My oxycon has a total of 3 filters;
1) A coarse one that keeps the dust bunnies and hair and larger what-not away from
2) the intake air filter.
The 3) output also has a super fine bio filter that keeps any micro sized stuff from going the oxycon directly into the patients lungs.
People talk about having the 'sieve beds' repoured, especially if they are talking about taking your money to do it, but I don't see the need unless you are overhauling an oxycon that has been in a flood or in tropical humidity so thick and for so long that you suspect mold has taken up living inside the machine.
The used medical ones you can buy on Craigs List don't need anything like that kind of work unless you are trying to make usable as a medical device again which then needs to be serviced by a certified technician and then it needs to be prescribed by a doctor.
The units used for patients do (or did) need to be overhauled about every two or three years.
Filers get replaced and often they put in new tubing because it tends to get brittle and crack or loosen in a few years even if not used.
For non medical use, we can replace the tubing with stuff you can buy at the hardware store.
I found this site some years back that might help;
http://www.frankshospitalworkshop.co...equipment.html
Also the oxygen purity drops off when you push it to the maximum out put but even at its best it only get 97% pure oxygen.
When you run even a brand new medical 5LPM one at 4LPM it outs out about 97%.
But if you push it to 4.75LPM the quantity (volume) of the out put goes up but the purity goes down to as low as 91% and often it can get down to 86% and this only gets worse the more thousands of hours it gets used.
Commercial welding supply companies tanked oxygen often have much higher purity approaching 99% but they can afford industrial units that are tens of thousands of dollars to do that.
Btw welding shops sell and rent small tanks in a range of sizes down to 20 pounds in weight so you can get tanked oxygen in lots of different sizes. You just have to be willing to pay more per cubic foot of oxygen in the smaller sizes.
But it is nice, when you are creating master pieces and are willing to pay for the convenience of having lots heat when and where you want it.