Differences between a backplate and wing (BP/W) and a weight integrated BC

April 11th, 2008   Filed Under Article  

Here is an overview of what I like about my backplate and wing (BP/W) over a weight integrated BC. I am a recreational diver, so I do not need the double-carrying abilities.

-   The shoulder straps attach to the backplate at the bottom (like a backpack), as opposed to being attached to the cummerbund/waist belt in a weight integrated (WI) BC. This allows a wider range of adjustability since the shoulder straps length can be adjusted without affecting the height or tension of the waist belt that much. With a WI BC, when you tighten the shoulder straps, you are really raising the cummerbund/waist belt higher on you since the shoulder straps have to be attached to the weight integrated belt in order to help hold up the weights. For me, being a tall person, I find this amount of adjustability of the BP/W makes it feel more “in place” on me than a WI BC.

-   Because the waist strap of a BP/Wcan stay lower on my body than a WI BC, and because the BP/W waist strap can be not as tight as the cummerbund of a WI BC, to me the BP/W feels less restrictive since I don’t have an elastic band velcro’d around me and constricting my diaphragm when I am trying to inhale.

-   In terms of trim in the water, I find that a WI BC positions the weights too far up on my body, sometimes causing floaty feet depending on what exposure suit I am wearing. I prefer to wear a weight belt in order to get the weight lower on me, so for me the WI aspect of a WI BC is pretty much negated. I also prefer to wear a weight belt because I like the idea of being weighted independently of my rig in the event I need to remove and replace it underwater, but that is not something that is particular to a BP/W.

-   The addition of a crotch strap makes a big difference. This is part of the equation of not having to have all of the straps really snug like with a WI BC. I recently added a crotch strap to my WI BC and it made a big difference in comfort. If you are concerned about a wedgie, your wetsuit is far more of a wedgie machine than a crotch strap will ever be.

-   Having two cam straps to hold the tank at two points makes a difference in terms of keeping the tank from wiggling around. It also helps prevent the tank from slipping out if one of the straps is not as tight as it should be.

-   The better WI BCs do have a kind of a backplate in them…it is a plastic plate sandwiched between then material that makes the harness and the bladder material. The backplate of a BP/W is typically larger and, for me, gives more of a solid feeling of the rig on my back in the water.

-   A WI BC, because of the amount of plastic used, tends to be positively buoyant by a few pounds or so (varies by BC) in the water. A BP/W, while weighing a similar amount on land, will be anywhere from slightly to 5 pounds negative in the water (depending if the backplate is steel or aluminum/composite. I don’t like having to carry extra lead just to compensate for a floaty WI BC.

-    I like the simplified inflate/deflate valve on a BP/W. I think some of the WI BC vendors are actually complicating the BC design by making the inflator buttons more stylish.

-  I have found that I do not need all of the  extra dump valves found on a WI BC.

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Dry suit pros and cons, part 1

October 10th, 2007   Filed Under Article  

Considering going dry? Here are some factors I was looking at when I made my decision to go dry.  I will split it up into multiple posts.

Pros:

User-variable compression
A wetsuit compresses the deeper you go. The more compression, the less thermal insulation. I have noticed that with a particular wetsuit setup I can be fine at 30 feet but then feel cold past 70 feet with similar water temperatures. So, that means that in order to be warm at 70 feet I need to layer up more neoprene before the dive. There are some disadvantages to this; more neoprene means wearing more lead in order to sink. Once the suit compresses some, buoyancy decreases, but I still have all that lead that required to initially get me down and keep me at 15 feet at the end of the dive. Now I am essentially overweighted for the rest of the dive. The deeper I go, the more overweighted I am since the wetsuit continues to compress more, and the deeper I go the less insulation I have.

A dry suit has a low pressure inflater hose connected to is so the diver can add air to compensate for the compression in the insulation layer caused by changes in depth. This allows the diver to maintain the same level of insulation regardless of the depth as the diver can add or release air to maintain the same volume of the air “bubble” in the suit as the diver ascends or descends. The other advantage of controlling the compression is all air in the suit can be vented in order to “vacuum pack” the suit. This reduces the volume of the suit, which reduces the amount of water the diver displaces, which makes the diver less buoyant. This is useful in managing the surface to 15 foot depth range - less weight is required to descend at the beginning of the dive along with maintaining a safety stop and slow ascent to the surface at the end of a dive (when the tank will weigh less since there is less compressed air in it). So with a dry suit you don’t have to pack extra lead for the shallower depths where there is less compressing force than at depth. Weighting for a wetsuit is pretty much what is required to keep you at 15 feet at the end of the dive, whereas weighting for a dry suit is more like the weight required to be neutrally buoyant during the dive with the desired amount of air in the suit. User preference and undergarment materials can affect the size of the bubble in the suit. So a dry suit isn’t so much about keeping the water out as it is about keeping the air bubble in.

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Tips on buying a SCUBA tank

October 1st, 2007   Filed Under Article  

Making a purchase decision on your own SCUBA tank can be a confusing one. Aluminum or steel? High pressure or low pressure? What size? Here is what I learned in doing research for a purchase decision:

Aluminum versus steel:
As a primary/back cylinder, the common aluminum tank (AL - the Dept of Transportation code for an aluminum constructed compressed gas cylinder) is the 80 cu.ft. A 100 cu.ft. can also be found. There is one primary advantage of AL tanks - they cost less than steel tanks. Other than that, most people go steel over AL as a tank they wear on their back (a stage bottle or pony is a different story). In terms of buoyancy, AL tanks are not very negative when full and are positively buoyant by about 4 pounds when at 500 PSI. That means packing more lead in order to sink. One case where an AL can be beneficial is in the case of warm water diving where large capacity steel tank will have enough negative buoyancy that a diver in a thin wetsuit could still be negative even after dumping their weights. That is not a concern for me since I am a big guy and get cold easy, so I have more neoprene than most people and all that foam requires more weight to sink me; Plus where I live I will only be doing cold water diving with the tank. There are neutrally buoyant 80 cu.ft. AL tanks on the market, but they are more expensive than regular AL tanks. AL tanks have very poor resale value, and they tend to fail hydro long before a steel tank will. Most AL tanks are rated (will be full at) 3000 psi, with some rater at 3300 psi.

Steel (AA - the Dept of Transportation code for a steel constructed compressed gas cylinder) tanks have a thinner wall than AL tanks. Most AA tanks are more negative (for a given volume) at the beginning of the dive than AL tanks, and AA tanks end the dive still negative while weighing similar to an AL tank of same compressed volume on land. AA tanks come in a wider range of volumes and sizes than AL tanks do. AA tanks have good resale value because they last a long time and therefore hold their value well. It isn’t unheard of to buy a new one, dive it for a bit and sell it for 80% of what was paid for it. AA tanks cost anywhere from 2 to 3 times the cost of an AL tank.

High pressure or low pressure?
AA tanks pretty much come in two flavors, low pressure and high pressure. Low pressure (LP) is defined as 2400 psi. LP tanks on the market today have a plus (+) rating, which means they can be filled 10% over the 2400 psi, or to 2640 psi. High pressure (HP) tanks are rated to 3442 psi. HP tanks are relatively new to diving; they have been around since 2003 or so. Here are some fundamentals on LP or HP tanks before we move on:

Since a HP tank can also become a LP tank (with smaller volume), I decided to choose a HP tank of appropriate size so that the LP volume would meet my needs. That way I would always get enough of a fill that I need for most dives. My plan is to have it filled to around 3000 psi the majority of the time, and then occasionally take it up to 3442 psi if I need the extra volume.

One disadvantage of a larger tank is if the volume in it is more than you need (i.e. you come up with a bunch of air left) then you are lugging around the extra weight of that air in the water. For example, a 130 tank when full is about 11 pounds heavier than when at 500 psi. If a diver consistently only uses 5 pounds of that air, then they are carrying around an extra 6 pounds that has to be compensated by having more air in their BC…and the more air in the BC the more buoyancy is hard to control. Why not just drop 6 pounds off the weight belt in that scenario? Proper weighting means the diver can hold a safety stop with 500 psi, so if you don’t breath the air in the tank, then it is just extra weight. So….the biggest tank on the market is not necessarily the best for everyone. It is more like – get the tank that has physical dimensions that fit you and that has a LP volume that will work for you, and then choose the fill based on what you will probably need for the dive. This is why a diver may choose to short fill a high capacity tank and turn it in to a LP tank.

See the Variable Capacity of Cylinders section on this page from Dive Rite Express: http://www.diveriteexpress.com/gas/steel.shtml

Tank dimensions:
Different capacity tanks have different dimensions. Some are close in volume but different in shape. The diameter of the tank will be either approximately 7.25 or 8 inches, and the length varies by tank. As an example of two similar volumes with different sizes, the XS SCUBA 120 cu.ft. is 7.25 inches by 28 inches long. Their 119 tank is 8 inches by 24 inches long. For comparison, an AL tank is 7.25 by 26 inches. Length can play a part in comfort and trim. A shorter, fatter tank will put more weight high up on the diver’s torso. A tall diver probably wants to get a longer tank since they have a longer torso, and a shorter diver probably wants a shorter tank so they don’t feel like is it hanging between their legs in the water. I am 6’4”, but I chose not to get the 120 as I believe anything longer than an AL 80 will feel a tad long on me.

Which size did I choose? I dived a LP 95 and a HP 100 and felt those volumes were the minimum I want to be able to do the slow ascents that I want to do. In terms of volume, a 119 is perfect but it is a bit short for a tall person. I was torn between the 120 and 130 cu.ft. tanks as far as a purchase decision. I decided to go for the 130 since I was a little concerned about the length of the 120. So the 130 is three volumes in one tank – according to the Dive Rite Express web site regarding tank selection, a 130 is also a low pressure 104 and mid pressure 117. It all depends on how much air you put in it.

Additional reading:
Dive Rite Express page about choosing a tank
http://www.diveriteexpress.com/library/tanks.shtml

Dive Rite Express page about tank volumes
http://www.diveriteexpress.com/gas/steel.shtml

XS SCUBA specs page for their tanks
http://www.xsscuba.com/tank_steel_specs.html

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Self-reliance

September 28th, 2007   Filed Under Article  

I like the idea of being more self sufficient in the water so I can rely less upon whomever I have for a buddy. Not that I want to dive completely solo, it is just that even with a buddy you may very well be effectively diving solo. Here is why:
- One of the reasons for having is buddy is in the event of an out of air situation. That means the buddy is carrying my emergency air supply. What if they don’t manage their gas supply well enough to leave a little extra for me? What if I am bigger than the buddy and I suck down their tank? What if they are ahead of me and I can’t catch them on one breath of air?

- Counting on a buddy to help me means relying upon them to have the skills and the mindset to do it. To me, that means everyone should aim to be at least rescue trained.

- In the past I have gone on dive charters alone and been assigned an “insta-buddy” wherein the boat divemaster says everyone needs a buddy and pairs up buddy-less divers. How much can I rely upon this other person?

I remember one insta-buddy incident in particular. I was on a dive charter in the Turks and Caicos islands and I got assigned an insta-buddy on the boat. After we jumped in to the water, we descended and he took off. He never looked back. I followed for a bit, but when I realized I couldn’t catch him, I gave up and just had a nice solo dive. Back on the boat I didn’t bother saying anything to him because after seeing what kind of buddy he was I felt safer diving solo. I had my underwater housing for my 35 mm camera and a big strobe unit to drag around, so I liked the idea of just cruising around and snapping some pics. Of course this brings up the issue of having mutual dive objectives, so my bad for not discussing them…but he should not have taken off like that. The next dive was the same thing. We descended from the surface over the wall. He flips heads down and starts finning hard straight down along the wall. I thought maybe he saw something, so I start following. At 80 feet I stopped and watched him disappear below me. I ascended a bit and had a nice solo wall dive. I would have been putting myself at risk if I were to keep up with him. He was putting me at risk by not being a buddy. Screw him.

What is involved in being more self-reliant? There is solo diving and then there is being self-reliant. Being self-reliant is part of diving solo. I want to be more self-reliant and still dive with someone else. It just means I don’t need to rely upon them as much, and if they are more self-reliant then they don’t need to rely upon me as much either. I see steps towards self sufficiency being:
- Carrying a larger primary air supply, and a redundant secondary air supply

- Proficiency in underwater skills

- Comfort underwater in dealing with a variety of conditions and incidents

- Knowing your limits

When I searched for other sources of info on the internet about solo diving, I found the objectives they list corresponds to my list.

Air supply: Under the buddy system, my buddy carries my emergency air supply. Self-reliance means I carry my own. I think this means having a larger primary tank so I have extra time to figure out how to free myself if I get tangled up in something, and it also means carrying a redundant air supply and regulator in case I have a first stage failure or go out of air. I recently purchased a 40 cf tank and regulator setup and the redundant setup, and a 130 cf for my primary. On the subject of air supply, I also believe in using nitrox in order to reduce nitrogen loading. I will sling the 40 like a deco bottle. A 40 cf tank might sound kind of big compared to the compact Spare Air units out there, but I am a big guy and therefore have a larger lung capacity. I want enough air to be able to make a slow and safe ascent from a deeper dive. I think of a Spare Air more as an escape bottle to get a few breaths in while doing a controlled emergency swimming ascent (CESA). I also have other plans for that 40 - like filling it with a richer blend of nitrox than my back gas and using it as a washout gas while ascending (once I am above the max depth for the nitrox mix).

Proficiency in underwater skills: Being able to maintain composure if someone accidentally knocks my mask loose with their fin or similar incident means extra safety for me since I won’t have the urge to bolt to the surface or grab on to a buddy. Being able to remove my BC underwater to help untangle it is important too. For me, taking the Divemaster course to re-visit the open water skills have helped give me assurance that I can handle these kinds of incidents on my own. Another skill is being able to do a free ascent while having control over the ascent so I can decide my ascent rate and where I am going to do my safety stop. Navigation is important as well; being able to make a nice underwater tour and end up back at the boat makes for a nice dive, as opposed to ending the dive with a long surface swim through kelp.

Comfort underwater in dealing with a variety of conditions and incidents: To me this means having some experience in dealing with a number of conditions, like current, kelp, low visibility, etc.

Knowing your limits: Experience will give some indication of what your comfort level is for a number of things. This is not only personal limits, but also adhere to recreational diving limits. I physically feel better after making a slower than “normal” ascent, so I like to incorporate a slow ascent in to my dive plan.

Related links:
The Buddy System Reexamined
http://cisatlantic.com/trimix/other/solo2.htm

Going Solo - The Solo Diver Debate
http://www.o2technicaldiving.com/articles/going_solo_the_solo_diving_debate/

Solo Diving the and Recreational Diver, Part 1
http://www.airheadsscuba.com/soldvrarta.pdf

Solo Diving and the Recreational Diver, Part 2
http://www.airheadsscuba.com/soldvrartb.pdf

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Another vote for Nitrox

September 24th, 2007   Filed Under Article  

Fast forward several months and several dives. I have since completed the PADI Divemaster certification. I have also been doing more diving. My recent dives have been:
- The dives for the Divemaster course
- Four dives in Cabo San Lucas
- Eleven dives on a 3-day live-aboard trip in the Channel Islands (Souther California)
- Seven dives in San Diego’s “Wreck Alley”
- Two dives in Monterey, CA.

The two Monterey dives and five of the seven Wreck Alley dives were on nitrox, the other dives were on air. I have noticed that I seem to have overcome the headaches and other CO2 buildup symptoms I had before. Two of the Cabo dives were pretty strenuous. We were fighting a strong current to get back to the boat. I thought for sure the headaches and such were going to set in, but they didn’t. The Channel Islands dives all went fine as well.

Having said that, I am even a bigger supporter of nitrox for recreational diving now that I was before. I was influenced by:
- Learning more about the physiological aspects of diving from the Divemaster course
- Conversations with a tech/cave/deep dive/trimix instructor
- My own increased focus on safety after a recent motorcycle accident
- Reading stories of divers getting DCS (the bends) while diving within recreational limits

Diving withing recreational (rec) limits involves nitrogen loading. Even though rec limits are no (mandatory) decompression diving, the more nitrogen a person has in their system, then the more opportunity for them to be one of the cases of people who get DCS even while following rec diving tables. The no decompression Recreational Dive Planner (RDP) tables are a model for a large cross section of people. They are not an actual representation of what exactly is going on in any human body, let alone your body. The less nitrogen you have trying to get out of your system, and the slower the pressure change from depth to surface (i.e. a slow ascent), the less likelihood of a person getting DCS (all other things being the same…because there are other things that lead to DCS, like exertion level, being cold and hydration).

From what I have learned, using nitrox reduces the amount of nitrogen being absorbed by the body when compared to using air for the same dive profile. The less nitrogen absorbed, the easier it is to get rid of it and the less chance of DCS. Another safety feature I see is in the case of making an emergency ascent. The less nitrogen I have taken on during the dive, the less likelihood of DCS being an issue after an emergency ascent with a missed safety stop.

One thing to keep in mind with nitrox is the max operating depth (MOD) of a given nitrox mix wherein the oxygen partial pressure in the body becomes toxic. The higher the oxygen percentage, the shallower the MOD. This mean you have to have an idea of the dive profiles you’ll be doing in order to get the appropriate mix percentage.

For a given dive profile within the no deco limits of the RDP air table, nitrox will give an increased level of being conservative with regards to nitrogen loading. If a person dives to the “edge” of the nitrox RDP tables (tables adjusted for the reduced nitrogen loading of a given nitrox percentage), then the only advantage nitrox is offering is either a longer bottom time or shorter surface interval than air, and there is no increased level of conservatism for avoiding DCS since the diver is pushing the limits.

The way I have been diving is to set my computer to nitrox mode and setting the conservative variable on my computer to its max value. One issue with using the conservative factor on the computer is that I can be well within no deco limits, but because of the conservative setting on the computer I end up in deco mode with the accompanying red flags and longer no fly penalty. This recently happened while doing a nice long and slow ascent from a 95+ foot dive. The computer didn’t seem to credit me with the slower than 33 feet per minute ascent (from around 75% of my max depth), so according to the computer I had an eight minute deco obligation between 10 - 12 feet until I could finish the dive. Sure doing this “deco stop” was extra conservative and didn’t hurt, but I would rather have more control over the planning of my conservative ascent and extra safety stop time. I think from now on I will leave my computer conservative variable turned off and just end the dive well withing the no deco time limit of the gas mix I am diving. Even though my computer does not credit me for a deep stop or a slow (10 foot per minute) ascent, I will still continue to incorporate a slow ascent in to my dive plan and just know that the ascent is just another area of conservatism for me while staying within the no deco time of the computer.

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