Mentally Estimating Deco in a Cave
So there you are, 40 minutes into a cave dive, currently at 100ft/30m. You check your computer, and it’s showing a no decompression limit (NDL) of 10 minutes. You didn’t expect the cave to go this deep. Your heart starts to beat a little faster, you can feel your brow start to sweat under your mask as you think to yourself “I didn’t bring any O2 for deco, if I turn right now it’ll still be 40 minutes until I’m out! How much deco am I going to have to do? Do I have enough back gas to finish it?
So what do you do? Do you have to turn the dive immediately? Can you continue into the cave if you still have penetration gas remaining? Unfortunately, these are questions your dive computer (even if it’s a Shearwater) simply cannot answer. However, by following a few basic rules of thumb, you can quickly calculate the answers to these questions in your head and adjust your plan.
Now, before you get all defensive and insist what I’m about to tell you is witchcraft nonsense and try to come up with examples of where it doesn’t work, I’ll agree with you to some extent. Using this method to calculate decompression isn’t perfectly accurate, but no decompression science is. It works within a fairly narrow depth range and requires the use of standardized gasses. It’s used as an easy way to estimate an approximate NDL or decompression obligation, which can be verified either with desktop/mobile software prior to the dive and/or on your dive computer while the dive is in progress. It’ll get you pretty close, and will be pretty conservative. So once you actually start your ascent, nobody is saying you can’t follow the computer to the surface. But, you’ll know what to expect when you get there if the parameters of the dive have changed at all. As my dad used to say, “measure with a micrometer, mark with chalk, cut with an axe”. So if we think of decompression science being inexact (cutting with an axe), there’s no need to waste our time measuring with a micrometer while in the water.
In this article, we will discuss how to use average depth compared to 32% Nitrox tables to mentally estimate NDL or decompression obligations using a couple of real life examples from popular cave systems.
32% Nitrox Tables, The Rule of 130 (imperial) and Ratio Deco
The basic foundation for this skillset comes from having an understanding of the 32% nitrox tables. We won’t get into all of the benefits of using standard gasses, but for most cave diving locations, 32% is considered the standard gas which you will find readily available at most fill stations.
If you look at the National Oceanic and Atmospheric Administration (NOAA), you’ll see a very clear trend in the 60ft/18m-100ft/30m depth range. Starting at 100ft/30m with an NDL of 30 minutes, for every 10ft/3m shallower, the NDL increases by about 10 minutes (until you reach 60ft/18m where it jumps up to almost 100 minutes). This makes memorizing these tables extremely easy, and we can incorporate an easy rule.
In imperial, you simply take your baseline of 130 (a baseline number that is not relevant to the dive at all, but constant with our NDL calculations for this depth range) and subtract the depth of the dive in feet, you will arrive at the NDL for that depth (imperial). In metric, it is slightly more complicated (YES, an application where imperial is easier than metric. It actually exists!). You take a baseline of 30 (the NDL at 30m) and subtract 10 minutes for every 3m of depth reduction.
Example (imperial): 70ft dive
130-70= 60 minutes of NDL
Example (metric): 21m dive
(30-21m)/3= 3m steps
30+30= 60 minutes of NDL
Now, caves come in a wide variety of depths, so why are we looking at such a narrow range? Well, for the most part, this is where these rules are most relevant. Any shallower than 60ft/18m, and it’s unlikely you will incur any significant deco obligation since the NDL begins to rise exponentially. Any deeper than 100ft/30m and we are now considered “deep” cave diving, where we would begin to incorporate different bottom gasses and new rules would apply.
Great, so we can now mentally calculate NDLs, but what if our dive exceeds the NDL? Can we mentally calculate what our obligation will be? Yes, we can create a rough estimate. If you punch the numbers into any old dive planner application (the dive planner function on your Shearwater, MultiDeco, Baltic, etc.), you’ll notice that for every minute you exceed your NDL, you’ll incur around 1 minute of decompression obligation at 20ft/6m on 32%. Most planners may have you making a couple of very short stops (1 minute or less) starting at 40ft/12m or 30ft/9m, which is really just slowing your ascent a bit, and the cave profile generally forces us to do anyways. When mentally calculating our deco, we simply look at the 20ft stop.
Now, I say “around” because no, it’s not exact, and will depend slightly on the exact algorithm you are running as well as the depth (at 100ft/30m it is slightly more, whereas at 60ft/18m it is slightly less). But on average, it’s about a 1 to 1 ratio of exceeding NDL to decompression obligation. This is where the term “Ratio Deco” comes from.
But what about deco gasses? I don’t deco on bottom gas! Well, if we now punch a variety of plans into the old planning software adding 100% oxygen as a decompression gas, you’ll notice the obligation at 6m/20ft is roughly cut in half. That’s pretty easy to memorize, if you ask me. Why 100% instead of 50% or some other mix? Well, for this depth range, almost all of our deco is at 20ft/6m or shallower where oxygen is the most effective gas available.
Using average depth
OK, I know, caves aren’t square. They go up and down, sometimes with a wide range of depths on a single dive. Well, remember, decompression science is inexact. While our dive computer is certainly a micrometer and calculating NDL or deco obligations on the fly using our exact depth, when we are roughly calculating our deco mentally, we are cutting with an axe. Instead of trying to use specific depth and constantly recalculating, we can instead use the average depth of the dive. Since the cave profile will likely be the same on the way out as it was on the way in, at any moment in the dive, our current average depth will be very close to what it will be back at the entrance if we turn the dive at that moment. At any point on the dive, we can take a peek at our average depth and mentally run through the calculations above and estimate our deco to determine if we need to turn the dive, or if we can proceed into the cave based on the resources we have available. Then, when we reach the exit, we won’t be surprised by what our computer is telling us to do, it will be roughly what we had estimated.
But, is the average depth displayed ALWAYS accurate (within cutting with an axe tolerances) in relation to deco obligation? Well, yes and no. Sometimes it’s important to “weight” the average depth a bit deeper in order for it to be more accurate.
For example, the dive you are planning will require significantly more time in the shallow section of cave close to the entrance. Ginnie Springs in High Springs, FL is a good example of this. Installing the primary reel and progressing through the first section of cave where it is much shallower can be more time consuming on the way in, where the exit through that section will be very quick on the way out. This will create an artificially shallow average depth and you will be burning NDL/building deco obligation faster than what the average depth would tell you since you will not be spending as much time in the shallow section on the way out.
For example, it takes you 10 minutes to install the primary reel and another 5 minutes to proceed through the first 300ft/100m into the cave where your average depth will display about 40ft/15m. The cave then quickly drops to around 90ft/27m. If you still have 20 minutes of penetration gas available, at the turning point of the dive you will probably be showing an average depth of about 70ft/21m, giving you the impression that you have about 20 minutes of NDL remaining. The exit from where the cave will begin to shallow will be much faster than the swim in and likely only take about 5 minutes as opposed to the 15 minutes it took to get in, so you’re assuming you’ll make it back with no deco obligation. You reach your primary reel and begin to ascend to the entrance, glancing at your computer and noticing your average depth is actually 80ft/24m and your computer wants you to do 5 minutes of oxygen deco. What happened? On the way out, you didn’t have the time in the shallow section that you had on the way in to “erase” the deeper portion of the dive and off-gas a bit.
How do we weight the average depth to compensate for this issue? We can use the “reset average depth” function on our computers at a point moving into the cave to create a deeper average. We do this at a point where we have cleared a time-consuming shallower portion of the dive, often times this is most convenient once you’ve installed the primary reel and tied into the permanent guideline. It also depends on the profile of the cave and anything that might slow your entrance, but not impede your exit, such as high flow. In the case above, had the team reset average depth after taking 10 minutes to install the primary, the average depth would now be starting at 50ft/15m for the remaining 5 minutes of the shallow section. This will weight the overall average depth more accurately to closer to 80ft/24m, so you’ll either know you need to turn a bit early if you’re not planning to do any deco, or that you’ll be expecting deco if you continue into the cave.
Putting it all together
Let’s look at some examples of this method being implemented in real time. The first example is a dive I recently did in a “new to me” site, Cenote Kaan Ha in Tulum, Mexico.
When looking at the map and formulating our dive plan, it appeared based on our available gas and chosen route, the average depth would be somewhere around 50ft/15m. Great, around 100 minutes of NDL. Knowing our gas consumption and estimated range, it looked like we might make it to a slightly deeper section with some penetration gas remaining. We decided to bring some oxygen for deco in case we were able to spend a bit of time in the deep section.
Turned out, we made it there quite efficiently, and were able to poke around in the deep section for a while. When we turned the dive at just shy of an hour of penetration, we were at a depth of about 70ft, where we had been for about 10 minutes or so. The NDL displayed on the computer was about 20 minutes. Clearly we had a longer trip out than 20 minutes, so the assumption based on that information would be about 20 minutes of decompression on oxygen. However, as you can see in image 1, the average depth at that moment was about 55ft/16m. Rounding that up to 60ft/18m (cutting with an axe), we could estimate that our NDL was in fact somewhere in the 40 minute range (100 min NDL minus 60 minutes for the time we had already spent at depth). If our exit took as long as our penetration, we would be expecting about 10 minutes of decompression on oxygen (60 minute exit minus 40 minutes of NDL equals 20 minutes of back gas deco. Divide that by 2 and we have 10 minutes on oxygen).
In this example, we had a bit of flow at our backs on the way out and the numerous restrictions we navigated on the way in were easier to pass now that we knew the easier way through. Our exit was a bit more efficient and we exited in about 45 minutes. As you can see by the computer display in image 2, you’ll notice that our deco obligation was about 1 minute, which is quite close to what we would have estimated:
58 minutes of penetration + 45 minute exit = 103 min bottom time at roughly 60ft
100 min NDL minus 103 = 3 minutes of back gas deco
3 minutes of back gas / 2= 1.5 minutes of Oxygen deco.
Another good example can be seen in the dive graph pictured in image 3. This was a Cave 1 level dive (limited penetration and no decompression) at Little River Spring in O’Brien, FL. As you can see from the profile, the dive was made to a maximum depth of about 100ft/30m. When the dive was turned at about the 20 minute mark, we were at 94ft and the NDL displayed was about 13 minutes. Uh oh, no deco allowed in C1! But knowing that the average depth of the dive at that point was less than 70ft/21m, everyone was comfortable knowing we would not exceed our NDL.
Also, by looking at the graph, you can see that the initial shallow portion of the dive (up until the minute 15) is a bit longer than it is on the exit. this would be an excellent example of a dive where it would be important to weight the average depth by resetting it once the primary reel has been installed.
Using Shearwater functions to help
Our dive computer is generally stupid in regards to cave penetration since it doesn’t know how long it will be before we can begin our ascent. So, does Shearwater provide any useful tools for us to help?
Shearwater’s flexible display options allow us to add things like average depth to the main screen so we have quick access to this valuable information. But, that’s not all, there are additional tools such as @+5 available to help us estimate what our deco obligation will be if we stay at our current depth for an extra 5 minutes. The Teric also has an “NDL @“ function, which will display a table showing what your NDL will be at for various depths from that moment forward on the dive. We can use this function if we know our current average depth and how long it will take us to exit. We can pull up that menu and see that for a range of average depths that we have approximately x minutes of NDL remaining, regardless of what our current depth is. Essentially, doing what we have just done in the examples above.
What our Shearwater does not (currently) do, is show TTS if we begin our exit now, taking into account the time and depth of the exit will be similar to the penetration. Do yourself a favor and bug shearwater, suggesting this type of function. I think I’d call it something like “cave exit TTS” 😉
Moving forward (deeper caves, ccr, etc.)
Cave dives are not limited to 100ft/30m of depth or just diving open circuit. Can these estimates be implemented on deeper dives or those incorporating rebreathers? Yes, absolutely, we just do it slightly differently. On deeper dives, we simply use different ratios to estimate our decompression obligation. On CCR dives, we can use constant PO2 tables instead of the 32% Nitrox tables. While the rules might be a bit different, the theory is pretty much the same.
At the end of the day, your computer will be the most accurate measure of NDL and deco obligation once you begin your ascent. However, now armed with a valuable tool, you can make better estimates of what that will be prior to reaching the entrance of the cave.