It is possible to keep the O2 level high using cannulas instead of a mask and when combined with the O2D2 pulse regulator, but you have to monitor carefully and regularly with an O2 saturation meter at that kind of altitudes. It is not ideal. I prefer to stay at around FL180. With passengers in the back I never go that high.
Now as for the limit of FL250. I think it is interesting to see that the Mooney 252 is certified to FL280. It is at these altitudes that you would need to deliver oxygen under pressure to stay ok in a non-pressurized cabin. I do not have the feeling that the Cirrus SR22T cannot go any higher when approaching FL250. I just have never tried as it is the limit stated in the manual.
AeroPlus wrote:
It is at these altitudes that you would need to deliver oxygen under pressure to stay ok in a non-pressurized cabin.
Really? I thought that limit was more like FL400. At least that’s what my Human Factors textbook says!
The Mooney 252 was certified to 280 because that’s the only altitude it achieves the advertised speed at – but even the test pilots said just because it’s certified it’s not necessarily a good idea to take it all the way up there.
The table below shows the Supplemental Oxygen Requirement to maintain MSL air equivalence:
At FL280 you can still cope with a normal mask. Above FL340 you need pressure breathing.
There are also many glider pilots doing higher altitudes than FL250 with the EDS in mountain wafes. To my knowledge the highest was in the US around FL450, at least unpressurized. That said I also do have doubts, if that is a good idea…
I for one would also regard FL250 as my limit, simply because many did that already – and survived 
lenthamen wrote:
Above FL340 you need pressure breathing.
Your table is for MSL equivalence.
If you’re satisfied with the oxygen you’d get at FL100 then you can go to FL400 without pressure breathing.
In addition to possible problems with oxygen interests me another question. The TB21 is certified for FL250, the Skylane TC only up to FL200.
I have been to FL 220 without any problems concerning engine parameters, Achim has reached FL240 with his Skylane. I would like to go up to FL 250 but I am a bit concerned about a damage to the engine. Can a turbocharger at heights overtighten or there are other risks for the engine, as long as the temps stay in the green ?
highflyer wrote:
Achim has reached FL240 with his Skylane.
Hey, I would never admit to that. Only thing I would say is I know somebody who knows somebody who has heard about somebody who allegedly…
Can a turbocharger at heights overtighten or there are other risks for the engine, as long as the temps stay in the green ?
The TC should reach its max speed much earlier than that. Just look at the temps. You might get ignition problems because your mags are not pressurized and the thinner the air, the more difficult to generate a spark. However, that usually kicks in much higher. The turbocharged Cessna T210 has been above FL300 without problems. Personally I would have no problem doing that.
PS: you should change your nick, maybe wannabe_highflyer? 
I know someone in another forum, who took his Skylane TC to FL310. Uups-just realized, that this is not sufficient “I know someone who knows someone stuff”
achimha wrote:
FL240 in a high altitude test of my Cessna.
no further comments on this.
