SSI Extended Range (XR) Nitrox Diving

… or “how can I safely extend bottom times on deep dives?”

Deep Diving. Dives deeper than 100 feet. What does that conjure up for you? Wrecks, hydro-coral, unique photo opportunities, and more? How often do you go Deep Diving? And what is it that keeps you from diving below 100 feet? Most likely, the answers are “not very often,” and “because of the limited bottom time we have in which to explore these dive locations,” respectively. Enter Extended Range (XR) Nitrox Diving.

For most of us, Deep Dives are limited by nitrogen on-gassing.  As we dive deeper, we on-gas nitrogen more quickly. And, as Recreational Divers, we are limited to “no stop” diving, in other words, dive profiles that allow us to immediately return to the surface without any mandatory stops to off-gas, or decompress.  But what if we could plan and execute limited decompression dives?  What would that do for us?

XR Nitrox Diving enables us to do just that.  It is a Recreational Specialty in SSI, or you may consider it a first step into exploring the world of Technical Diving.  As a Recreational Specialty, we are still limited to 130 feet as our maximum depth, but we are now trained to plan and execute decompression obligation dives, utilizing an optimal Nitrox mix, that require a maximum of 15 minutes of decompression time.  After training, we are qualified to use up to 50% Nitrox as our decompression gas (although, for safety, we calculate our decompression stops using our back gas – more on this later) as well.  And, it requires a minimal amount of new equipment to execute – basically, a decompression stage cylinder with first and second stages, a 7 foot hose for our primary second stage, submersible pressure gauges on each first stage for redundancy, and a surface marker buoy with finger spool.

At this point, you’re probably saying, “Okay, enough of what it is, why would I want to do it?” To illustrate, it is much easier to look at some dive profiles. But first, let’s talk about what an “optimal nitrox” mix is, and how we determine it.

In Nitrox diving, we increase the amount of oxygen, while decreasing the amount of nitrogen, in our breathing gas to lower our nitrogen on-gassing during our dive.  Our “optimal nitrox” mix is the mix with the highest percentage of oxygen that we can use at the depth to which we plan to dive.  We can utilize the T-Formula below to calculate our optimal nitrox mix for the dive we are planning to execute.  In the T-Formula, the dots represent division signs, the “x” is for multiplication, and the relation of the three values enables us to calculate the third, given the values for the first two, in any combination.  Thus, we can calculate a Partial Pressure of a gas by multiplying the known Fraction of that gas by the Ambient Pressure (in terms of Depth).  Or we can calculate the Fraction of a gas by dividing the known Partial Pressure of the gas by the Ambient Pressure.

t-formula

For our optimal nitrox mix for a given depth, we know our Depth and, when planning Nitrox dives, we plan for a Partial Pressure of 1.4 atm for that depth (as you recall from, or will learn in, your Nitrox Diving Specialty).  We can find the optimal nitrox mix for a dive to 120 feet by utilizing the T-Formula as shown below, resulting in a fraction of 0.30, or 30% Nitrox.

supporting-equations

Now that we know our optimal nitrox mix, let’s look at three dives to 120 feet: one using air, one using optimal nitrox, and finally one using optimal nitrox with up to 15 minutes of decompression time (an XR Nitrox dive).  Our examples below use V-Planner, with a small amount of conservatism set.  Compare these to your dive tables for profiles one and two.

Dive Profile One: 120 feet depth, utilizing air.  Results: we must begin our return to the surface after 9 minutes of dive time have elapsed.

dive-profile-1

Dive Profile Two:  120 feet depth, utilizing 30% Nitrox (optimal).  Results: we must begin our return to the surface after 13 minutes of dive time have elapsed (a gain of 4 minutes at depth).

dive-profile-2

Dive Profile Three:  120 feet depth, utilizing 30% Nitrox (optimal), allowing for maximum of 15 minutes of decompression using back gas (30% Nitrox) as our decompression gas.  Results: We must begin our return to the surface after 25 minutes of dive time have elapsed (a gain of 16 minutes over air, 12 minutes over nitrox, no-deco), making decompression stops at 30, 20, and 10 feet.

dive-profile-3

Remember, we said that our decompression times are set utilizing our back gas (whatever optimal nitrox mix we have in our main cylinder) rather than our decompression stage (which can be up to 50% nitrox).  This is for two safety purposes at this Recreational level: 1. If we can’t switch to our decompression stage for some reason, we are still safe and 2. If we do switch to our decompression stage (at a higher Nitrox percentage than our back gas), we are actually off-gassing faster than our decompression times needed, meaning we are degassing more and increasing our safety profile.

In the final example, the decompression gas required is only 14 cubic feet (based on a surface air consumption rate of 0.5 cu ft / min), meaning a 19 cu ft pony bottle would be sufficient for the decompression stage cylinder, not the bulky, full-sized cylinders we associate with Technical Diving.

From these profiles, we can see that, with some basic training and minimal additional equipment, we can dramatically extend our dive times at depth, safely, enabling us to explore more of the adventures that await us in the deep.

There are some prerequisites to enroll in an XR Nitrox Diving specialty. These are:

  • Minimum age of 16 years
  • A Dive Medical exam based on the Medical History section of the Student Record
  • Deep Diving Specialty certification
  • Nitrox Diving Specialty certification
  • 24 Open Water dives logged

If deeper adventures are what you’re looking for, take the first step. Enroll in an XR Nitrox Diving course.

Author: mikecisdivers

Owner of Channel Islands Scuba, Thousand Oaks, CA.

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