Deep diver specialty

Lot's of beautiful wrecks and reefs are located deeper than the 18 meters you're allowed to dive when having an open water diver certification or even deeper than the 30 meters with the advanced open water diver certification (padi). So if you ever want to dive those wrecks or reefs, extra training is required. The last two weeks I've taken the deep diver specialty course so I can dive those spots in a safe and responsible way.  I want to share with you what I've learned during this course.

When doing a specialty course, you learn in two ways, from the textbook and from the dives. And I must say, I've learned the most from the last dive. Because for that dive, I had to make the dive plan, do the briefing and go through the safety procedures.

From the textbook

Before the dive
Before you even put on your gear set a goal that is realistic for you and your buddy. Take into account the expected circumstances underwater and both your experiences with them. Making a 35 meter dive in tropical water just isn't the same as making a dive to that depth in cold, dark water.

A goal and a plan
First of all you need a goal for your dive and a decently set up plan. Don't just go in and see what happens. It will put you and your buddy at great risk. Make a decent and conservative plan to visit that great wreck and stick to it. Because of the short bottom times at depth (no deco time), make sure you can descend and descend very close to the goal of your dive.

Buddy check

Well, of course, you should check your gear before each dive, but before a deep dive you should do this even more carefully. Gear failure at depth can cause a lot more problems than at 10 meters, and it's a long way to the surface. So, do a very thorough buddy check...

While diving
Check your air regularly. At 30 meters your air consumption is twice as much as at 10 meters. So check it regularly and that of your buddy as well. And for that matter, maintain contact with your buddy very often, so you'll notice the signs of nitrogen narcosis. Turn back well before you're low on air. You still need to do a safety stop, so make sure you both have enough air for that.

Ascending
When you're at the spot where you want to ascend, release your safety sausage. The safety sausage makes people on the surface aware that there are divers below. It will reduce the chance of being run over by a boat. Then ascend slowly and maintain (almost) neutral buoyancy. At 5 meters, do at least 3 minute safety stop. And then continue to ascend.

The dives

From the four dives we made in the course doing different kinds of exercises like answering diving theory questions and doing math. Also we looked at colour changes underwater and the effect of pressure on object filled with air. And for the first time, I had to use a safety sausage.
But the most educating dive was the final one. I had to prepare the dive, determining bottom time, descending and ascending procedures, agree on hand and light signals. And of course, navigate the whole dive.

Also I had to prepare emergency procedures and had to do the briefing.


The dive itself was cold (5 degrees at the bottom) and dark. But with our lamps, visibility was very good. And everything went as planned.
This course has made me look at a dive in a more serious way and taught me a lot, especially when I have to lead a dive.

European Shark Week

This week it´s European shark week. Project Aware is working together to with the shark alliance to ask the European parlement to sharpen European shark finning ban.

And because shark fins should be on sharks and not in some bowl of soup, I support the shark week with the post in my blog.

You can find more on the European Shark week on Project Aware.

Esspecially for Shark week, I looked on the net for some shark cartoons to make you think about us people..

More shark cartoons on http://www.shaaark.com

Physics in diving (2): Underwater pressure and gas compression

We can all feel it in our ears when we descend for a dive, or when we ascend at the end of a dive. The pressure changes with depth.


Water pressure
At sea level, the pressure is 1 atmosphere or 1 bar. This pressure is caused by the weight of all these kilometers of air in the atmosphere above us.
In water it works the same. The weight of the water above us determines the pressure working on us. As a rule of thumb, every 10 meters we descend, the pressure we endure increases with 1 bar.
It is easy to calculate the pressure at any depth. Just divide the depth you are at by 10 and don't forget to add 1 for the pressure the atmosphere is already giving you.

So at 10 meters the pressure is 10/10 + 1 = 2 bar. In the table there is more.

Depth	Pressure
Surface	1 bar
10 meters	2 bar
20 meters	3 bar
30 meters	4 bar
40 meters	5 bar

So why is it important to know the pressure underwater? For one, when diving with enriched air, the pressure is needed to calculate the partial pressure.
An understanding of pressure , together with gas compression, is also needed to better understand the changes in buoyancy when changing diving depth.

Gas compression

Gas compression is very convenient in diving. It makes that you can stay underwater as long as you do. By compressing air into your tank you can take air with you to stay underwater for a long time. How does that work?

Boyle's law
Boyle's law states that at any given temperature the product of the pressure and the volume of a certain amount of gas, is constant:

P1*V1 = P2*V2 = constant 

So let´s say your underwater at 10 meters and the air in your BCD is has a volume of say 9 liters. When you descend to 20 meters, the water pressure will increase from 2 bar to three. The air pressure in your BCD will take on the same pressure. This pressure will make the volume of the air in your BCD change from 9 liters to 6 liters. 

The other way around, when you go to the surface the water pressure will decrease from 2 to 1 bar and the volume in you BCD will increase to 18 liters.

Boyle´s law can be corrected for temperature influences. This results in the combined gas law:

P1*V1/T1 = P2*V2/T2 = constant

Because when diving, temperature differences aren't that big (unless you pass through a thermocline), I will explain the temperature influence with an example of filling your tank.

You might have noticed that after you've fill your tank at the dive center, your tank feels a bit hot. Let's say that you filled your tank to 200 bar. 

Then later on when you have set up your gear to go diving and check your air supply, you might notice that your tank has cooled and when you check your air supply you will notice that you have less than 200 bar in your tank.

That is the temperature effect working. The volume of your tank is still the same, the temperature has dropped a bit, so only the pressure can decrease to make the formula work.

Pressure, compression and buoyancy

So when you change depth, the water pressure changes and the volume of air in your BCD changes with it. These changes have a direct influence on your buoyancy. Your buoyancy is dependent on your weight and volume. As your weight doesn't change when you change depth, only the air volume in your suit and BCD has an influence on your buoyancy.
When you go deeper the volume in your CD will decrease, as will your buoyancy. That is why you will have to add air to your BCD to compensate for the volume lost by the pressure. Also, when you go up, you'll have to release some air, otherwise your buoyancy will be too positive and you will shoot to the surface, with all risks that come with that....


Relates subjects:
Physics in diving (1): Buoyancy