Some calculations I have done as I blend my own Nitrox/O2.
I currently rent a G size medical grade cylinder $270/yr, and the gas costs $170. It contains 10m3 which is 10,000 litres O2. As I don't have a pump I can only use 170 BAR if mixing 32%. That equates to 2 cents/litre purely relating to gas cost, but if you figure in the yearly rental its 5.2 cents/litre (assuming only 1 cylinder of gas/year. If more gas is consumed per year the cost overall drops as the rental is a fixed yearly cost/cylinder).
Thus an 11 litre cylinder @ 200 BAR 32% will require 28 BAR of O2 or 308 litres. 308x0.02=$6.16, with rental costs the actual cost is 308x0.052=$16/cylinder (not including air/filter costs). My top up air is double filtered to ensure sufficient quality (additional cost of a filter plus consumables) so air costs are more than normal scuba grade air costs (I double filter all my air as all my cylinders are oxygen cleaned regardless of proposed oxygen content).
A LDS advertises Nitrox at; Oxygen 3.5 cents/litre ($10.72 for an 11 litre cylinder), $5 for the air, $10 to blend=$25.72. So I consider that there is not as huge margin in it as one might imagine. They are after all a business and must make a profit to survive
I can probably get slightly cheaper rental from another gas company which I will try this in future. I could also use industrial grade oxygen which is cheaper, however given I am playing with my life, I like using medical grade. I believe the actual physical difference is that medical grade, on every occasion the cylinder is evacuated before filling, filled and then tested. Industrial grade its just refilled and not evacuated before filling or tested (except on a random cylinder basis perhaps). I believe the oxygen source is the same for both grades of oxygen. The real difference being that the medical grade is guaranteed to be 100% oxygen and no mixed gases and is certified as such. The industrial grade (while probably 100% oxygen) could potentially have other gases thus the actual gas content is not known or certified but only assumed to be 100% oxygen.
With relation to using a compressor for nitrox blending, yes its done all over the world, however the risk being that it has the potential to promote combustion within the compressor during fills (high oxygen, fuel [oil], temperature [from the compressing process]), and it also increases wear on the compressor (which I picked up from my Dive Technician course). A compressor in poor condition promotes oil combustion within the cylinder, thus just another risk we must consider before going down this path.
So one might ask, why do I do it? Why not just go and get a fill from my LDS;
Well convenience is one, I use varying mixes from 21-100%.
I know what gas is used to fill my cylinders as I do all my own fills. Thus no contamination from suspect air sources.
I get correct fills every time and no short fills.
I inspect, test, clean my own tanks thus save in these costs as well.
I like having hands on so I understand the process fully, thus can often see when others are not providing as good a service as they should.
I would love a Haskel booster pump so I can use all the oxygen from my cylinders but they are in the order of $2500+ and my minister for finance has drawn a line in the sand (although I sneaked a nice new Bauer compressor past her last year).
---------- Post added April 5th, 2015 at 01:45 PM ----------
An interesting link;
Oxygen Labels