Re: [APML]: Storing TechPan

[Brad Wallis <wallis@pepe.jpl.nasa.gov>]

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...

Brad Wallis
http://voltaire.csun.edu/wallis_provin.html