I think Tbone might have given you a good short answer, but, since this basic scuba, and your post shows some fundamental misunderstanding of basic principles (and of your computer) I would like to explain what was happening so you or others can learn from it.
I dive a Galileo Sol, which operates the same as your Luna. I must say strongly that the DM did not understand how your computer works, and did you a huge disservice by adjusting your PO2 max to dangerously high levels, when PO2 has nothing to do with what the computer was telling you. I am shocked that a trained DM would do what you described. However, I must also say, you put yourself at unnecessary risk by not understanding how your computer works, and what it was showing you. Please, RTFM!!! Your very life depends on understanding your computer--for which you, and not a DM, are solely responsible.
“RBT” is an acronym for “remaining bottom time.” It is exclusively and only a GAS CONSUMPTION calculation that is part of the computer’s air integration feature. It has nothing to do with NDL, and it has nothing to do with the PO2 of your mix.
The computer shows you your tank pressure, and also “RBT.” The RBT is a calculation of the amount of time you can stay at depth before your tank pressure reaches the level that, if you immediately ascend properly and do all stops, you will still reach the surface with a reserve tank pressure that you set before the dive. To give a practical example, if you set 500psi as your “reserve pressure on the surface”,the RBT will show “zero” when you must begin your direct ascent, including all stops, to reach the surface with 500 psi still in the tank. So, zero RBT does not mean “out of gas” and should not trigger an urgent response, but it does mean “begin your ascent” and you should do so in a calm orderly way.
As you can see, RBT has no relation to NDL. NDL is related to decompression only, and is not a guarantee of adequate gas to do the dive. So, even if you have a lot of NDL, if you are breathing through your gas quickly, you will have to surface before your NDL is reached, because you used up your gas. The computer lets you know when your gas is so low that it is time to head for the boat with your pre-set safety margin. In fact, since going OOG is far worse than exceeding your NDL, be happy you have a computer that gives you not one but two notifications related to your gas supply (actual pressure in tank, and RBT).
As you can also see, RBT also has nothing to do with the MOD of the mix, which relates solely to oxygen toxicity and not gas consumption. This is where your DM made a dangerous mistake and you did not catch it because you were not familiar with the computer . By trying to artificially get more time, you put your max Po2 into a dangerously high setting of 1.6 for the mix.S ince you set it, the computer will use this as “acceptable” to you and not warn you until you actually exceed 1.6. This is risky given the consequences of an ox tox seizure. If, for some reason, you had better gas consumption or a bigger tank on the next dive and so stayed down longer or deeper, you could have been exposed to high PO2. Please, set your computer back to 1.4. However, you absolutely did the right thing by following the computer’s information and not second-guessing the gas RBT on the second dive, even if you thought something was off and your first solution did not work.
Now, perhaps what happened will be clearer. You are diving approx. 80-90 feet.Don’t know what cylinder you had but maybe the ubiquitious AL 80? Also don’t know your starting pressure. Those two factors determined your available gas.It does seem pretty fast to go through half an AL 80 in 10 minutes at 90 feet, but the computer adjusted to show more RBT as you went shallower, which is entirely as expected and indicates the algorithm was functioning. Maybe, you were just using more gas than usual on these dives.
However, Tbone suggested that you have your reserve pressure set too high. If so, the RBT algorithm will send you up early maybe even at 1200 or 1300 psi. You need to check your reserve setting. If, after setting the appropriate reserve, the RBT still seems off, have the computer checked.
Finally, your post shows confusion and misunderstanding of the significance of, and differences between, NDL, MOD and gas consumption. I would suggest that you review your OW and nitrox materials in addition to thoroughly reading your manual.
Don't take this wrong. I am a highly advanced rec diver, diving the limits of rec depths, with nitrox mixes, in currents, drifting, doing safety stops in the open ocean while drifting, all of the above at night, etc. When I got my Galileo, I spent three hours with the manual, and went into each and every menu setting to understand how it worked, what it showed, what I needed to set, etc. I did this more than once until the computer was second nature. Once you have it set up, the Galileos are incredibly easy and intuitive during the dive. But, you need to know the basics, and have the computer set to show you what you need to see.
