Re: [APML] Film Creep

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

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...

----- Original Message ----- 
From: "George Anderson" <georgea@cam.org>
To: "Discussion of Film Astrophotography" <astro-photo@seds.org>
Sent: Sunday, September 28, 2003 10:49 PM
Subject: Re: [APML] Film Creep


> Hi Steve
>
> I don't shoot medium format so a vacuume back is not a problem for me,
> but a thought did come to mind.
> Any chance that the suction from the vacuume back is greater than your
> nitrogen purge? In other words is the vacuume back also pulling in damp
> outside air as well as sucking out all of your nitrogen? Have you tried
> experimenting with higher flow rates?
>
> George Anderson
> Montreal Canada
>
> Clear skies and good health
>
>
> Steve Walters wrote:
> >
> > Hi Jon,
> >
> > Thanks for the suggestion but it already has a vac back. I had problems
with
> > the original vac pump that allowed the film to lift up during exposures
but
> > I replaced it and solved that problem.
> >
> > In any case, a vac back will not keep the film from changing dimensions
due
> > to temp or humidity levels. It can only keep it flat.
> >
> > Steve...
> >
> > ----- Original Message -----
> > From: "Jon Kolb" <jkolb@datawest.net>
> > To: "'Discussion of Film Astrophotography'" <astro-photo@seds.org>
> > Sent: Sunday, September 28, 2003 12:15 AM
> > Subject: RE: [APML] Film Creep
> >
> > > Hi Steve,
> > >
> > > > Any suggestions on solving this problem would be very welcome.
> > >
> > > I haven't tried to verify your calculations, but I will say that going
> > > with a vacuum back may solve the problem.  I bought the Hutech version
> > > that replaces the standard Pentax 67 back, and although I thought it
was
> > > a little pricey, it sure helped cure some of my shots of the same
> > > symptoms you describe.  Understand, however, that I have pretty dry
> > > conditions to begin with - my meter said the dewpoint was -3.5F Friday
> > > night with an ambient temp of 28F.  Still my camera sweeps close to
the
> > > ground with my long refractor, down where it's wetter, and the vacuum
> > > back did make a very noticeable difference once I started using it.
> > >
> > > Sincerely,
> > > Jon Kolb
> > > Adventures in Astrophotography
> > > http://home.datawest.net/jkolb/
> > > jkolb@datawest.net
> > >
> > >
> > >
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> >
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