Descent MKI diving at Altitude

Seems it is telling you the right thing.

Air pressure at 6,500' is around 80% of that at sea level.

PPO2 for air at sea level is ~0.21.  At 6,500' it will be around 0.168 (0.21 * 0.8) with is less than 0.18.

I had this same thought. However, even atmospheric  pressure is lower and therefore less O2, O2 still makes up 21% of the atmosphere.

I tried setting set the O2 level to 18% and dived again. Same result as far as the low PPO2 alarm.

PPO2 stands for partial pressure of O2.  It is not the same thing as percent and expresses the amount of oxygen in the breathing gas in terms of pressue.

At sea level, the air pressure is around 1 atm.  With air being around 21% O2, the PPO2 is 0.21.  that is, the partial pressure is 0.21 atm.

At 10m, the pressure is around 2 atm.  The same 21% gas mix will now have a PPO2 of 0.42 -> 0.42 atm.

20m on air, PPO2 is 0.63 and so on.

Reduce the pressure and the PPO2 goes down proportionally with the air pressure.

At 6,500' the air pressure is around 0.8 atm so PPO2 of air will be around 0.168 or 0.168 atm.

Setting you O2 to 18% just made the situation worse.  Your Mk1 would have thought that the PPO2 was now 0.18 * 0.8 = 0.144.

If you want to stop the alarm, you'd have to change the trigger point for the alarm, not trick the Mk1 into thinking you were breathing a different mix.

Some extra information....  Why is PPO2 important?

Chemical reactions and so physiology is dependent on partial pressures not percentages.

Most assaults on Everest are done by people on oxygen.  That's because even though the air is still 21% O2, the partial pressure isn't enough so they don't get enough oxygen.  Those that do it without oxygen have acclimated themselves to a lower PPO2 environment (increased the red blood cells in their blood).

One of the main reasons commercial aircraft are pressurised is so people can breathe (the other is so they don't get bent). It is also why if there's a sudden loss of cabin pressure, oxygen masks are lowered - otherwise they wouldn't be able to breathe eve though the air is still 21% O2.

PPO2 is also important for increasing pressure.  Oxygen is toxic at a PPO2 of around 2.0.  For example, if you dived on pure oxygen to 10m, you would almost certainly have a severe toxic reaction. 

Now I say almost certainly as there appears to be some variation is tolerances between people.  When I was at Uni a mate of my volunteered for some trials and he was actually fine breathing O2 at 2 atm in a pressure chamber (don't try this at home). 

Because of the variations, dive computers are usually set to a PPO2 of 1.40 for alerts and calculated MOD (Maximum Operating Depth) is based on 1.4 so there is a large margin of safety.

If you dive on 32% Nitrox, you MOD is 33m.  This is because the PPO2 at that depth is (33/10 + 1) * .32 = 1.376.

Thank you for the help. I have posed this question to various divers and instructors and yours is the first answer that makes sense. 

Thanks again.