Meet new scuba divers, maintain a virtual dive log, participate in our forum, share underwater photos, research dive sites and more. Members login here.

Selecting Best Mix
Scubablog - 5/26/2014 8:54 PM
View Member Articles
Category: External Blogs
Comments: 0
 Here’s a question for all the recreational NITROX divers out there. How do you go about choosing your mix for a given dive? I know it sounds like a stupid question, since we all went through NITROX training, and we all remember the simple math required to select the best mix, but there is some common sense judgment that I’m not sure is being completely passed on in all cases.

The reason I ask is this. I was on a dive in Key Largo recently with a dive operator who will remain nameless. I was down there with my long time dive buddy Derek Sullivan and we were on the way out to dive the Spiegel Grove wreck. Once we got our gear on board and began setting up, we realized that there were a large number of people on the boat who were using this as a certification dive for a NITROX course that had been given by the charter operator. Not only was the divemaster going from student to student analyzing their mix for them, (the students should have been doing their own analysis) the operator had provided the students with 32%. Now, if you’re not familiar with the Spiegel Grove wreck, it’s 130 feet to the sand with the main deck being at around 90 feet. We all know that we need to choose a mix that will keep our PPO2 at or below 1.4 to be safe. Now, assuming that the students will stay on the main deck at around 90 feet, their PPO2 will stay around 1.2. Great, right? Not so fast! What happens when one of them drops a GoPro over the side or down into a hold? What if a dive computer or an expensive light drops into the sand, will they just watch it fall? Not likely. What is the magic number we want to steer clear of during the working portion of any dive, 1.6 correct? Ok, well, if someone drops something they value into the sand at 130 feet with 32% NITROX, their PPO2 will go to 1.57% when they go to retrieve it. That’s dangerously close to 1.6, and WAY past our conservative number of 1.4. Is that a guaranteed O2 hit? Of course not, but if the water is cold, they have been working hard against a stiff current and maybe they’re nursing a bit of a hangover, is it possible? Yep!

I think most of us take a little license with the PPO2 from time to time. I’m not going to hang around a dive shop while they re-mix a fill just because it would put my PPO2 at 1.45. I’ll dive it. But, I wouldn’t intentionally dive a mix that would put my PPO2 at 1.57 for any of the working portion of a dive. But I feel like that’s exactly what this operator set its students up to do. This is just my opinion and it isn’t the first time I’ve seen people choose a mix that’s right for the intended dive without taking the possibility of an unintended trip to the sand into account.

It also concerns me when I see on a message board discussing an upcoming dive, someone ask, “Hey Bob, what is the recommended NITROX mix for this dive?”, or “What mix are you going to use?”. Again, in my opinion, anyone who doesn’t know how to determine the best mix for a dive shouldn’t be breathing NITROX.

It just seems to me that any diver who is advanced enough to have a NITROX certification should:
  1. Always personally analyze the gas they’re breathing.
  2. Fully understand the math required to determine the best mix and why it’s important.
  3. Choose a mix that’s suitable for the worst case scenario in case something goes south.
Recreational scuba diving has an amazing safety record and I genuinely believe that it is in large part due to the fact that most instructors scare the bejesus out of students with “don’t do this or you’ll die” statements. “NEVER HOLD YOUR BREATH” and “NEVER DIVE ALONE” are a couple that come to mind that serve to keep a novice diver from doing something stupid until they have enough experience to be able to put both of those statements into a workable context. Why then are we telling novice NITROX divers to choose a mix suitable for the dive they intend to do when there are so many things that could change their plan on the fly?