Dive planning - how to do it?

What is RMV and how to calculate it?

The abbreviation RMV means the minute gas consumption on the surface, counted in liters. Everyone has a slightly different RMV and should calculate it. The easiest way is to dive to, for example, 20m and swim at a normal pace at this depth for 15-20 minutes (deviations from the depth should not be more than +/- 0.5 m). Of course, you should write down the "state" of the manometer before and after, I know from experience that saving may fail.

Example: Let's assume that in 20 minutes on 20 meters we used 100 ata in a 2x10 set. What is our RMV?

We calculate: 100 ata x 20 liters = 2000 liters - that's how much gas we used for 20 m in 20 minutes.

So we use 2000 liters per minute / 20 minutes = 100 liters - that's how much gas we use at 20 meters per minute So we still have to calculate the consumption on the surface we were at 20 m with an absolute pressure of 3 ata, we want to know how it will be on the surface i.e. at an absolute pressure of 1 ata. So 100 liters / 3 ata = 33 liters / minute - the result is a bit big but I selected the data randomly.

So we have our surface consumption RMV = 33l/min

What does it give us? Well, when planning a dive at, for example, 50m with a bottom time of 20 minutes, we can calculate how much gas we will use ... So 20 minutes x 33 l/min x 6 ata = 3960 L

ATTENTION!!! THIS IS ONLY AND EXCLUSIVELY THE CONSUMPTION AT A DEPTH OF 50 M, YOU HAVE TO ADD TO THIS SUBSCRIPTION, ASCENT AND DECO!!! As well as the necessary stock, more on that later.

For an average scuba diver, such consumption, depending on: gender, training, equipment taken (resistance), ranges from 15-25 l/min. It is also worth tracking how our consumption changes in connection with any work, stress, etc. increase in consumption by 5-10 l/min!!! It's worth taking into account...

My RMV is 18 l/min, it takes 20 l/min to count, but with any kind of stress or work it jumps to 25 l/min.

It is also worth noting that the RMV per deco will be a little lower, because we are rather stationary on deco. Mine is 15L/min.

Bottom gas planning

Let's now move on to planning the consumption of bottom gases. We already have our RMV, for the purposes of this reasoning we will assume that it is 20 L/min.

As you already know, gas consumption at a given depth is counted in liters by multiplying our RMV (surface) by the relative pressure prevailing at a given depth (e.g. 40m = 5 ata) and by the time of staying at a given depth.

Example 1. 

How much gas will we use during 20 minutes of staying at 40m? i.e. 20 l/min x 5 ata = 100 l/min - so many liters/min. we will use it at a depth of 40 m, so in 20 minutes we will use 100 l/min x 20 min = 2000 l.

How does this relate to our diving and diving cylinders?

As you know, we dive with different cylinder sets. It can be a single cylinder, e.g. 8, 10, 12, 15 liters or a two-cylinder set, e.g. 2x10, 2x12, 2x15 etc.

Assuming that we fill cylinders up to 200 bar (1 bar = approx. 1 ata), we can "stuff" the appropriate amount of liters of gas in a cylinder of a "given" capacity. For example, an 8 l x 200 bar cylinder = 1600 l, a 2x10 x 200 bar set = 4000 l, etc. Our diving, however, consists not only of the bottom phase, but also of the descent and ascent stage and decompression. For the time being, we will not calculate the reserve of decompression gases in the side cylinders (more on this later) and for the sake of simplicity, we will use linear decompression, i.e. slow continuous ascent.

So let's assume: descent speed of 10 m/min, ascent speed of 5 m/min (linear decompression). So, we will dive to a depth of 40 m in 4 minutes, and we will emerge in 8 minutes. How to calculate our consumption during ascent or descent? The matter is simple... For the descent and ascent stage, we take the average depth, and the rest as in the bottom consumption calculation. So for dipping...

Average depth = (40 m + 0)/2 = 20 m, i.e. consumption: 20 l/min x 3 ata x 4 min = 240 l.

And for the ascent...

Average depth = (40 m + 0)/2 = 20 m, i.e. consumption: 20 l/min x 3 ata x 8 min = 480 l.

Hence, summing up for the entire dive, we will use: 240 l + 2000 l + 480 l = 2720 l.

ATTENTION!!! The given dive and decompression times are modeled for the purposes of the example. An attempt to complete such a diving profile may result in severe injuries or death!!!

Please note that this is the amount of gas we will use for an example dive... We have calculated our gas consumption for the entire dive to be 2720 l. Now it's time to answer the question of how much gas we should take with us. As you know, in decompression diving we use the so-called the 1/3 rule.

What does that mean? Well, the gas content in the cylinder is divided into 3 equal parts, and then we plan the dive in such a way that 1/3 of the gas is used for immersion and diving, 1/3 for ascent, and another 1/3 for the so-called "just in case". So with a properly planned and carried out diving, at least 1/3 of the gas we took with us underwater should remain on the surface.

so for our example...

2720 l. - gas used for diving 2720/2 = 1360 l. - so much gas we should have left (1/3 

So 2720 liters + 1360 liters = 3980 liters, which gives us 4000 liters - that's how much gas we have to take with us underwater.

i.e. 4000 l. / 200 bar = 20 l. - i.e. a set of 2x10 l.

I would like to draw your attention that diving is a partner sport... so we have to include our partner in our calculations... and here is a little problem... if the RMV of the partners is the same then the calculation will not change, but what about when the partner's RMV is greater than ours?

Well, then we have to add 1/3 of the partner's consumption to our consumption!!!

Let's show it with an example...

2720 l. - this is our consumption (diver no. 1)

3400 l. - this is the consumption of our partner (diver No. 2), i.e. 1/3 of the partner is 1700 l. So how many gases do we take under water?

2720 l. + 1700 l. = 4420 l., i.e. 4420 l./200 bar = 22.1 l, i.e. a set of 2x12 l. - this is what diver no. 1 takes.

3400 l = 1700 l = 5100 l, i.e. 5100 l / 200 bar = 25.5 l, i.e. a set of 2x15 - this is what diver no. 2 takes.

Decompression gas planning

Okay, how do we plan decompression gases? Let's start with what decompression gas is. It is a gas taken under water in an additional, usually side cylinder, which, as the name suggests, is used for decompression. Most often this gas is some kind of nitrox - which allows us to shorten the decompression. Ok... now that we remember what deco gas is, let's consider how much gas we should take under water.

Consumption is calculated using the same method as for bottom gas by multiplying the RMV by the absolute pressure at a given depth and the time spent at the depth. Then we add up all the components and we get some gas consumption ... let it be 1000 liters. Now what? Do we add 1/3 (our or partner's)? No... the rules are a bit different here.

Since the decompression gas is used only for decompression, when adding the reserve, we must take into account the loss of gas by the partner, and thus the need to "share" our gas. So in this case the rule of 1/2 applies, i.e. we only use half of the gas.

The partner wear rule also applies here. If the partner has a higher consumption than us, we add his 1/2.

Example

1000 l. - our decompression gas consumption, the next 1000 l. is our 1/2, so together we have to take 2000 l of deco gas, i.e. 2000 l./200 bar = 10 l.

But...

1500L - Our partner's consumption, so his 1/2 is 1500L, so the partner needs to take 3000L and the diver using 1000L must bring 1000L + 1500L = 2500L.

MOD, END, EAD, CNS

I will try to introduce you to the rules that govern the selection of gases in diving. We will explain some "magic" terms such as MOD, END, EAD and CNS and their influence on the selection of one gas and not another. We will, of course, count a few examples explaining these concepts and their effect on the gas selected for the dive. These considerations will require recalling information about the effects of various gases on our body, mainly oxygen and nitrogen.

At the beginning, we will deal with the concept of MOD and what it entails with the impact of oxygen on the diver's body.

For starters, what is a MOD?

MOD is an English abbreviation of the phrase Maximum Operating Depth meaning in Polish Maximum Operating Depth, which in translation into "human" means the maximum depth at which we are allowed to breathe a given gas.

Okay, but where do we get this from?

The concept of MOD is inextricably linked to the effect of oxygen on our body, and more specifically to the effect of oxygen under elevated pressure.

As we all know (and if someone doesn't know why, please wait for the part devoted to the CNS) oxygen under a certain partial pressure is lethal for our body...

Different diving organizations adopt different levels of allowable partial pressure of oxygen, usually between 1.4 ata and 1.6 ata (why I will explain in the section on CNS).

Briefly for now...

Personally, when diving, the partial pressure of oxygen at the level of 1.4 ata when I do light work (i.e. I just swim) and 1.6 ata on decompression (then I don't actually move), when doing heavy underwater work, I recommend not exceeding oxygen partial pressure of 1.2 ata.

Okay, so what does that mean...

Let's assume we're doing a normal standard dive and I take 1.4 ata as the max ppO2 (partial pressure of oxygen).

Example 1.

I have EAN 36 in my cylinders, how deep can I dive with such gas, i.e. what is my MOD?

EAN 36 is in other words nitrox with an oxygen content of 36%, i.e. ppO2 = 0.36 ata - on the surface we want to find out at what depth ppO2 will reach 1.4 ata

For the record...

The air pressure at the surface of the sea is 1 ata and increases by 1 ata every 10 m of depth.

We are only interested in oxygen, whose partial pressure in our example at the surface of the sea is 0.36 ata, and at a depth of 10 m - 0.72 ata, 20 m - 1.08 etc... I hope everyone remembers these dependencies.. .

Back to our example...

We need to make a proportion (Primary school level)

1 ata / 0.36 ata - this ppO2 prevails on the surface, i.e. at a pressure of 1 ata = x ata / 1.4 - this ppO2 is to prevail at the depth we are looking for.

That is

1ata/0.36ata = x ata/1.4ata

hence

x ata * 0.36 ata = 1 ata * 1.4 ata

So

x = 1.4/0.36 = 3.88 - what does this mean to us?

Well, our ppO2 will be reached at a depth corresponding to a pressure of 3.88 ata.

Okay, how deep is that pressure?

Bearing in mind that the surface pressure is 1 ata and every 10m of depth we gain 1 ata, we can quickly calculate ... [3.88 ata - 1] * 10 = 28 m (subtract 1 ata of surface pressure and multiply by 10 meaning meters that add 1 ata of pressure).

Thus, with EAN 36 in the cylinders, we cannot exceed the depth of 28 m during the dive ... so our MOD = 28

Example 2

Okay, now we would like to dive to a depth of 35 meters, which is MOD = 35.

What gas to choose so as not to exceed the assumed ppO2 = 1.4 ata?

First, we have to remember that at 35 m there is an absolute pressure of 4.5 ata (1 ata for every 10 m of depth + 1 ata surface pressure - clear?)

So, back to elementary school again and we're making a proportion...

4.5 ata/1.4 ata - we want this maximum ppO2 at a depth of 35 m (i.e. 4.5 ata) = 1 ata/x ata - such ppO2 will be on the surface (i.e. 1 ata)

hence

4.5 ata/1.4 ata = 1 ata/ x ata

That is

4.5 * x = 1 * 1.4

So

x = 1.4/4.5 = 0.31 - this is the partial pressure of oxygen that our mixture should have on the surface, i.e. it should consist of 31% oxygen ... such a mixture is, for example, EAN 31

MIX PLANNING

We will now deal with the second parameter limiting the selection of the breathing mixture, this time due to our ability to perceive and act under water, i.e. the narcotic effect.

The parameter defining our narcosis is END - Equivalent Narcosis Depth, which in Polish means Equivalent (i.e. equivalent) Narcotic Depth, which means that we "feel" as at such a depth. So, for example, END = 30 at a depth of 60 m means that we "feel" as if we were at 30 m.

Okay, what is this drug?

Without going into too much detail (I hope I'll be able to persuade Agatka to write something about it in medicine), each gas has "some" narcotic properties, which increase with the increase of partial pressure. In the simplest example of air, that gas is nitrogen.

To simplify calculations, he assumes that air consists of 21% oxygen and 79% nitrogen.

Example 1.

What will be the END for EAN 32 at 30 m?

In EAN 32 we have 68% nitrogen, i.e. ppN at 30 m = 0.68 * 4 = 2.72 - such ppN we have at 30 m so

END = 2.72/0.79 = 3.44 - or 24 m (0.79 is ppN in surface air)

i.e. that diving at 30 m on EAN 32 will "feel" like at 24 m.

Example 2.

Let's assume that the drug level we accept is 30 m (which is in line with the recommendations of most diving organizations).

Assuming that the narcotic gas is only nitrogen (which is not necessarily true, but more on that later), let's calculate the maximum partial pressure of nitrogen we allow for our dives.

that is ours

ppN = 0.79 * 4 = 3.16 - at 30 m we will have such a partial pressure of nitrogen, i.e. nitrogen at a pressure of max. 3.16 ata produces an acceptable narcotic effect.

Example 3.

Well, now we want to dive to 50 m, but the END is to be equal to 30 m, i.e. at a depth of 50 m. ppN = 3.16 ata

In order to know what mixture will meet our assumption (END = 30) we need to calculate what partial pressure of nitrogen on the surface corresponds to our assumed ppN.

That is

3.16/6ata = 0.53 - that's how much nitrogen should be in our gas (53%)

What does this mean to us?

Well, that 47% of the mixture should be occupied by "something" else

OK, but what?

Certainly oxygen, after all, we need something to breathe.

But we remember from the previous part that max. ppO2 = 1.4 ata, i.e. for 50 m 1.4/6 = 0.23 - this is the maximum amount of oxygen in our mixture (23%), so "we are missing 100%

100% - (53% + 23%) = 100% - 76% = 24%

So we need to add 24% of "something" to our mix

The "something" in diving is helium. Why?

Well, because the narcotic potential of helium is much lower. To put it simply, we can assume that helium as an inert gas is not narcotic. And this "magical" way created a mixture of 3 gases: Oxygen (23%), Helium (24%) and nitrogen (53%), called trimix. Ok ... let's remember the depth of 50 m and the gas called tx23/24 - we usually give the fractions of oxygen and helium in the record - nitrogen can be calculated by subtraction. The gas calculated in this way for a given depth is called the best mix. In diving, for various reasons, standard mixes are also used for different depth ranges.

But it would be too easy if that was all

Well, it turns out that oxygen is also narcotic!!!

According to the book "The Physiology and Medicine of Diving" by Peter Bennett and David Elliot, ed. 4 W.B Saunders Company Ltd, London 1993 for oxygen we assume the narcosis coefficient equal to the narcoticity of nitrogen (coefficient = 1)

That is

Example 4.

in our example tx 23/24 depth 50 m, in this approach END will change...

0.53 + 0.23 = 0.76 - this gas fraction will be narcotic,

so at a depth of 50m it will be this

0.76 * 6 = 4.56 - partial pressure of narcotic gases

So our END will be

4.56/0.79 = 5.77 - which corresponds to a depth of 47 m!!!

but, but... it's not that bad...

since we assume that oxygen is narcotic, we must be consistent...

in the air is also narcotic, so narcotic is not 79% of air (nitrogen) but 100%.

So in our example, END = 4.56/1 = 4.56 or 35m.

So this trimix helped a bit

Example 5.

We calculate the best mix for a 50 m dive, assuming the narcosis of nitrogen and oxygen and END = 30. The oxygen percentage will not change for us and it will still be 23%

What about nitrogen?

First, we must remember that if we accept anesthesia at the level of 30 m for air, the gas pressure that we allow will change, because there is more or less 100% oxygen and nitrogen in the air. If so, the impassable barrier for us is the partial pressure of nitrogen and oxygen in total at the level of 4 ata.

so we calculate...

6*(0.23 +x) = 4 - at a depth of 50 m (6 ata) Oxygen (0.23) + nitrogen (x) is supposed to exceed 4 ata, i.e.

0.23 + x = 0.66

x = 0.43 - that's how much nitrogen can be in our mixture (43%)

So let's calculate helium...

1 - (0.23 + 0.43) = 0.34 - and that much helium (34%)

and what did we get?

our best mix is tx23/34, for comparison in the example not assuming oxygen narcosis our best mix was tx 23/24...

To make it even harder...

Helium is also narcotic!!!

According to the book "The Physiology and Medicine of Diving" by Peter Bennett and David Elliot, ed. 4 W.B Saunders Company Ltd, London 1993 for helium we assume the narcosis coefficient equal to 0.23 of the narcoticity coefficient of nitrogen (coefficient = 0.23)

Well, let's calculate it

Example 6.

Let's calculate the END for our original tx23/24 mix at 50 m, assuming the narcosis of all three components of the trimix

END = 6*(0.23 +0.53 + {0.23 * 0.24})/1 = 6*0.81 =4.86 - or 38.6m

As you can see, assuming the narcotic nature of all 3 components of the trimix, END moved almost 10 m down!!!

What's the conclusion?

I drew one, END I count assuming oxygen and nitrogen narcosis.

Another conclusion, assuming oxygen toxicity, nitrox does not reduce narcosis. Another conclusion... you can go bankrupt on helium

And now to confuse you a bit more...

Bennett believes that the narcotic effect of oxygen and nitrogen together is greater than either of them alone

Richard Pyle argues that the narcosis of oxygen and nitrogen depends on the percentage composition of the mix.

We will now deal with the concept of EAD or Equivalent Air Depth, which in Polish means Equivalent (equivalent) Air Depth.

The EAD parameter is used only for nitrox dives. It shows the equivalent depth of an air dive for the nitrox dive we did... phew... that was a hard definition.

I'll try to explain with an example...

Let's assume that we have EAN 36 in our cylinders and we want to dive to 20 m. What will be the EAD?

When calculating the EAD, the parameter of interest to us is the nitrogen content.

So first we calculate the ppN in nitrox at 20 m depth

ppN = 0.64*3 = 1.92 - this is the ppN for our EAN 36 at a depth of 20 m.

Okay, now what depth for air does this ppN correspond to?

EAD = 1.92/0.79 = 2.43 or 14.3 m. We would have to dive to this depth on air to correspond to EAN 36 diving at 20 m.

What is it good for us?

E.g. to calculate decompression with only standard air tables...

In our example, we take the standard air tables and check what decompression we would need to do for a 15m dive with a bottom time of 20 minutes.

And all the philosophy...

Finally, of course, a small task for you

We dive to 30m with a bottom time of 20 minutes. What nitrox? What EAD? What decompression based on Buhlmann tables?

Well, and we still have CNS and then a surprise

Let's start with the definition of what is CNS... actually CNS Oxygen Toxicity

CNS - or Central Nervous System which in Polish means the Central Nervous System, Oxygen Toxicity or Oxygen Toxicity.

In other words, we understand the abbreviation CNS as the toxic effect of oxygen on the Central Nervous System.

As you all know, i.e. I hope you know, the toxic effect of oxygen on our nervous system depends on the partial pressure at which we deliver this oxygen to the body.

To briefly recall the basic information...

At a partial pressure of oxygen below 0.5 bar, no toxic effects on our Central Nervous System have been found. Therefore, it is assumed that due to the CNS, we can stay under ppO2 indefinitely.

We measure CNS as a percentage. The maximum "dose" is 100%, however, we try to plan our dives so as not to exceed 80%.

The increase in CNS per minute is shown in the table below

When looking at this table, it is worth noting that the CNS increment per minute is not a linear function. I think that by looking at this table you already understand why in diving we assume the maximum ppO2 limit while staying at the bottom is 1.4 bar, and 1.6 bar during decompression. I especially pay attention to the limit value of 1.6 bar and the consequences of even a slight exceeding it in the context of CNS increase.

Pay attention to what happens when we perform accelerated decompression on 100% O2, we will have problems with buoyancy and we will fall to 7 m ... our ppO2 will then be 1.7 bar, which means that within a minute we will increase as much as 10% CNS per minute!

Okay, but how do we count this CNS?

Task:

Let's calculate the CNS for a 24-minute stay at 30m on EAN 30.

So the ppO2 for EAN 30 at 30 meters is 0.3 * 4 bar = 1.2 bar

Now we find in our table the increase in CNS for ppO2 = 1.2 and we have 0.47% CNS/min, which gives us 24 min * 0.47% CNS/min = 11.28% CNS - so to our safety limit we have still far away...

What if we dived to 40 m on the same EAN 30?

ppO2 for 40 m is 0.3 * 5 bar = 1.5 bar

Let's look at the table and count...

24min * 0.83% CNS/min. = 19.92% CNS

All right, what if we thought (please don't do it under any circumstances, because it's dangerous!) to dive on this EAN 30 at 50 m?

ppO2 for 50 m is ) 0.3 * 6 = 1.8 bar!!! looking at the table...

24 min * 50% CNS/min = 1200% CNS - we are dead!!! probably after 2 minutes!!!

It is also worth paying attention to repetitive dives. In the case of CNS, we have a rule that every 90 minutes, half of the current CNS saturation "disappears".

For example, we have 60% CNS... after 90 minutes we have 30% after another 90 minutes. 15%, after another 7.5%, etc...

I hope everything is clear!

And now the task to solve for you:

On the basis of the chart below, illustrating our diving, please fill in the table.

Table manual...

For example, we dive to 40 m at a speed of 10 m/min, i.e. for 4 minutes. so for calculations we assume that 4 min immersion at 40 m is equal to 4 min stay. at 20 m.