Okay, there are a few things to consider with a booster. Boosters need drive gas and this is normally supplied one of three ways:
1. Shop air compressor with a good filter and a dryer on it regulated to 150psi max.
2. LP banks and a high flow regulator, regulated to 150psi max.
3. Direct from your scuba compressor, with a high flow regulator, regulated to 150psi max.
The second option is my preference as the air is cleaner, but is also dryer and you will need to lubricate the cycling spool a little more often. This is a 10 minute job and I do mine every 50 hours of use. The third option will need a compressor with 25 to 30cfm capability and I like to be able to hear my booster cycling and compressor noise will generally not allow this.
The greater the pressure difference between the input cylinder and the output cylinder, the more drive gas you will use as your booster becomes less efficient. If I am boosting into a cylinder that has 150 bar to start with and my supply cylinder has 130 bar, the boosting is very quick. If my supply cylinder has 30 bar in it, the booster is much slower. I have the Haskel AGT15/30 and input gas does not need to be down regulated within the ranges that we generally use in scuba. Any other booster will need to be regulated on the input gas or interstage stall will occur - but not all the time, it depends on the input gas pressure, the higher this is, the more likely you will get interstage stall. With the Haskel AGT15/30, you don't need to worry about this.
A new booster will set you back USD$6 to $8K, so most people buy a second hand one and have it oxygen cleaned. A second hand oxygen clean one in good condition can be bought for USD$4.5K.
The next thing you need to decide is how you will set it up, do you want to boost back through the panel or just boost into individual tanks. I boost back through my panel and it saves a lot of time with not having to change whips.
I use the booster for oxygen and helium, but use a nitrox stick for the last 10 bar of oxygen from a supply cylinder and pump the last 10 bar of helium straight through the compressor using a helium balloon on the intake. For small amounts of helium, this is fine as the final stage doesn't heat up too much to cause any problems. The booster will drain a supply cylinder down to less than 1 bar, but this use is highly inefficient in terms of drive gas and wear and tear on your booster. To give you an example, with 130 bar supply and 150 bar output, the booster may move 1 bar every stroke, drop the supply pressure down to 10 bar, it may take 10 strokes to move 1 bar, drop the supply pressure down to 5 bar and it may take 25 strokes to move 1 bar, drop the supply pressure down to 2 bar and it may take 35 strokes to move that last bar. So as you can see, even with the best booster available for our pressures, there is a point where you have to decide to try an alternative. An alternative is to double boost, that is, using an empty cylinder to boost into and then boosting from that into your final cylinder. There is a general rule of thumb that says anytime your pressure ratio (input to output) is greater than 1:10, your boosting will be inefficient in terms of volume and drive gas. I am talking about 2 stage boosters here too. If you look at single stage boosters, the ratio is less and you can expect to not be able to practically use the last 40 bar of any supply cylinder without double boosting.
As far as filling for tec divers, personally, I wouldn't be interested in an 80% mixture. Most techies are using either 50% or 100% to do their deco with and rebreather divers will want 100% too. But that won't really affect you as your booster will need to be oxygen clean anyway and will handle 100% happily up to 210 bar. I don't boost 100% oxygen beyond this - ever but know of people who have boosted happily to 250 bar - not worth the risk in my opinion.
Good sources of boosters are High Pressure Technologies, talk to Peter or Perry, these boys know everything there is to know about boosters and sell them, service them and modify them, both new and second hand. They have access to Maximator, Haskel and HI. There is a new Chinese booster called a SITEC, but I know a few people who own them and they have been nothing but problems and run above and below the stated specs. Stick to Haskel, HI and Maximator in my opinion.
Anyway, hopefully this helps somewhat.
