Doing It Right Diving
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DIR - An essay
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How much lead?
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Gas switching procedure
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Average depth for deco?
Using the min deco table
Nitrox Class Part 1
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How Much Lead Do You Need?

It is very important to get your weighting right. For instance, if you have a problem at depth and your are too negative you can't get up. On the other hand if you dont have enough weight you can't stay down and complete a decompression stop when you are getting low on gas.

You weighting need to be a balance between these two extremes and when you get it right you have "a balanced rig". And it can't be calculated, it has to be adjusted in the water.

Weight changes during the dive

There are two things that will change your weighting during the dive.

/pics/misc/weight_of_gas_large.png

Number one, that affects everybody, is the weight of the gas in your bottles. As you breath it down and the pressure drops your tanks will become lighter - more buoyant or less negative depending on their weight in the first place. In the graph above you will see the weight of 32% nitrox in a 12 liter 232 bar bottle. 1 kg is about 2.2 pounds and the size of the cylinder is similar to a 80 cuft bottle.

As you can see the difference between a full bottle and an empty one is almost 3 kg, about 6 lbs. If you were wearing doubles the difference would be doubled.


/pics/misc/lift_at_depth_large.png

The second thing affects your weighting is material that will compress at depth, for instance neoprene drysuits or neoprene wetsuits. When you go deeper the bubbles inside the neoprene will compress and the lift (positive buoyancy) from the neoprene will decrease.

The graph is calculated using a 7 mil XL full neoprene suit just as an example but actual values may vary depending on neoprene quality, thickness and size of the suit. Precruched or compressed neoprene should show only very small amounts of compression.

As you can see the difference between being at 30m/100ft or at the surface is about 5kg / 12 lbs in this case.

Two opposite extremes

Since we have the weight of the gas and suit compression that affects us during the dive we need to think about two opposite situations when we consider our weighting.

One is that we are at the end of our dive with virtually no gas in our tanks and we try to hold a decompression stop close to the surface. In this situation we are the most buoyant and we need to make sure we have enough lead to just stay down.

The other situation is at the beginning of the dive when we have full tanks and have just reached the bottom. Now we are the most negative and if we have a problem, like our wing doesn't work, we need to be able to swim the negative weight up. If that is not possible we need to have droppable weight that we can get rid off in order to be able to get up. For a nightmare scenario imagine jumping of a boat with your tank valve off quickly sinking to the bottom with no gas to breath or no gas to put in your wing.

Determining the right amount of weight

Our weighting consist not only of lead but also of tanks, backplate, lights etc. Basically we need to figure out how much weight we need in total and then how much of that we need to be able to dump in an emergency.

Step one is to dump the gas in your tank so you have about 10-20 bar/150-300psi. Drop down to 3m / 10ft. You should be able to stay down but just barely. If you can't get down however, you need to add more weight.

When you are down below have you buddy check the amount of gas you have in your wings. The wings should be empty and if you have a drysuit it should just hold a comfortable amount of gas.

By looking at how much gas you have in your wings while neutral you can guesstimate how much more lead you need to remove. Each 10 cm (4 inch) cube of gas equals about 1kg (2.2lbs) of lead.

Now, if you have small weights on a weight belt you can take some of until you find the amount of weight so that you can just stay down.

If you have removed all possible weight and still are too heavy you could change from a stainless steel backplate into an aluminum one. That will be almost 3kg / 6 lbs less weight. If you have done that too you need use other tanks that are less negatively buoyant. That is actually the primary reason that steel tanks are not recommended for wetsuit diving because often they are simply too negative and there is no way to get the weighting right.

Determining the amount of droppable weight

After we have figured out how much total weight we need, we need to figure out how much can be permanently attached and how much we need to be able to dump in an emergency.

With full tanks drop down to the bottom. If you have a shell drysuit the depth doesn't really matter but if you have a wetsuit or neoprene drysuit you should go deep enough so that the suit compression is significant. About 20-25m or 70-80ft should probably do the trick.

Now empty your wing completely and try to swim up. If you can't, you must drop enough weight until you can. And if you have nothing to remove, you need to reconfigure your gear.

Droppable weight could be in the form of a weightbelt but also a heavy light cannister could be considered as droppable weight. Most of the latest NiMh cannisters are not that negative though.

Using weight belts

Usually most non decompression divers use some form of droppable weight, like weight belts. The problem with doing decompression diving is that an accidental loss of the weight belt means that we wont be able to complete our decompression and could end up seriously hurt or dead. So a lot of technical divers, especially those diving shell drysuits, avoid using weight belts unless it'a absolutely neccessary.

If you use a thin wetsuit or shell drysuit with a single tank, you most likely don't need a weight belt to be able to reach the surface in an emergency. However at the surface you might not be able to gain positive buoyancy without dropping some weight. So consider having 2-4kg / 5-9lbs of your weight detachable or be prepared to remove your gear entirally and let it sink to the bottom.

If you feel any insecurity about whether you should dive with a weight belt or not I suggest using one. But if you have a large amount of lead consider putting some of it on your tank instead.

Stages and decobottles

When doing your weighting test you should not have any stages or decobottles on you. Because they are detachable they are not part of your weighting system.

You should also use bottles that will be slightly negative when full and slightly positive when empty. That means aluminum tanks are the DIR choice in most cases.

If you have a problem and are too heavy you can give some of your negative bottles to you buddy. If you are too positive you can dump them because if they are that positive they are also empty. They will end up on the surface - get them another day.

Conclusion

Get your weighting right and you will be in a much better position to handle emergencies, like wing failures and other problems. Also not carrying more weight than neccessary means it will be easier for you to control your bouyancy because you don't have to fill and dump as much gas from your wings during the dive.

Remember though that you need to do this all over again when you change something in your configuration like tanks, primary light or drysuit underwear.

Also keep in mind that most of the things on this site relates to DIR diving as we use specific equipment and procedures. Not all of the logic and arguments will apply if you change some parts. If you want to use fancy words you could say that DIR is a holistic approach.

Good luck!

Peter





This page was last modified 14 September 2006
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