Best Practices – Dive Planning Part 2

By | Knowledge Base, Training

Duane Johnson - Technical Diving Instructor

Duane Johnson - Technical Diving Instructor

In my previous article about dive planning, I discussed how divers can calculate no decompression time without the use of dive tables or dive computers. In this article, we will discuss how to calculate multi-level dives using that same approach. In addition, we will discuss how to calculate repetitive dives when using this method. So get ready to eBay that dive computer.

The biggest selling point dive stores make for dive computers is that they calculate your NDL every 1 second. When doing multi-level dives, this is a benefit over tables as dive tables only have one depth. Unless you are using the PADI wheel dive planner. However, we can use average depth and compare that with the results of the 120 rule to determine our NDL for a multi-level dive.

Disclaimer
Calculating average depth is not that difficult of a task and can be done rather quickly in your head. However, it does take some discipline. You MUST check your depth gauge at regular intervals. You MUST be able to remember your previous depth and NDL calculations. If you do not have this simple discipline, the you should stick with your dive computers or tables.

First off, when we look at how averages are calculated, there is some form of interval of measurement. When we talk about average depth in scuba diving. That interval is time. We must check our depth gauge at regular intervals to calculate the average depth of all those intervals. I regularly check my gauges every 5 minutes. I do this for every type of dive, from a shallow 30 foot dive to a 200 foot dive on trimix. For deeper dives, I will check my gauges more frequently. But 5 minutes is the longest I will let it go. So every 5 minutes, I’m checking my depth, bottom time, and tank pressure. We’ll talk about gas supply in a later article.

So let’s walk through a simple example so you can see how it works. I’m on a dive to 100 feet on air (yeah, yeah, I know. Humor me!). After the first 5 minutes I check my gauges, I’m at 100 feet. So my average depth from the surface to 100 feet is 50 feet. Now we are taking into consideration that we did a descent of about 50 feet per minute. If the descent is faster, then we can pad some more depth to the average depth to account for being at depth a little longer. So, after the first 5 minutes, my average depth is 50 feet. In 5 more minutes, 10 minutes into the dive, I check my gauges and I’m still at 100 feet. My average depth is now the average between 50 feet and 100. This results in an average depth of 75 feet after the first 10 minutes.

Let’s stop at this point and plug average depth into the 120 rule. After the first 10 minutes of the dive, I have an average depth of 75 feet. Using the 120 rule, I know I have a total of 45 minutes of NDL time. However, I’ve used up 10 minutes so far. Thus, I have 35 minutes remaining on my NDL.

By now I hope the light bulb has gone on above your head. The question that should be on the tip of your tongue is “But if you stay at 100 feet for more time, won’t your average depth increase?” Ding, ding, ding!!! Absolutely. So let’s continue our example and plug average depth into the 120 rule at each interval. At 15 minutes into the dive, my depth is 90 feet. So the average depth is the average between 75 and 90. This is going to be 82. But lets round up, for conservatism, to make the numbers more manageable. Let’s call our average depth 85 feet. Plug that into the 120 rule, we get a total of 35 minutes total NDL time, subtract our our 15 minutes we’ve been on the dive already, we have 20 minutes of NDL time remaining.

Now let’s ascend to our next level in our multi-level dive. At 15 minutes into the dive, we ascend to 60 feet to check out part of the reef there. Assuming we ascend at a rate of 30 feet per minute. This may be slow, but let’s do it for the sake of argument. To go from 90 feet to 60 feet, our travel time is 1 minute. Then we will be at 60 feet for 4 minutes before checking our gauges again. At 20 minutes into the dive, we calculate our average depth between 90 feet and 60 feet. This gives us 70 feet average depth and 30 minutes of NDL time remaining (120-70=50-20 minutes of the dive already=30 NDL time remaining). So by moving up to the next level of our multi-level dive, we just gained 10 more minutes of NDL time.

As you can see, the more time we spend at the deepest part of the dive, our running average starts to get deeper and deeper. Then when we move shallower, our total average depth is weighted more towards our deeper section of the dive because we spent more time there.

At this time in our example, we are most likely coming close to hitting our gas reserves and need to ascend. Hopefully, you have seen how we can calculate average depth and apply it to the 120 rule. We can do this for square and multi-level profiles. Where average depth gets tricky is in saw tooth type profiles. However, those profiles are mostly found in cave environments and we’ll save those calculations for your cave class.

In the next article, we’ll discuss ascent techniques.

Dive Safe,
Duane Johnson
Precision Diving

Best Practices – Dive Planning Part 1

By | Knowledge Base, Training

Duane Johnson - Technical Diving Instructor

Duane Johnson - Technical Diving Instructor

In this series, we will take a look at how to create a solid dive plan. First, let’s take a look at what is being taught in the traditional open water courses. When I went through my open water course, I was taught that the most important component of a dive plan is our No Decompression Limit, or NDL. The NDL was the factor that controlled how long we could stay under water. As I began my journey towards better diving, I learned that this was wrong. The controlling factor of how long we can stay under water is determined by how much gas we are carrying, not our NDL. This article will not discuss gas management. That will be discussed in another article. Instead, I want to discuss how NDL can be determined in another way. That is, we don’t need to rely on tables or a dive computer. We can calculate NDL times in our head. [Read more...]

Basic 6 Skills of Scuba Diving

By | Training

The Recreational Scuba Training Council (RSTC) is the training council that determines the minimum skills that need to be taught for scuba diving classes. Most of the big training agencies such as PADI, SDI, and SSI are all members of the RSTC and therefore have adopted the minimal skills required by the RSTC. In the basic open water class, there are 20 skills that students are required to master. Some of these skills are more useful than others such as regulator and mask skills that we use on every dive. There are 6 basic skills that we must master in order to lay a strong foundation for sound diving.

The basic 6 skills of diving are progressive. So as we go through the basic 6, please remember that every skill will be a small piece of the next skill. The basic 6 are the following:

1) Regulator Removal and Replacement – This skill is simply taking the regulator out of your mouth, re-inserting it in your mouth, and clearing the water out of the reg. When we take the regulator out of our mouth, we don’t want to cover the front of it with our hand. This may restrict access to the purge button when doing air shares. Remember, each skill builds upon the previous one. So let’s get into the habit of doing it right the first time. Some training agencies teach that if the regulator is out of your mouth, you should be blowing tiny bubbles. The theory behind this is that if you start to ascend while holding your breath, then the risk of rupturing a long is strongly increased. However, if you hold a normal breath and your buoyancy doesn’t change, then the risk of rupturing a lung is not high. This is simple physics. Gas expands as we ascend, but if we are neutrally buoyant, the gas doesn’t expand. Of course, if you start to ascend, start blowing those tiny bubbles.

2) Regulator Switch – This is pretty simple. We remove the regulator from our mouth and clip it off to the right chest d-ring. Then put the backup regulator in your mouth and clear it. Are you starting to see how skill #1 comes into play? If can’t put a regulator into your mouth and clear it, you can’t switch to another regulator (buddies or your own backup). We clip off the long hose to make sure it isn’t hanging down to catch on something. It also builds the muscle memory for doing a gas switch in technical diving.

3) Regulator Recovery and Replacement – If your regulator comes out of your mouth for any reason, it should not be a time to panic. Simply put the backup regulator on the bungee necklace into your mouth, clear it, and then find the primary reg. We can find the primary regulator by rolling slightly to our right. That should drop the regulator down and we should be able to find it by feeling our right shoulder with our left hand. If we cannot find it, then we can simply find the long hose as it is coming around our torso and follow it with our hands. The second stage is guaranteed to be at the end of the long hose. :) Once we find the primary regulator, remove the backup and insert the primary and clear it.

These first three skills build the foundation for even more critical skills such as doing air shares and gas switching.

4) Mask Flood and Clear – This is one that everyone should have learned in their open water class. Simply flood the mask by cracking it from the top to let the water in. To clear the mask, crack the bottom seal, exhale through the nose and look up slightly. This will fill the mask with air and force the water out of the bottom. If you try to clear by cracking the mask seal from the top, the air will force water to the bottom of the mask. Thus not getting all of the water out of the mask. This skill is where the low volume masks are a great benefit. A low volume mask doesn’t have much of an air space between the class and your face. This means that it doesn’t take a lot of air from your nose to clear the mask. Also remember never to over exhale when clearing the mask. This causes your buoyancy to change.

5) Mask Removal and Replacement – This skills is critical in the event your mask comes off and you have to switch to a backup mask. Not having a mask on, should not be a situation for panic. Situational and buddy awareness comes into play here. If you see your buddy’s mask come off, you should be right there with your backup mask out. To perform this skill, simply flood your mask as mentioned in skill #4, but do not remove the mask just yet. This allows us to get used to the cold water on our face. If we were to yank our mask off and that cold water hits us, our first response is to inhale. Many times, through our nose. That means we suck in some water. But if we keep the mask on, let it flood for a breath or two, then take it off it helps to reinforce that we need to remain calm when the colder water hits our face. Now we can take the mask off our head. When replacing the mask, first locate the nose pocket. This gives us a reference point as to the orientation of the mask. This makes sure that we are not putting on the mask up side down. Do you see the link from skill #4 to now?

6) Modified S-Drill – The S-Drill is also known as the safety drill. The full safety drill, simply put, is the air sharing drill. The modified s-drill is the mechanics of donating the long hose, switching to your backup, and re-stowing the long hose when done. The modified s-drill, you donate the primary regulator with your right hand, by grabbing the hose, and “unwrapping” the hose from around your head. When you grab the hose, your palm is facing your. To donate, remove the regulator from your mouth, raise your hand straight up then in front of you. While doing this, rotate your hand such that your palm is no facing away from you. This will put the regulator into the correct position such that the out of gas diver just has to open up, insert the regulator, and clear it. Remember to keep the mouthpiece facing down as to prevent free flow. So once you have donated the primary and extended it in front of you, switch to your backup and clear it. No move the regulator to your left hand, while keeping it extended in front of you. With your right hand, form an “O” with your thumb and index fingers around the hose. Trace the hose back to your right hip. If you have a canister light, un-tuck the hose from under it. If your hose is tucked into your waist belt, pull it out. Then windmill your right arm up and to the front of you while holding onto the hose. This will clear the long hose from behind the wing. Now the long hose is fully deployed. To re-stow the long hose, trace the hose with your right hand, windmill the hose back behind you so that it is behind the wing, tuck the hose under your canister light or into your waist belt. Now you form a small bend in the hose, with the reg in your right hand now, such that it is just big enough to go over your head and not get caught on your valves behind you. Loop the hose over your head and switch from your backup to the primary and clear it. Finally, trace the long hose so that you can verify it is in the correct position. Don’t forget to turn your head to the left. If the reg feels like it wants to come out, then somewhere the long hose is too tight and needs a little more slack so you can turn your head comfortably.

Here is video clip showing the modified s-drill:

These are the basic 6 skills of diving that form the foundation for many other skills. As you can see, each build upon previous skills. The modified s-drill now uses skills 1-3 as part of the drill. As with all skills, these should be done while neutrally buoyant and with good trim.

Please feel free to ask any questions or post any comments.

Dive Safe,
Duane Johnson
Precision Diving

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The Myth of the Mandatory Visual Inspection

By | Knowledge Base

I see many dive stores that have started the practice of requiring visual inspections to be performed if a tank comes in with less than 500 psi. This practice is just ludicrous in my opinion. If a scuba cylinder unexpectedly loses gas, then yes, a visual inspection should be performed to see if there is something wrong. However, if you breath the tank down to 400 psi, then there is really no need for an inspection.

First, let’s start off with some of the rules and regulations regarding the inspection of scuba cylinders. By Unites States law, all compressed gas cylinders, including scuba, must be visually inspected at the time of hydrostatic testing. Hydrostatic testing is required every 5 years. So, by law, scuba cylinders must be visually inspected every 5 years. The scuba industry has adopted a recommendation by the Compressed Gas Association (CGA) that the cylinders should be visually inspected annually. By following this recommendation, the number of failures in scuba tanks has been greatly reduced. So, scuba cylinders do not need to be visually inspected annually. However, the annual inspection is a good thing and the practice should be continued.

The argument is that if the scuba cylinder’s pressure drops below 500 psi, then potentially, water or hydrocarbons can enter the cylinder. Ever watch Mythbusters? Many times when they need to propel something, the use air pressure. This air pressure is relatively small, less than 100 psi. Now, if less than 100 psi can propel a small object at hundreds of feet per second, do you really thing that water or dust can enter a scuba cylinder? Not really. Should we be concerned about water entering our scuba cylinders, yes. Especially if we use steel cylinders. That water can cause rust and possibly contaminate our O2 clean cylinders.

If you have taken a gas blending or Nitrox course, you should know how Nitrox is made. One of the most popular methods of blending Nitrox is called partial pressure (PP) blending. With PP blending, the blender first adds pure oxygen in the cylinder, then tops off with air. But in most instances, the scuba cylinder must be drained to empty in order to get the most accurate blend. So if a dive shop that has the policy of requiring a VIP every time your tank goes below 500 psi and they PP blend Nitrox, does that mean they are doing a VIP, and charging your for it, every time you get a Nitrox fill? That’s a very expensive Nitrox fill.

The truth is, that you do not need a VIP if you scuba cylinder falls below 500 psi. If it does so mysteriously, then it does need to be inspected. If you breath it down to empty underwater, it wouldn’t hurt to have it checked just out to be safe. But you shouldn’t have to pay for a VIP if the tank is at 300 psi. This is just another way for a dive store to unnecessary charge their customers. In this tough economy, the more money in your pocket, the better. That $15 for the mandatory VIP could be a couple more air fills so you can go dive again. Ask the person behind the counter why they have that policy. Ask them to prove their arguments. But most of all, be an educated consumer.

I welcome your questions and comments.

Dive Safe,
Duane Johnson
Precision Diving

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