Gas hypersensitizing the film removes water and oxygen from the film via vacuuming and replaces it with a hydrogen nitrogen mix.  This makes the film receptive to dim light for a much longer period than without hypering.  In the case of Tech Pan it effectively doubles the ISO rating.

I started off with the the modest thought slapping together a hypering tank out of PVC and using a hand vacuum pump.  As projects often go, I ended up with a miniature bomb shelter and an industrial strength setup.  I can only hope that, someday, I will do it justice with at least one decent astro photo ;).  Click on any photo to see it much larger.

How it came to be...

While I have quite a few SCUBA tanks I really didn't want to cut one up.  I remembered a dive buddy who had a old J-Valve tank that was waaaaay out of hydro (safety certification).  He was willing to part with it for a steak dinner after our next dive.

Since all I had was the tank and an idea I needed to find a local machine shop that could handle the cutting, milling and drilling. Northrop Machining in St. Petersburg,. FL seemed to have what I needed.  The first step was to cut the tank to see how thick the walls were.  This one was 9/16".  Once we knew this we finalized the design and metallic carnage really began.

The basic idea was to use a single O-ring to make the seal between a groove in the milled tank top and a 1/4" aluminum plate with a shallower matching groove for the top. On the outside of this groove would be eight small bolts to hold the lid down during positive pressure.  Two holes were tapped in the lid for vacuum/gas and the pressure gauge.  While not a high-vacuum setup it will max out the gauge at 30" and hold it for days without noticeable leakage.

 The first lesson learned was that SCUBA tanks, at least this one, are not actually round.  This makes the machining of the groove a little tricky (=$$)  

Above left is the finished tank with a Sunbeam heating pad around it..

Below is the CAD drawing (PDF-5)  to make the tank.

Some of the other pieces...

This is one of two temperature regulators I built for this project.  This one is set to 50C and the other for 40C.  The other one is a slightly simpler design using am extension cord with a built-in LED to indicate power instead of the red LED on this one.  This design is from Rockett Crawford although the schematic is here now.   I'll try to get a photo of the other design on the page soon.  It was pretty easy to build and works well.

The forming Gas was ordered from a local welding supply company (Crumpton Welding Supply) before the tank went to the machine shop.  Due to some administrative mix-ups it was the last piece of the puzzle received after a three month wait.  The good news is that with 80 cu. ft of the stuff I'll probably never run out.

I wanted the hypering tank to sit in another container for insulation.  While in a ACE Hardware I ran across a little galvanized trash can that was perfect.   I added some fiberglass insulation which isn't in the photos.