Deco For Beginners Part Two
So what is decompression and how do we deal with it?
Not a problem a certain Mr Bhulman gave a simple enough answer:
pamb. tol. = (pt. i.g. - a) ·b
or
pt. tol. i.g. = (pamb / b) + a
See simple I don’t know what all the fuss is about. Deco on the fly is easy too all you need to do is work out this equation for the given parameters of the dive and you can do it in your head:
pt. i.g. (tE) = pt. i.g. (t0) + [pI i.g. - pt. i.g. (t0)] ·[1 - 2-tE / t1/2]
pt. i.g. (t0) - pressure of inert gas in the tissue at start of exposure
pt. i.g. (tE) - pressure of inert gas in the tissue at end of exposure
pI i.g. - pressure of inert gas in the breathing mix
tE - duration of exposure in minutes
t1/2 - half-time in minutes
I hope you’re impressed. It took weeks to learn how to cut an past that
OK so all this is fine and if you have a masters in math read no further, but for those like me who still spend ages trying to figure out what 82p a ltr is for a gallon of petrol apart from being a rip off, this is basically how it works.
Every thing in life needs balance, gas pressure is the same. Imagine your lungs as an oil drum and your body as a second oil drum. They are connected at the bottom by a tube. If liquid were pored into the first drum (your lungs) it would cross over through the tube until both drums have the same amount of fluid in and achieve a balance. Now exchange liquid for partial presure of gas.
Breathing normal air at normal atmospheric pressure there is a balance between the amounts of nitrogen absorbed into our bodies and the amount in the air we breath. No further movement occurs through the connecting tube of our two barrels. However when we get into the water the air we breathe becomes compressed. At a depth of 10m we have a 100% increase in the amount of nasty Nitrogen in the first barrel (The gas in our lungs) so it starts pushing through the connecting tube and into our bodies.
The tube is fairly small so it takes a while to do this. The deeper you go the greater the pressure from the first drum and the faster the gas will pass through to the second drum. Not only that but as the gas is compressed it gets smaller and passes through the tube more easily. This will continue to occur until the pressure of the gas in your body is equal too the pressure of the gas you are breathing, or until you are full. Full is called saturation. When you are saturated with the gas you can physically absorbed no more.
Now some bits of the body can absorb gas quickly and some are slower. As a result our oil drum analogy has to be duplicated until the fast and slow bits of the body are properly represented. Originally they did 8 samples. These days we are more familiar with 16 samples hence Bhulmans decompression tables ZHL 16 B. 16 sets of oil drums. Each set with a different diameter connecting tube allowing the flow of gas at different rates. These are referred to as the fast (or leading) tissue compartments and the slow tissue compartments.
So having absorbed the gas during our dive we now start to ascend. The pressure on the gas we are breathing is reduced rather like someone bailing out the first oil drum before the contents are able to pass through to the second. If as you raise in the water column, the amount of absorbed gas (in terms of partial pressure) stays less than the amount of gas being breathed there will be no problem ascending to the surface.
So after a 5m dive for 20mins you can pop up like a cork and not much will happen.
However if the amount of gas in your body is greater than in that being breathed the gas will be rushing to escape before you get to the surface. A level of imbalance is acceptable but go past that level and things will start to go wrong. Blockages will occur in the flow of the gas due to the gas expanding as pressure is removed. The expanding gas will form bubbles. These bubbles will block the passage of blood that’s carrying the all-important oxygen and can tear tissue and pinch nerves as they expand. This is the bends.
In order to prevent this happening we must allow time for the gas to escape. Slowly ascending to the surface allows some time but if the levels absorbed are too great we may have to stop once or even several times to allow the escaping gas time to catch up before the bubbles get too big. Slow ascents mean less than 10m/min up to 15m then less than 5m/min to 6m then less than 2m/min from 6m to the surface. Remember that between 10m and the surface the bubbles will double in size so it is very important to control this phase of the dive.
Off gassing can be very slow using air as a decompression gas. Air has a LOT of nitrogen in it. 79% is a big number. If there is a lot of nitrogen in our bodies but there is also a lot of nitrogen in the gas we are breathing the rush to balance the two levels is more of a crawl. So the trick is how to speed up the transfer of gas without removing so much pressure that the gas expands and forms bubbles. Simple, get rid of the Nitrogen in the breathing gas.
To do this we replace it with oxygen. This is far friendlier to our bodies and can be metabolised, so it’s great. Well sort of. Unfortunately too much oxygen can hurt us in a completely different set of ways so we have to keep the levels in the mix below a maximum guideline of 1.4pp02 for breathing gas and 1.6 pp02 for decompression gas. Further to this we can only be exposed to that maximum guideline level for a set amount of time. The time at 1.6pp02 is 45mins. The time at 1.0 pp02 is 300mins. So you can see that as long as we don’t stay at 1.6 for the whole dive we have plenty of time to off gas.
Now the goal posts are moved. If we remove Nitrogen from the gas we are breathing our body will be desperately trying to make up for the imbalance. If we do this at depth the Nitrogen gas will still be small and will travel easily through our connecting tube. Unfortunately we cant do it all at depth because there is unavoidably still some nitrogen left in the breathing gas. So we move ever shallower in the water so we can increase the amount of Oxygen in the mix and reduce the nitrogen further and further until at 6m we can if we choose breath pure Oxygen. This really upsets the nitrogen which rushes out of our bodies as fast as possible in a futile attempt to balance the 0 levels in the gas we are breathing.
So now we understand how it works it’s a simple case of balancing the ascent rates and stops against the level of gas absorbed.
Unfortunately no.
It’s not quite that simple. The rate of absorption is not a constant. Fat will absorb nitrogen quickly so people with a higher body fat ratio will require greater decompression than skinny gits. Kate Moss is the perfect diver
Levels of hydration in the body will alter the rate of gas transfer. A properly hydrated diver will off gas efficiently and a poorly hydrated diver will not. Blood flow is critical. Nitrogen will on gas faster in cold conditions but will off gas slower due to restricted blood flow. Levels of electrolytes in the body effect the surface tension of the forming bubbles and speed up / slow down decompression, others feel that people with thinner blood will off gas faster than people with thick blood. So much so that some decompression divers take Aspirin to thin the blood before a dive. Empirical data would appear to support this theory but strangely other anti-inflammatory drugs like Neurofen were found to be less effective.
One thing is for sure it is better for a diver to have eaten some proper food and to have properly hydrated before a dive. Proper hydration is considered the consumption of fluid required to replace that lost during the course of a day. For temperate regions this is approximately 2300ml and for hot countries 3300ml. With exercise water loss can double or more. To ensure proper hydration a diver should consume between two to three litres of water a day for a minimum of 48 hours prior to a dive. Sipping from your 1ltr of Evian on the dive day just doesn’t cut it. Electrolytes are salts that amongst other things; regulate the fluid flow across cells. As you can imagine this is fairly important for off gassing. Electrolytes are easily replaced using isotonic sports drinks. These are fast acting and drinking them on the way out to the dive is a good plan.
The other big variable in humans is C02 retention. Some people do some don’t. One thing is certain smoking massively increases CO2 retention and so does faulty or poorly performing regulators, skip breathing, slow breathing and exercise. Increased breathing resistance causes the production of raised levels of CO2. The production of high levels of C02 causes increased blood flow required to flush out the C02. This in tern increases on gassing of nitrogen. When the C02 is flushed blood flow returns to normal so the off gassing during low stress ascents is at a reduced rate. As a result the calculations used to estimate the deco on gassing phase of the dive could be wrong.
SO in summery:
· Hydrate properly for the dive and in the period leading up to the dive
· Avoid direttics like alcohol and caffeine
· Use electrolyte drinks like isotonic Sport drinks in the hours leading up to the dive
· Eat properly to keep blood sugar levels balanced
· Use Nitrox when ever possible
· If doing decompression diving try to use rich Nitrox as a decompression gas
· Keep Warm prior too and during the dive
· Add decompression time for high exertion dives
· Add decompression time for cold dives
· Consider adding deeper stops to your ascent even on no decompression dives. Or keeping your ascent rate below 5m/min from 15m up.
· Ascend from 6m to the surface in no less than two min's on a no decompression dive and 6mins on a decompression dive.
· Don’t smoke
· Make sure your regulator breathes properly and avoid skip breathing.
All this will help, but the only way to guarantee you will not get a DCI hit is don’t dive.
ATB
Mark Chase