I did a lot of Cascade filling of my low pressure Twin 38s before I upgraded to a compressor. I did it (and STILL do it) because I wanted to dive the same set of tanks several times at the same dive location. If you're still interested in a cascade system, here's what I learned about cascade filling:
1. If you are filling low pressure tanks (like my 1800 psi Twin 38s), standard 3000 psi aluminum 80s are perfectly sufficient for cascade filling. They are cheap to buy (Joe Diver sells them for $125 ea), you can handle loading them in and out of the car to refill them at the dive shop, and they're commonly accepted for visuals and hydros at all dive shops.
2. When cascade filling, you get the best bang for your buck when filling EMPTY tanks. This goes against conventional recommendations to leave a 500 psi safety margin. But remember cascade filling is simply balancing from high pressure to low. So, if your cascade tank only has 500 psi left and the tank you want to fill already has 500 psi... you can't put any more into it. Also, if your typical dive only uses half a tank, then don't waste your cascade pressure filling all the way to full pressure. Fill it a little over half so that you'll be near empty when you finish your dive.
3. Use good judgment on the empty tank thing. For example, I dive mostly in shallow water lakes, 15-30 feet deep. Running out of air while diving in these conditions isn't such a big deal... Especially when you know its coming. In fact, I think its good to know what running out of air feels like. I use my watch or banjo gauge to time the approximate air time to arrive close to the dock when empty. I pull the J-Rod when I'm out and use the reserve to finish the dive close to empty.
4. A few of us used cascade filling at the Sea Hunt Forever show in Silver Springs Florida last year. We dove the show in 30 minute shifts and my Twin 38s lasted almost exactly right for two shifts including a pull on the reserve rod. I made of total of 9 dives over the weekend, refilling via cascade from only 5 scuba tanks: Two 72s at 2500 psi, two 80s at 3000 psi, and one HP80 at 3300 psi. The 9th dive I only had enough cascade capacity to fill my 38s to 1500 psi... But that was plenty enough to last for the final 30 minute shift!
DIY Gauge Conversion for my Cascade Hose.
- I replaced the original brass yoke and bleeder valve with the body and gauge from one of my vintage USD Tank Checker gauges. Sometimes these tank checkers can be had for a bargain price (this one was $16).
- I used to use a tank equalizer hose with a banjo pressure gauge to cascade fill some of my vintage scuba tanks. I got to looking at the connections on the "T" and those on the tank checker and got the idea to try this little re-configuration. Getting the checker gauge apart was the hardest part. I had to cut some of the rubber overmold off the flats of the gauge connection to get a wrench to mate reliably. But otherwise, it went together slick as a whistle and it works perfectly for cascade filling.