First, good for you. Asking about this here is really smart. I've been reading posts here and there is some really good advice.
I just completed my AOW and have several dives to 90-100 feet and am planning several with max depth of 120 in the near future. I will be diving a single HP 120, no pony. Diving with permabuddy.
How experienced is the permabuddy? This could help if they have a lot of experience. By max depth of 120' do you mean you want to go to 120' or are you saying that you have set a personal limit of 120' (even though most agencies are okay with 130'-140'). I have set a max depth of 110' for myself; it is based on some of the wrecks I would like to dive. What is your 120' based on?
I have planned my rock bottom PSI and EANx mixtures at 1.3 PO2 at max depth and wanted any other helpful information/preperation that was out there.
Think about worst case scenario. As others have pointed out, is the 120' a hard limit or will the environment let you go further? Calculate the depth for 1.6 PO2 and keep that in mind. Assuming your 1.3 PO2 is based on 120' max depth then you are diving 28% nitrogen. This gives you a max depth of 155' before oxygen toxicity. If the environment will let you go deeper, you have added things to worry about.
Nitrogen Narcosis - Never experienced it. Tips or advice for a first encounter?
Read more postings on scubaboard.com. Did you test yourself? How? If you are just going on how you felt and you were narc'd you might not have realized it. Slightly narc'd is like slightly drunk. You don't realize it until you miss that stop sign or red light.
To really know you need to do things that make you think. Some things instructors have tried are:
- open a combination lock
- tie a knot with a piece of rope
- do simple math
The open combination lock is pretty easy to do. You open the lock on the surface while someone times you. Do it again at depth and see how much slower you are.
Or take three pieces of rope and tie three different knots while someone times you. Go to depth and do it again. Don't do it with one piece of rope. Do it with three pieces of rope. Keep the results. Check the knots when you get to the surface. If they weren't right, do it again but 10' shallower.
For simple math, if I put my thumb up it means ADD. If I put my thumb down it means SUBTRACT. If I show you my palm I want the total. I will then do something like 2 fingers, thumb up, 1 finger (= 3), thumb up 2 fingers (= 5), thumb down, 3 fingers (= 2), palm. You then respond with the current total of 2. Someone else is timing us and recording the values and result. I'll do it again at depth.
If you and your permabuddy are doing these, get anoehr buddy pair to monitor time and air. If you are both narc'd you might not realize how long these exercises are taking you until you are out of air.
If you are doing something like these test and you did fine then you know you are not narc'd. Additionally, remember the person testing you might be narc'd. This is why I suggest bringing the results back to the surface for knot tying or having a third person time you for the hand math.
Loss of buoyancy at depth - Do you adjust continously on descent or at depth?
I'm REALLY surprised you have to ask. If you have been to 90'-100' you should know the answer to this. Actually, if you have any experience diving to 60' you should know the answer to this.
NDL - Not pushing it to the limit (ie 1 min remaining at the begining of ascent) do I need to account for longer ascent time in NDL? At 120 ft, using 2 minutes to reach my deep stop at 60 feet, how much NDL would you start your ascent with? (ongassing/offgassing threshhold)
Your wording is a little unclear but I'm going to assume you mean, you will NOT be beginning your ascent with only 1 minute remaining and you want to know how much air you need to be able to do an 30' per minute ascent and have plenty of air left. Kind of depends on your SAC rate.
Anything else to think about?
Your buddy. Is your buddy better or worse at air consumption? Are they diving HP 120 with no pony?
What kind of exposure protection are you wearing? The thicker the wetsuit the more compression at depth, the better your buoyancy needs to be.
Is there a risk of free flow? Do you know what to do if a free flow happens?
Are you doing anything else which can add risk? E.g. wreck penetration?