Re: [APML] Film Creep

["Steve Walters" <s-walters@comcast.net>]

Hi George,

I have built a similar circuit, it's on my website.

Steve...
http://www.starrynights.us

----- Original Message ----- 
From: "George Anderson" <georgea@cam.org>
To: "Discussion of Film Astrophotography" <astro-photo@seds.org>
Sent: Monday, September 29, 2003 9:31 PM
Subject: Re: [APML] Film Creep


> Well if your vacuume back holds the vacuume with the pump shut off (more
> or less) then my idea is out to lunch (not the first time).
> There is a nice controller for a dew heater on Don Clements site and it
> is also mirrored on the ATM site, http://www.atmsite.org , do a sort by
> author and look for Don's name, if you are into the brew your own for
> electronics.
>
> George Anderson
> Montreal Canada
>
> Clear skies and good health
>
> Steve Walters wrote:
> >
> > HI George,
> >
> > Thanks for the suggestions!
> >
> > When the film is down flat, the vac has almost no leak at all. My little
vac
> > pump gets down to about 15" Hg. As an experiment, I put a small 1 cubic
foot
> > cannister in-line with the vacuum system, pumped it down to 15" Hg,
turned
> > off the pump and left it alone. The vac held for about 3.5 hours. I
think
> > that is very very low leak rate.
> >
> > I increased the N2 purge flow rates from about 0.3 cubic feet per hour
to
> > 1.0 cfph on my last outing in an effort to be sure the moist air was not
> > getting into the camera. The film backs have a purge fitting that fills
the
> > cavity containing the film and there is a second purge point on the OAG
body
> > that is in front of the film plane. The 1.0 cfph is divided between
these
> > two locations. I estimate the volume of the filmback and OAG to be about
63
> > cubic inches which is a mere 0.036 cubic feet. So the purge is fully
renewed
> > roughly every 2 minutes. If I run the purge any faster, I'll need a
bigger
> > bottle, I'm only taking a 20 cubic foot tank to the field so I only get
20
> > hours of purging. If the dew heater stops this problem, I'll try
lowering
> > the purge rate back to 0.3 cfph.
> >
> > In reading the Kodak literature on ESTAR properties (again, I don't
think
> > this is the correct backing for E200 but it's instructive), the
coefficient
> > for thermal expansion is only slightly greater than the expansion for
%RH.
> > For ESTAR, these are 0.001% / degF and 0.0008% / %RH. That means that a
> > change of 1 deg F is about the same as 1% RH. So it's possible that as
the
> > temp drops during the early part of the night, my problems are temp
related
> > and then when temp stabilizes, dew forms on the camera and it shifts to
> > humidity problems. So when I add the dew heater, maybe I should also add
a
> > controller that keeps the camera temp stable, not just above the air
temp as
> > I was planning to do. That's not very hard, just something else to do
and
> > more wires to manage....
> >
> > Steve...
> >
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