Browsing Tag ascend
The 5 main trends in decompression
Decompression theory has a reputation of being complicated and hard to learn.
In my experience, it doesn’t have to be!
We can reduce the complexity if we just keep in mind these 5 main trends of decompression theory.
*Note: this article isn’t meant to be a thorough understanding of decompression theory. The main goal is to simplify decompression theory in layman’s terms.
Trend 1 – The deeper you go, the more nitrogen you take up because of the higher pressure of the gas you breathe.
Whenever you scuba dive, you are breathing gas at ambient pressure.
This means that you are breathing more molecules of nitrogen for every breath that you take when deeper than sea level.
The deeper you go, the more molecules of gas you take in, even though the volume of gas in each breath doesn’t change.
This extra nitrogen is absorbed into your body and eventually, will need to be released when you ascend to the surface.
Trend 2 – The longer you stay, the more nitrogen will get absorbed into your tissues.
If you spend longer in the water at depth, then the more breaths you will take and more nitrogen will be absorbed into the body.
Trend 3 – If the gas you use has a lower nitrogen content, then there will be less nitrogen available to be absorbed into your tissues.
Let’s compare two gases, one with a lower nitrogen fraction than the other.
Which gas will cause more nitrogen absorption into the body given that all else is the same (number of breaths taken, depth and duration of dive)?
I think the answer would be fairly obvious.
This basic idea is the reason why Enriched Air Nitrox (EAN) gas mixes exist.
A commonly available EAN mix is 32% oxygen and 68% nitrogen.
This provides an 11% reduction in nitrogen content as compared to normal air.
Choosing an Enriched Air Nitrox mix is always a good idea for reducing your decompression obligation.
Trend 4 – The slower you ascend, the more excess nitrogen will leave your body.
A less commonly known trend would be how fast you surface from each dive.
By ascending slower, you’re allowing time for the nitrogen in your tissues to leave your body.
A faster ascent rate also causes any gas bubbles in your body to expand.
It will also cause the release of excess nitrogen as bubbles instead of being transferred back to the lungs for exhalation.
By slowing down our ascent rate, we can control the size of these bubbles and at the same time allow nitrogen to work its way out of our tissues.
Trend 5 – The longer the surface interval, the less nitrogen you will have before your next dive.
Once on the surface immediately after a dive, there will always be some excess nitrogen in your tissues. This is the concept of residual nitrogen.
The more residual nitrogen you have in your system, the more conservative you should be on your subsequent dive.
Thus, the longer you stay on the surface before your next dive, the more time the excess nitrogen has to leave your body.
The less nitrogen in your system, the better it is for your decompression obligation on your next dive.
Putting it all together
So what does this mean for divers?
How can we navigate the complexities of decompression while staying safe?
The 5 main trends in decompression
General rules
Once you understand these trends, then how do we put everything together?
I consider these 5 trends as adjustable variables in my overall decompression obligation.
I can tweak or adjust one or more while leaving the others in place, allowing me to ‘adjust’ my decompression obligation based on the dives I’m planning to do.
Planning a day of diving
Keep the number of dives per day to 3 (roughly 9 am, 1 pm, 4 pm, with a break for lunch).
You can add on a dawn dive and a night dive as these would extend the beginning and end of your diving day.
Let’s look at how this relates to a decompression table.
Let’s look at a PADI RDP table and trace a line that follows a surface interval of 1 hour and 2 hours.
The PADI RDP table showing a 1 hour and 2 hour surface interval line
We can notice that we end up with a pressure group of J and C respectively if we have a starting pressure group of Z.
With a more realistic pressure group of between G and M, you would end up with a pressure group between A to E after a 1 or 2-hour surface interval respectively.
With a 2-hour surface interval between each dive, 3-day dives are all that you have time for anyway.
What about if you happen to miss a safety stop?
If you do happen to miss a safety stop, then change up one of the other parameters on your next dive.
You could do a longer surface interval, do a slower ascent, stay shallower, or dive for a shorter time.
You can still change the depth and length of your dive if you are on a fixed dive schedule.
Always ascend slowly
For ascents, I would always take my time to ascend, limited by the gas that you have remaining.
However, I only do this after I’m shallower than 6m. Any deeper than that and you would still be taking on nitrogen into your system.
If there’s a nice shallow reef with things to look at while you are at your safety stop, and you also have plenty of gas left, then why not stay and enjoy the view?
Personally, I would ascend slowly (9 m per min) to half of my depth, then move at 3 m per min up to 6m.
Once at 6m, provided the team has gas, I’d take my time and enjoy the sights until everyone’s computers have cleared or we get bored!
We’ve been known to hang around at 6 m for 15 to 20 mins!
And always dive Nitrox!
As for gas choice, if you ask me, there’s no reason to dive air if there’s nitrox available.
If I can reduce my decompression obligation just by changing the gas I dive with, it’s a no-brainer.
There’s so much benefit in diving nitrox, and these days it doesn’t cost much either.
There you have it!
Some simple and easy to remember rules for planning a day of diving while keeping yourself safe.
Now, get out there and know your decompression obligation!
When to deploy a surface marker buoy (SMB)
The answer might seem obvious, but many divers neglect to shoot the bag early, and end up surfacing a long way away from their dive location and their boat. I’ve heard many stories of divers drifting for hours because of their boat crew could not locate them. Don’t let that happen to you.
I would always consult the diving conditions when deciding when to deploy my surface marker buoy. Generally, you should shoot the bag when you are starting the ascent in a normal dive, or whenever you are leaving the reef, whichever happens earlier.
However, if the conditions are bad, such as when there’s a strong current pulling you away from the reef, then you would need to deploy your SMB as early as possible before you get swept into the open ocean. Don’t delay getting the SMB to the surface!
Consider a situation where this wasn’t the case, for example, if the diver ascended to a safety stop before deploying the SMB, or in a situation where the spool wasn’t long enough to reach the depth the diver is at.
In these situations, the time taken to ascend to a shallower depth, coupled with the current, could mean that the diver drifts far away from the dive site. Given that the boat would be waiting for the diver to ascend near or on the dive site, this could mean the boat captain will not be looking out towards the ocean where the divers eventually surface.
Needless to say, it’s much harder for the boat captain to spot divers floating in the open ocean than next to the reef. Personally, I wouldn’t take it on faith that the boat captain will always know where to look for you when you surface. Do your part as well to guarantee that he can see you!
Finally, if you’re wondering whether to use an inflated SMB throughout your dive, read this article that discusses it in depth.
Choosing the best surface marker buoy (SMB) for your dive
Surface marker buoys are handy for a number of occasions, whether it’s to signal your position to your boat at the end of your dive or during emergency ascents.
When you need immediate help or when someone in your dive group needs to get to the surface quickly, you’d want to be able to deploy the bag without it causing other problems or making matters worse. That’s why choosing a suitable SMB for your dive is so important.
There are three main factors a diver should consider when choosing a surface marker buoy: its size, method of inflation and the length of its line.
- What size do you need?
To be useful, the marker must be visible on the water’s surface, so it should be as large as you can manage. My emphasis is on what you can manage. You could have the largest marker on the planet, but if you can’t inflate it properly, it would simply be a limp piece of plastic floating on the surface. That certainly won’t be noticeable from a distance — unless the boat crew is consulting Google Earth.
Therefore, the size of the SMB you choose should be the largest that you can reliably fill from depth. Personally I prefer a smaller, thinner bag that will stand upright when filled with a relatively small volume of gas. I use the Halcyon 1m SMB on all my dives. This bag will fill up completely with only a single regular breath at a depth of about 10m. If you are at a shallower depth, then you’d need to blow in a larger breath of air, but it’s definitely still possible to fill up the bag quickly and send it on its way without much stress.
Now you might be wondering, what if you’re diving at sites with large waves and surge? Wouldn’t a larger (more visible) surface marker be better? In response I would ask: Is the dive worth putting yourself in such a situation? If the answer is yes, then make sure you are prepared for it with a larger surface marker.
Remember that a larger bag must be filled with a greater volume of gas in order to stand upright on the water’s surface. This means you’d need to either blow more gas into it (by exhaling or purging from the regulator) or release the bag when you are at a greater depth to allow for more gas expansion. The latter would be a good option, but the downside would be the much longer line that you would have to reel in as you ascend.
I would normally only inflate these larger surface markers once arriving on the surface and the boat is further away. The bigger marker will be easier to spot from a distance, but will also be harder to manage while underwater.
- Which method of inflation should you use?
Open circuit SMBs are some of the commonest in the market. To inflate an open circuit SMB, a diver typically purges air from his alternate air source into the bag. However, I do not recommend this method of inflation for a number of reasons.
The large amount of air purged from the alternate air source would likely cause a fairly significant and sudden buoyancy change. In an emergency or stressful situation, a sudden change in buoyancy could easily spiral out of control.
The force generated by the large volume of gas also means that the SMB will take off towards the surface really quickly. If the diver does not manage the line well, the line could get tangled with the BCD or tank, and drag you up to the surface. Nobody wants to be a human SMB!
I would always opt for the oral inflation method, because it is quicker and safer. The amount of gas that you exhale would not cause a significant buoyancy change (unless you inhale an extra-large breath of air right before). This makes the buoyancy change much more manageable. You wouldn’t need to find your alternate air source, too.
If like me, you prefer the oral inflation method, what’s left is to choose an SMB that works best with it. It’s possible to orally inflate an open circuit SMB, but due to the line being attached near where the open end of the SMB, there is a higher risk that your regulator might get caught in the SMB line or the SMB itself. Definitely not my first choice.
There are many SMBs on the market that are designed for easy and effective oral inflation. They normally have an oral inflator with a one-way valve. This, in my view, is the best one to use because it guarantees that the gas goes directly into the SMB. It’s also really easy to use. The Halcyon SMB I use does not have a locking mechanism, so you won’t need to fiddle with extra bits to inflate the bag.
- How long should the line be?
Some SMBs come with a 5m line, which means you can deploy the SMB only at the 5m safety stop. Obviously this is limiting, as you might run into other situations where you need to deploy the SMB at a greater depth. My advice: Always get a longer line that’s wound around a spool.
SMBs do not always take a straight path up towards the water’s surface, and might ascend at an angle when there is a current. As such, always make sure your SMB’s line is longer than your deepest planned depth. This means that if you are doing a 30m dive, then a 30m spool is not going to be sufficient. A 45m spool would be a better choice. I would use a 30m spool for dives to depths shallower than 25m.
In summary
My recommendation is to get a small thin SMB that you can reliably fill via oral inflation, and use a spool with an appropriate amount of line. Also, remember to practise deploying your SMB after buying one! A seasoned diver can easily get a bag deployed in less than 15 seconds, so keep practising until it becomes second nature!
Should I hold on to a line while descending/ascending?
Let’s start our discussion by looking at the line itself. What kind of line is it, and why is it at the dive site in the first place?
It could be a mooring line, which is usually anchored to a large concrete block on the seabed. Boats secure themselves to the mooring using this line.
Or it could be a diving shot line, which is typically deployed to mark a dive site. A boat may also sometimes use it to locate a submerged wreck. This type of line is often tied to a weight and is not securely anchored to the bottom.
If the line was not deployed by you or your boat crew, it could very likely have been placed by a previous group of divers to mark the location of their ascent or descent. In this instance, the line is often connected to an inflated surface marker buoy at the surface, with its spool is tethered to a rock or coral structure at the bottom. Such lines serve as a reference for divers visiting a particular dive site repeatedly for survey or conservation work, among other purposes.
From these examples, you probably have a sense of when it is appropriate (and not) to hold on to the line. In situations where the line is not anchored securely to the bottom, touching or tugging on the line may dislodge the weight or move the line, and render the location reference useless.
However, even if the line is securely anchored, I still wouldn’t recommend holding on to the line unless the current is so strong that a diver is unable to kick to maintain position next to the line.
I think it’s important that we use the line as a visual reference during a descent or ascent, and not as a means to control or manage buoyancy. A diver should actively manage his buoyancy whether he is holding on to a line or not.
Consider the situation where a diver is holding on to a line and ascending, but does not bother or remember to deflate his BCD. The diver does not realise that the BCD has inflated to the point that he is very buoyant. This could result in a runaway ascent if the diver, for some reason, loses his grip on the line.
Ideally, a diver should avoid holding on to the line during an ascent or descent. Instead, he should use his kicks and buoyancy to stay close to or within visual range of it. If the line is about an arm’s length away, one strong kick would bring the diver close enough to grab the line if need be. The rest of the team then forms up beside the diver, so everyone is able to grab the line too, if the situation calls for it.
Finally, when ascending next to a mooring line, divers should remember that there is likely a boat tethered to it. If there is a current, both the boat and the diver will be on the same side of the line, which means the diver is ascending directly below the boat. This can be dangerous if neither the boat crew nor the divers are aware of their respective positions. If this is the case, then make sure you check upwards to see if there’s a boat on the line, and if so, then shoot an SMB and drift off the line to be on the safe side.
Moral of the story: Don’t hold on to any line while ascending or descending if you can help it!