Another Piece of DM Decompression theory for you!

[BLUESEASONBALI]

This time i'll put the question in BOLD so you can understand it!

Explain what is meant by a multi-tissue decompression model and the number of tissues used in the creation of the RDP versus the U.S. Navy models?

Decompression theory is based on the principle that gas pressures want to equalize themselves until reaching equilibrium. This affects us when diving because when we go underwater, the increasing water pressure (hydrostatic) causes a rise in the ambient (aka absolute) pressure around us. Our scuba regulators work by providing breathing gas at that surrounding pressure.

Our breathing gas enters our body at higher pressure than the gasses which were already absorbed into our body (which were there as a result of living in a gas-filled environment here on Earth). So, our tissues begin absorbing more gas- although in solute form- and try to reach equilibrium.

Here’s a question- do you think your femur absorbs gas at the same rate as the plasma in your blood? Maybe they do, I’m no decompression physiologist, but the point of a multi-tissue decompression model is that we need to account for the fact that our body has different components which possess different properties of perfusion, vascularity, etc. You don’t need to know what all those words mean, just that the human body has different bits.

So, the guys at DSAT (and the guys in the USN) followed this thinking in developing their respective decompression models. The USN table uses six different theoretical tissue compartments to figure out how long (or how short) a dive can be at a certain depth. DSAT’s RDP uses 14.

Top tidbit of the day: The USN table was originally designed for navy divers doing one or two (possibly decompression) missions in a single day. To control surface interval credit, choose a uniform rate (half-time) of 120 minutes. The RDP allows for more repetitive diving by running washout at 60 minutes. This means more time diving and less time on the surface getting sun burned!


BLUE SEASON BALI

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[BLUESEASONBALI]

3. When diving above sea level, explain why it critical to know the altitude at which the dive is to take place.

One of the critical aspects of diving safely is the effective management of inert gas tension and gas loading. In recreational diving, the gas we are concerned with is nitrogen. As we go down, our bodies begin taking on higher concentrations of nitrogen.

Many of the procedures we have to follow when diving have to do with managing how this gas will get back out of our bodies. Examples of this are ascending slowly and making a safety stop. Our dive planners (such as the DSAT RDP) are based on the assumption that we will surface to a surrounding (aka “ambient” or “absolute&#8221 pressure of one atmosphere.

Why does it matter? Well, we are assuming that there will be one whole atmosphere of pressure holding that nitrogen in our bodies enough for it to come out slowly. If the difference in the nitrogen pressure inside our bodies and the ambient pressure outside our bodies is too high, bubbles can form in the tissues or blood stream. This is the cause of Decompression Sickness or “the bends.”

By making sure we account for altitude when diving above 300m/1000′, we can alter our procedures, ascent rates, and calculate theoretical depths in order to make diving safer. If you would like to learn more about the special procedures needed for diving at altitude, check out the PADI Altitude Diver Specialty Course!

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[BLUESEASONBALI]

Explain why Pressure Groups from one model/table cannot necessarily be transferred to another model/table.

There are various tables and planners out there. Almost invariably the are built on a haldanean or neo-haldanean mathematical model. However, the number of theoretical tissue compartments in a given model- an their assigned properties like M-value and halftime- can vary.

One poignant example is the difference between DSAT’s RDP and the US Navy dive tables. They are based on the same mathematical concepts, but apply them selves in slightly different ways. For instance, as we discussed earlier, the RDP “washes out” at twice the rate in comparison to the USN table.

Perhaps the easiest way to see their incompatibility is by looking at them! If you give it a go, you wil notice that the RDP has PG’s A-Z, while the USN table has only A-K. It then seems logical then that they are most certainly incompatible.

One interesting thing to note is that the PG’s from all DSAT products are interchangeable because they all rely on the exact same mathematical application of the same decompression model. All of these DSAT products produce interchangeable PG’s: the RDP table, the RDP Wheel, the eRDP, and the eRDP-ML.