[APML]: Purging during exp. with N2

[Alfredo Zanazzo <alferd@mbox.lol.it>]

Brad Wallis wrote:
> 
> 
> re: purging during exposures with N2
> 
> Again, the problem is moisture ....  the purge eliminates the moisture...
> Alex Smith and his crew initially published results that showed that
> Hypered Tech Pan could lose as much as 50% of its speed during a 1 hour
> exposure in air with high humidity ...  When this came out ( in early 80's )
> it explained a problem that we had been seeing but had not yet tracked down..
> so we adopted a purge system ...  Tony Hallas discovered another version
> of this that we had never seen ... as the film soaks up moisture it tends
> to creep ... creating 'trailed' star images .. the film can also swell
> and buckle creating out of focus images also ... so Tony moved to a purge
> system a few years ago ...
> 
> with 35mm and a camer body that is sealed ( with a window or field flattener )
> you can get away with gassing the camera quickly before each exposure and then
> resealing the system ( this is a dirty solution .. but it helps )... you cna drill
> the camera thru the tripod mount hole and purge ... buy a flow meter at a surplus
> store and only use about 1 cubic foot per hour purge ( giving a bigger blast
> of dry gas at the start of the exposure ) .. you can do the same with the 6x7
> bodies ....  either FG or N2 will solve the moisture problem ...
> 
> if you can not purge ( or do not want to ) then I would suggest that you
> at least take the step of skipping a frame between exposures as the film
> that is adjacent to the frame being exposed is getting a nice blast of wet
> air during the exposure .... this is the effect that we kept seeing back
> in the early days of hypering ...  the adjacent frames were seemingly
> shadowed ... one half going much deeper in limiting mag than the other half...

Dedicated to Brad or other experts like him
>
Do you think that this unpleasant moisture effect applies also to closed
tube instruments like refractors, Cathadioptrics and Schmidts, or rather
to open tube classic cassegrains, newtons, RChretien etc?
In the first cathegory of instruments (the corrector plate's equipped
and refractors) would  possibly help to fix  a vacuum pump aspiring air
from the tube to outside?
It is an idea I am having now, I dont' know if it could work, and if
positive, how powerful should be the pump and how close to film.
Another point is what would be the warning humidity value out of your 
 tel. tube to make you upset about the final resault of your exposure?
You have mentioned that above a certain degree film is loosing half of
his speed and turns back to reciprocy failure.
The solution you suggested, to seal and unseal continuously your camera
would be (in my opinion) restrricted to small formats, and have also
other unpleasant effects: change of focus, ghost images and spot of dust
if some powder grains fall on the glass.
Maybe photographing with a glass filter  screwing in to the adapter
ring could act as sealer.
I dont' think that the sealing must be absolute, otherwise you would get
pressure and...poff!
Certainly for those who shoot in damp sites this is a very important
subject to discuss!!
Alfredo