
> Behalf Of Chris Albertson
> Sent: Thursday, May 11, 2000 17:17
> To: Tom Droege; tass@listserv.wwa.com
> Subject: Re: Dry Cameras
>
>
>
> I figured a wet bulb thermometer would be pretty reliable.  I think
> you only need to blow air over it if you are in a hurry to get a reading.
> One set up permanently could have a cadge built over it. As for a source
> of water, a soda one liter bottle would last forever almost.
>
> If you have some money to spend why not get a full on weather station
> complete with an RS-232 interface to a computer.  For about $400 you
> can get a pretty complete system.  These are easy to find.  I know of
> at least one place that sells them.
>
> Then there is the zero cost software-only dew point sensor.
> You get the aviation weather reports off the web and extract
> the dew point from the message.
>
> Tom Droege wrote:
> >
> > Chris and all,
> >
> > Yes, I think Chris is right about the dew point.  There are extra
> > thermometer positions available in the system so one could be
> dedicated to
> > a wet bulb thermometer.  I am using the AD-590 which is a
> similar device.
> > But I don't much like a themometer with a wet sock hanging on
> it.  For one
> > thing, you will have to blow air on it to get a reasonable
> reading.  I used
> > to make this measurement at a power plant.  You have this wet bulb
> > thermmeter on a chain and you sling it around to circulate the
> air around
> > it.  I would better like a relative humidity sensor direct, even if the
> > reading were not so good.  I am opting for as much reliability as I can
> > get.  The bears are apt to come by and drink the water, or some
> such thing.
> >
> > Tom Droege
> >
> > At 12:59 PM 5/11/00 -0700, you wrote:
> > >
> > >I asked this question on the list a week or so ago. I wanted
> > >to know how to measure the dew point.  I figured I'd control
> the cooling
> > >water temp to a little over the dew point too.
> > >
> > >The answer I got was simply that the dew point equals the wet-bulb
> > >thermometer reading.  (Easy to verify this with a glass thermometer.)
> > >Just put your standard air temp sensor in a wick over a container of
> > >water.  I think there is a part LMxxx that reads out temp directly.
> > >
> > >> Does anyone know a source for a cheap, not too accurate
> relative humidity
> > >> indicator?  I would like one powered by 5 or 15 volts, and
> which puts out
> > >> only a modest signal.  The ADC measurement system has a
> least significant
> > >> bit of 300 micro volts.  Something with say 3mv or more per
> % relative
> > >> humidity would be fine.  I think I can tolerate almost any
> scale factor
> > >> that such a device would have.
> > >>
> > >> I would like to sense the relative humidity.  I already sense the
> > >> temperature.  With the two, I think one can compute the dew
> point.  I would
> > >> then regulate the cooling water just above the dew point.
> > >>
> > >> Tom Droege
> > >
> > >--
> > >  Chris Albertson
> > >
> > >  calbertson@logicon.com                  Voice: 626-351-0089  x17
> > >  Logicon, Pasadena California            Fax:   626-351-0699
> > >
> > >
>
> --
>   Chris Albertson
>
>   calbertson@logicon.com                  Voice: 626-351-0089  x17
>   Logicon, Pasadena California            Fax:   626-351-0699
>