CMF Gas matching - 4 parts [reposted with working photo links].

Freef

Advantages and Dangers of Gas and Flow Matching on Constant Mass Flow [CMF] Semi Closed Circuit Rebreathers.

Introduction.

This article consolidates and expands on the information given in two previous articles that discussed the calculations used in determining the gas mixtures [other than those recommended by the manufacturer] that can be used with a CMF SCR rebreather. This article is based on the author’s personal experience with the Drager Dolphin CMF SCR, although if the flow rates for other models are known, these too can be used with the calculations to fine-tune the fO2 in the breathing loop. The reasons for wishing to calculate a non standard match between cylinder fO2 and flow rate are varied, for example wishing to dive deeper, using the unit as a gas extension tool or only being able to obtain a gas mix that is not specified in the manufacturers literature. However, there are dangers associated with this practice, and the risks that divers who work outside the manufacturers recommended flow rate/gas combination expose themselves to are also subject of this article. Calculations of cylinder durations are based on the UK standard cylinder size of 5L and a working pressure of 200 bar, and do not take into account any additional gas added to the loop on descent as the bypass valve operates.

The CMF SCR Design.

The design of the CMF SCR is remarkably simple. With the continual supply of oxygen rich gas to the breathing loop electronic or manual O2 addition is not required, and the Drager units can be dived without a means of monitoring the percentage of oxygen within the loop [fO2] at the manufacturer’s standard mix and jet combinations, although this practice is not recommended. Extra gas is introduced into the loop as required to maintain an adequate loop volume during descent or when a loop flush is performed by an automatic valve similar to the second stage of open circuit scuba equipment, or by manual addition through a valve similar to a drysuit inflation valve that is usually found on one of the counterlungs.

As there is a continual flow of gas into the breathing loop, the excess must be vented. On Closed Circuit Rebreathers [CCR] this is often achieved by exhaling “off the loop”, usually through the nose, or by loosening the seal between the mouthpiece and lips. When diving on a CCR the only time that off loop exhalations will usually occur is on ascent as the gas in the loop expands. Most SCRs have an overpressure valve [OPV] built into the exhale counterlung which allows the diver to stay on the loop through all phases of the dive. Some CCR designs are also incorporating this feature.

With the flow rate of the gas from the cylinder being fixed, the amount of oxygen injected into the loop will depend on the flow rate and the cylinder fO2. On the Drager Dolphin there are four standard jet sizes, known by the designed cylinder fO2 they are used with. It can be seen that the actual amount of oxygen injected is not constant:

It may seem strange that the actual volume of O2 injected varies so much, but the reason for this can be found by examining the SCR system. If the loop is as empty as it can be and the cylinder turned on then the flow of gas begins into the inhale counterlung. From here it is used by the diver and exhaled into the exhale counterlung. If the loop is not at maximum volume then the gas will pass through the scrubber and back into the inhale counterlung where it will mix with fresh gas. Once the loop is full. On each exhalation a portion of the gas is vented to the water, and this gas contains a volume of oxygen as well as nitrogen and carbon dioxide.

Most people will exhale 4% less O2 than they inhale at the surface. If the user is at the surface breathing 32% through the 32% jet at a rate of 10 breaths per minute, they will exhale 28%. If the tidal volume of the lungs is taken at 4 litres, every six seconds they will remove 0.16 litres of oxygen from the loop [the 4% used multiplied by 4L], or 1.6 litres per minute-their vO2. If the exhaled gas contains 28% O2 then looking at the flow rate from the jet and diver gives an average loop fO2 of 24%. So why 24% and not 30%, which is the average of 28% and 32%? The jet will supply 32%, and the diver 28%, but it needs to be remembered that the diver will be re using some of the gas they exhaled, which has less oxygen in it. In this example the diver is cycling 40L of gas per minute, and only 15.6L is being added to the loop as fresh gas from the tank, therefore 24.4L per minute will have been previously inhaled.

If the gas supply were to be turned off at the surface the diver in the example above would still use 1.6 L/min. If the total loop volume was 10L and contained 32% O2 [3.2 litres] the diver would suffer the ill effects quite quickly with a drop of fO2 to 21% [2.1 litres] and below. Each breath removes 0.16 litres of O2 from the loop, so after each breath the loop fO2 drops by 1.6%. At depth, the diver in this example still only uses 1.6 litres per minute of oxygen. At 10m there will be 6.4 litres of oxygen in the loop, so each breath the diver takes will reduce the loop fO2 by 0.8%. At 20m each breath will remove 0.53% and so on.

This is the reason that CCR’s are more gas efficient the deeper the diver is. As the quantity of oxygen consumed by the diver is fairly constant. If the divers vO2 is 1 litre/minute, only one litre per minute is added to the loop. It must be remembered that this is one litre at one bar per minute, so at 10m the volume added would be ½ litre-one litre at two bar, 50% more efficient.

CMF SCR’s have a fixed gas duration independent of depth, however they will vent more at shallower depths because of the fixed gas supply. As with CCR systems a fixed amount of gas is injected, and the flow rates are based on flow at the surface. For example the 40% jet on the Drager Dolphin injects 10.4 L/min at the surface. At 20m it will inject the same amount of gas, but because of the increased depth which gives three times greater pressure, the 10.4 litres of gas will be compressed and occupy a volume of 3.47 litres, so the OPV will operate less frequently.

Having a continual flow of gas into the loop presents a problem for the diver, in that the fO2 of oxygen they inhale will vary with workload, temperature and stress levels, as each of these has an effect on the respiration rate. A loop flush [exhaling off loop to completely refresh the gas in the loop] will cause a spike in the fO2 of the loop because of the addition of a large quantity of oxygen rich gas. It is because of these factors that care needs to be taken in choosing the gas for a given dive.

Finding the Oxygen consumption levels of the diver [vO2]


Before any adjustment of the cylinder fO2 can take place the diver needs to calculate their personal level of oxygen consumption. The term vO2 is used to describe the amount of oxygen in litres per minute a diver metabolises during diving. At rest, on the surface the average person will use exhale up to 17% oxygen, which is why mouth-to-mouth resuscitation will keep a non breathing persons blood oxygen at a life supporting level. A vO2 of 1-1.5 litres/minute is a good starting point for general calculations without tuning the gas mix-for example to assume a loop fO2 for dive planning. Under exercise the vO2 will rise, and a very fit person may reach a vO2 of up to three litres/min. It is also important to keep tracking vO2 as experience grows, as the value is likely to drop as the diver becomes more familiar and relaxed with the unit.


To calculate the vO2 of a diver four things need to be known: the percentage of oxygen supplied to the breathing loop [cylinder fO2], the flow rate of the gas [in litres per minute], the diver’s depth and the fO2 reading at that depth. The fO2 the diver is breathing will need to be determined first if the monitoring device gives a pO2 reading by using the ‘pressure T’, and from this the vO2 can be found using the following formula:


Using the following figures the vO2 of a diver can be determined:

Cylinder fO2 = 60%
Flow rate = 5.8 litres/min
Depth = 12m
pO2 = 1.12 bar

From the depth and pO2 it can be determined that the loop fO2 is 50.9%

Placing the figures into the formula gives:


Which gives a vO2 of 1.07 litres per minute of oxygen consumption.

While the single example above gives a vO2 of 1.07, this figure alone must not be used. Relying on a single figure will not give an accurate picture of the actual vO2 of the diver, so multiple readings must be taken. By taking at least five readings per dive over a number if dives prior to adjusting the cylinder and flow rate combinations the SCR diver will be able to determine the optimum mixes for a dive, while reducing the risk of an incorrect gas choice. Adjustments made to the cylinder fO2 should not be too drastic. Using a flow rate optimised for a gas containing 60% O2 and switching from a cylinder fO2 of 60% to 40% without any intermediate steps is not advised. The graph below illustrates the loop fO2 levels [vertical axis] that are obtained from various cylinder fO2 [horizontal axis] for a vO2 of 1 L/min



It is also important to consider a heavy workload dive. To simulate this a hard fin for at least two minutes against an immovable object at a fixed depth will raise the vO2 so that the higher vO2 can be taken into consideration when dive planning. Hard fining at an equivalent exertion level to sprinting will cause a rise in vO2 that will provide a good indicator of the maximum value that will be encountered by the diver. Once again, periodically repeating the exercise will give a double check of the peak vO2.

The following readings were taken by the author on a dive in October 2006, with a cylinder fO2 of 40% and a flow rate of 5.8 L/min.



It is worth considering these readings and the vO2 that is calculated from the depth and pO2 to understand what causes readings that appear to be incorrect. During the descent phase of the dive the bypass valve operates adding more O2 to the loop than the metered flow alone. This extra oxygen gives a higher pO2 reading, which leads to an apparently lower vO2.

The spike in vO2 at a depth of 15.5m is again easily explained as this was when an exercise test was being performed. A stationary object, in this case a submerged double decker bus, was used to push against for two minutes of hard fining. At 11.2m the author was stationary while watching fish, and relaxed allowing for a lower pO2 to occur compared to fining. Finally at 6.6m the diver was just returning to the loop after a switch to OC to inflate a delayed SMB to mark the ascent as is required by the dive site. This allowed O2 to be supplied to the loop without being consumed by the diver.

These readings were taken as an illustration of the factors that can lead to spurious vO2 readings, and when a diver is taking notes to determine vO2 then a period of settling at a depth is advised to take into account the extra O2 injected into the system during descent.

Once the vO2 of the diver has been found for both the normal and peak workloads then this can be used to assist the diver to begin optimising their gasses. The graph below illustrates the difference in loop fO2 [side axis] for various cylinder fO2 [bottom axis] at the four standard Drager jet flow rates for a vO2 of 1.5 [solid lines] and 2.0 [dotted lines]. At lower cylinder fO2’s it can be seen that the ‘spread’ between the resultant loop fO2s is greater than when a richer mix is used.

When compared to the vO2 reading of 1 in the graph above it can be seen that a 60% cylinder used with the 5.8 L/min flow rate produces a far lower loop fO2. At a vO2 of 1 litre per minute the diver would get a loop fO2 of 52%, at 1.5 46%, and at 2 L/min the loop fO2 would have dropped to 39%



There are some equipment factors that need to be remembered when calculating the divers personal vO2. The O2 cell is affected by factors such as temperature and any moisture on the cell itself. For this reason CCRs use three cells as the operation is based on having a constant pO2 in the loop which means a variable fO2 as depth changes. CMF SCRs use a fixed fO2 and a variable pO2. A well-designed CMF SCR will have the cell protected as much as possible from the effects of moisture. The Dolphin has the cell mounted at the top of the inhalation bag when the diver is horizontal in the water.

The O2 sensor also needs to be calibrated prior to each diving day. It is best to let the sensor settle in the atmosphere for a few minutes prior to calibration. A check on the sensor should also be undertaken on the surface by turning on the gas after emptying the loop as far as possible and checking the pO2 or vO2 reading. It should come within 1-2% of the cylinder fO2, but due to the fact that there will be air in the loop which will dilute the nitrox supply it is unlikely that the reading will match the supply fO2. Some divers make or purchase a device that they can pressurise to two bar or greater to check the accuracy of the monitoring device. It should be remembered that some displays use the loop gas transferred through the hose to keep them pressurised to ambient pressure.

It is also possible to use parts of the Dolphin loop to check O2 cell readings. It will be more accurate than checking on an assembled loop, but it is not possible to check above ambient pressure. The inhale counterlung is used, with the inhale part of the hose set, supply valve and the blanking plug for the O2 sensor port. The plug is placed in the connector for the scrubber, and the valve and O2 sensor in their normal places. After the loop is drained by inhaling through the open end of the mouthpiece [the valve is left shut], the O2 rich supply is turned on. The one way valve in the mouthpiece will open before the bag is full, and the reading on the pO2 meter the author uses matches the cylinder fO2. If this method is used as a check the evening before a diving day, and an on site check completed on the assembled loop, the correct functioning of the O2 cell and meter can be checked.


The other variable is the flow rate. CMF jets are based on the tolerance range that the jet can be used with, and for truly accurate vO2 readings the flow of the jet will need to be checked with an accurate gauge that can read flows to 0.1 L/min. Once the vO2 is calculated the diver must add in a factor of safety to prevent problems occurring while diving the unit.

__________________
David.

Currently owner of two differently sized ankles.

Freef

Determining the loop fO2 from a known vO2 and cylinder fO2.

Once the vO2 is determined over a series of dives, the fO2 of the loop can be derived from the gas flow rate, cylinder fO2 and the diver’s vO2. If it is not possible to obtain the ‘ideal’ cylinder fO2, the best jet to use the cylinder with could be found. Even with the correct mix and flow rate, determining the loop fO2 will assist in planning the dive. The following formula is used to find the loop fO2:


If we assume a cylinder fO2 of 50%, a flow rate of 7.3 litres per minute and a vO2 of 1.1, use of this formula will give:


Which works out at a loop fO2 of 41%.

Another example would be the diver has a cylinder containing 45% O2. Assuming the divers vO2 to be 1.1, and diving the Drager Dolphin there is a choice of two jet sizes either side of 45%, namely 50% [flow rate of 7.3 L/min] and 40% [10.4 L/min]. By using the formula above the loop fO2 can be determined at 35% for the 7.3 L/min jet and 37% for the 10.4 L/min jet. It may seem strange to use a flow rate designed for a lower oxygen level than is actually present in the cylinder, but this is the practice recommended by several training agencies.


Finding a cylinder mix from a desired loop mix and known vO2.

This is probably the most useful of the gas match formulae. With a known vO2 and a desired loop pO2 or fO2 for a given depth, it is possible to determine the required cylinder fO2 in conjunction with a known flow rate. The first reason for wishing to dive the unit in this was is gas extension-getting more dive time from the cylinder.

By using a slower flow rate than standard a longer duration dive can be undertaken. On the Drager Dolphin, with a 5L cylinder at 200 bar, and with surfacing with 50 bar in the cylinder, the gas duration is between 48 and 129 minutes, depending on the jet selected.

If a dive is to take place at 15m, the fO2 in the loop can be anything from to 21% to 56%. Although theory allows a lower loop fO2 than 21% to be used at 15m this is outside normal SCR diving practice. At a loop fO2 of 21%, with a vO2 of 1 the cylinder fO2 can be found for each of the four flow rates found on the Dolphin from the following formula:



It can be seen that an increase in dive time of over 2½ times can be gained from changing from the 32% jet to the 60% jet and increasing the cylinder fO2 by 9½%.

The problem of gas matching now appears, in that if the divers work load increased and their vO2 went up to 1.75 L/min, the cylinder mixes that were giving a loop fO2 of 21% would now give the following:





The figures for the 5.8 L/min flow rate [the 60% jet] are particularly noteworthy. At a depth of 15m the loop would supply the diver with a partial pressure of oxygen of 0.195. While this would sustain life, the partial pressure of nitrogen [pN2] would be 2.305, which is the nitrogen pressure that would be found at 20m, rather than the 15m that the dive is being conducted at. This obviously has an effect on the decompression obligations of the dive. Worse still, on ascent the loop pO2 would drop rapidly to 0.078 bar on the surface, too low to enable the diver to ascend safely.

So, for simple gas extension at shallower depths the chosen cylinder fO2 is based on the upper vO2 that will still leave a loop fO2 of at least 21%. From the examples above, and using a peak vO2 of 1.75 the cylinder contents would be as follows:



At the fairly shallow depth of 15m, all the above mixes are safe to breathe on open circuit, so if a loop flush was required while using the 60% jet the peak pO2 would only be 1.12 bar, well below the maximum of 1.4 bar that is the usual ceiling for the bottom part of a recreational or technical dive. If there is a risk of a hyperoxic situation occurring [for example during a loop flush] then the safest option is to use a higher flow rate with a lower cylinder fO2.


Deeper Diving Using Slower Flow Rate Jets to Control Loop fO2


The maximum recommended depth for diving the Dolphin is 40m using a cylinder mix of 32% through the 32% jet. This combination will give a loop fO2 of 27.3% for a vO2 of 1 L/min and 23.4% for 1.75 L/min. While this combination is the safest way of diving the Dolphin, there is only 48 min gas duration in the cylinder with a 50 bar reserve. It should also be noted that the maximum operating depth [MOD] of a 32% mix is 33m, so if a loop flush is performed at 40m the pO2 could in theory reach 1.6 bar, although this is unlikely as there is always a diluting effect with the gas that is already in the exhale side of the loop before the bypass valve operates, and it is difficult to clear every last remnant of gas when doing a loop flush.

When deeper diving with a lower cylinder fO2 it may be possible to get a hyperoxic mix in the loop as well as a hypoxic one, once a depth approaching 30m is exceeded. An example of a safe dive would be if a diver plans on a dive to 30m, using a tank fO2 of 38% and a flow rate of 10.4 L/min, [the 40% jet on a Dolphin] and a vO2 of 1.25 was used to plan the dive.

At 30m the loop fO2 should be 29.5%, and the pO2, 1.18 bar. If the diver is relaxed and their vO2 drops to 1 the loop fO2 will increase to 31.5% and the pO2 to 1.32 bar. The following table illustrates what a further drop in vO2 would do to the loop fO2:



It can be seen that the loop is not likely to go into a hyperoxic state during this dive with a more relaxed vO2 than was planned for unless the divers vO2 drops lower than would be expected for a base vO2 of 1.25.

If the vO2 were to rise then the following loop fO2 and pO2 would be expected:



Again, it can be seen that with this particular combination of cylinder fO2 and flow rate that the mix supplied to the diver is unlikely to drop to a hypoxic level, again unless the vO2 rises far higher than the diver would expect to experience.

For a typical gas choice that can be used at 35m, a cylinder fO2 of 43% and a gas flow rate 5.8 L/min, the effect of various values of vO2 are shown below:






From these figures the dangers of using non-standard cylinder and flow rate combinations can be seen. The loop can become hypoxic at high workloads and hyperoxic at low values of vO2. Bear in mind that a loop flush at 35m with the cylinder providing 43% will give a loop pO2 of up to 1.93 bar.

That said, with careful selection of gasses and knowledge of their personal vO2 allows divers to extend the usefulness of their SCR. The diver needs to know the oxygen window for the choice of cylinder fO2 for a given flow rate.

__________________
David.

Currently owner of two differently sized ankles.

Freef

Using the Oxygen Window to Determine Choice of Cylinder fO2


If the diver takes their time and uses a pO2 meter to determine their vO2 then a range of cylinder fO2’s can be found to match any given flow rate. As an example the author has determined his vO2 to vary from 0.80 L/min in normal diving conditions to 1.25 L/min when exercising hard. The dive details are entered into a spreadsheet of the authors devising, with these two vO2 values to determine the safe cylinder fO2:

Once the desired pO2, depth flow rate and high and low vO2’s have been entered the spreadsheet calculates the loop fO2 that will give the selected pO2 at the dive depth, and then the cylinder fO2’s that will give the loop fO2 required. Note that as well as the range of cylinder fO2s there are also loop fO2s that give the worst-case scenarios [lowest lines].

The first gives the loop fO2 that will occur if the lowest cylinder fO2 is used [which assumes that the vO2 will be the lowest value] but the diver is working at their maximum vO2. In the example above it can be seen that the loop is still above 21%-although a larger buffer is preferred to prevent problems with lowered oxygen and raised nitrogen levels.

The second reverses the vO2 and cylinder fO2 to give an indication of the loop fO2 if the diver assumes that the dive will be harder work than it really is. Assuming a high vO2 and working at a lower vO2, the loop fO2 will increase, and in this case it can be seen that the loop may contain 35.3% O2, which would give a pO2 of 1.59 bar at the depth the dive is planned to [35m] and the MOD [for a maximum pO2 of 1.40 bar] is also given as 29.7m. The following table gives the readings for the same dive on different flow rates.



It can be seen that range of choices of gas narrows with the increase in gas supply flow rate, from 5.6% on the 5.8L/min flow rate to 2.1% at 15.6 L/min. This is because the diver has the oxygen in the loop replaced at a faster rate on the higher flow rate jets. Once the range of flow rates has been determined the diver can make a choice of the gas they wish to employ for the dive.

For the vO2 ranges and depth given above the it is possible to choose a cylinder fO2 of 40% which is fed through the 60% to get best gas optimisation. This will also result in a worst-case pair of loop fO2s of 24% and 30%, with a MOD that is deeper than the planned depth. The choice of 40% is also close to the middle of the permissible fO2 limits.

The table below illustrates that as a diver goes deeper on a fixed jet the cylinder fO2 range widens. vO2 values of 0.80 and 1.30 are used to provide the maximum and minimum rates of consumption, with a supply fO2 of 40%,a flow rate of 5.8 L/min, and a maximum depth pO2 of 1.3 bar.



At 36, 39, and 42m the minimum cylinder fO2 must not be used as this would lead to a loop fO2 of less than 21%. Even at 33m the figure of 22.5% is below the minimum that the author finds acceptable [25%] for a buffer against the loop fO2 getting too low. So, realistically the oxygen window starts to close at 30m, with a minimum cylinder fO2 of 41.8%. Using this figure the 33-39m section of the table above can be revised thus:



Where it can be seen that the oxygen window has closed. Any deeper and the minimum required fO2 will exceed the maximum required fO2. The diver must also be working at their maximum planned vO2 [1.30 in this example] to prevent the risk of the loop fO2 rising. If the 42m dive was undertaken, and the diver in this example had their vO2 drop to 1, the loop fO2 would rise to 29.6% giving a pO2 of 1.54 bar. This shows how critical it is to choose gasses with care, and to understand how easy it can be to get a hyperoxic mix at depth, or if the diver tries a fix of assuming incorrect vO2 levels how quickly the loop fO2 can move outside the planned parameters.

It must be remembered that a loop flush at these depths will lead to a spike in the loop fO2. At 42m with a cylinder fO2 of 41.8 a flush would cause the loop pO2 to exceed 2 bar, which is an extremely dangerous situation.

The table is repeated below, with the same parameters except with a higher flow rate of 10.4 L/min which is the rate of the 40% jet on the Dolphin.



Again, it can be seen that at 36m or deeper the minimum fO2 is below 25%, so the figures can be recalculated to give:



Again, the effect of the oxygen window closing can be clearly seen.

If the two tables above are compared, for each of the depths given, the flow through the 40% jet gives a far better margin of safety compared to the 60% jet. A loop flush at the maximum depth of 42m would result in a peak pO2 of 1.79 compared to 2.17 when using the 60% jet. The MOD for the richest mixes is also deeper, with a 3.6m shallower MOD for a depth of 42m on the 40% jet compared to the 8.9m difference when using the 60% jet. It can be seen that the benefit of using the 60% jet is purely in terms of gas duration, and it must be remembered that the slower the flow rate, the greater the risk of the diver having an incorrect loop fO2.

The chart below shows the oxygen window more clearly with the minimum and maximum vO2s for the two flow rates compared. The red and blue lines are for a flow rate of 5.8 L/min [the 60% jet on the Dolphin], and the green and yellow for the 10.4 L/min [40%] jet. It must be remembered that just because the lines are further apart it does not make the choice of gas mixes safer. In fact, the risks increase with depth, especially when the low vO2 calculated cylinder fO2 bottoms out to prevent a loop mix of less than 25%. The only way to really minimise the risks of CMF rebreather diving is to dive the manufacturers recommended mix and jet size combination.


Deeper diving with gas extension is a higher risk form of diving the CMF SCR. On the Dolphin the maximum depth is realistically 40m whichever gas mix you choose. Anyone choosing to dive in this way must be fully aware of, and accept the risks in doing so. Bailout must be carried, and it must be of a size that will get the diver to the surface including any decompression stops. The standard 3L/232 bar cylinder in use in the UK will just about get a diver to the surface from 30+m when full. If the diver is on their second or third dive and they have a loop failure of any sort then the 3L may not be enough to get the diver to the surface safely. It must be remembered that the bailout is usually used to supply gas to the divers buoyancy control, be it a wing or drysuit as well, so there will be less gas available to the diver than when they descended.

It is possible to use a nitrox mix as bailout, but as this must be a safe gas to breathe at the deepest planned part of the dive, it is unlikely to be of benefit for decompression. Conversely, if a decompression mix is carried, this may be unsafe to breathe at the maximum depth of the dive, which is where bailout use must be planned from.

Further calculations regarding the size of bailout can be found in the section below entitled ‘Decompression on a CMF SCR’.

__________________
David.

Currently owner of two differently sized ankles.

Freef

Custom Mix Calculating Tables With Known vO2


Steve Sprague adapted a series of TDI tables for use when the diver wants to quickly calculate their loop fO2 from a known flow rate, cylinder fO2 and vO2 when access to a computer running a spreadsheet isn’t available. His original reworked table appears below. Note how the range of vO2 covers values of 0.40 to 2.50, and how dangerous the wrong gas choice can be, dropping the loop fO2 well below 0.21 [21%]. The range of flow rates for the chosen gas is also illustrated at the top of the table, as is the orifice fO2 designation and the ideal flow rate. A useful feature of the tables is that the coloured band that is fitted to the jets is used as the band at the top of the table. It was from this original table design the author calculated ones more specifically suited to his own vO2 levels, with smaller steps between the vO2 levels.








When compiling tables such as these, vO2 readings outside the normal vO2 range of the diver should also be used. By doing this the diver can quickly check for any out of range problems a given cylinder fO2 could give.

Calculating desired flow rates for variable supply valves.


Moving away from the standard Drager design, some manufacturers SCRs are equipped with a valve that the user can set themselves to determine the flow rate of nitrox and hence the loop fO2. CCR units such as the KISS work on a similar principle of adding a continual flow of pure O2 [rather than nitrox] to the loop to maintain a near constant pO2 with the diver adding O2 as needed.

The following formula is used to determine the flow rate when the vO2, supply fO2 and desired loop fO2 are known:



Which, when calculated out will give a flow rate of 5.29 litres/min.

Again, it is best practice to work out a contingency flow rate for higher and lower vO2 levels. Varying either of the desired loop fO2, cylinder fO2 or vO2 readings while leaving the other values constant can do this.

Dive planning when using CMF SCR


When planning a decompression dive on a CMF type SCR unit, the diver needs to plan for all of the factors that would affect a normal nitrox dive in terms of oxygen exposure and decompression while calculating the loop fO2. The Author always assumes that the dive will be slightly harder work than the previous one, and the loop fO2 is taken as 2% lower than calculated.

For example a dive using 50% through the 60% jet at a vO2 of 0.95 L/min would result in a loop fO2 of 40%, so the plan would be as if the dive were being conducted on 38%-the same as if the vO2 had gone up to 1.1 L/min. The diver must also make a note of the highest pO2 reading they get at the deepest part of the dive to calculate oxygen exposure. It is also worth planning the dive as it was being conducted on air as a back up in case the vO2 rises with the consequence of the loop vO2 dropping below 38%.


Dive computer use with CMF SCR


There is increase in popularity of dive computers that have the ability to read the loop fO2 from a cell and provide real time decompression information to the diver in the same way that the fixed O2 levels in open-circuit can be set on a normal nitrox or trimix computer. It is also possible to use multi mix nitrox computers where gas switching may be performed by the user.

However, consideration needs to be given to the wisdom of relying completely on these methods of decompression management. Dive computers are battery powered electronic devices that are submerged in water. They are also operated by a human who is not perfect. The author has twice mis-set a dive computer, but the error was realised due to having a knowledge of what the readings should have been, and has personally seen three computers fail while in use by others. Putting complete trust in a dive computer without the knowledge of how long you can spend underwater for a given decompression profile, or no decompression at all, increases the risk to the diver.






With pre planning and carrying a run time slate for any planned decompression diving, dive computers can be used to assist the diver in optimising their dive. With a multi mix computer it is possible to plan for a loop fO2 above 21%, a fall back of 21% if the vO2 rises, and if a third mix can be programmed in then a decompression gas can also be entered.




For example, if a dive to 36m is planned with a 40% cylinder fO2 and a flow rate of 5.8 L/min and a vO2 of 1 L/min, it is safe to assume a loop fO2 of 26% [2% lower than calculated for the divers vO2 for safety]. The diver would then make a note of the minimum pO2 they should allow before switching to the fallback of 21%. At 36m with a loop fO2 of 26% this is 1.19 bar. It is worth noting the expected pO2 at other depths as well to ensure maximum safety. If the pO2 drops it is simple to switch to 21% to continue to dive safely. Due to the spiking of loop fO2 on descent it may be advisable to keep the computer switched to 21% until the correct level of loop fO2 has been verified during the dive. A second bail out slate should be carried assuming that the loop fO2 drops below the planned percentage.

Decompression on a CMF SCR.

Many manufacturers prohibit the use of their SCR models for decompression diving. They also want only the correct gasses to be married up with the flow rates they are designed for. It should also be noted that most dive computers carry warnings that they are not to be used for decompression diving, so it appears that the restrictions may be more for a liability function than any shortcomings of the equipment.

Decompression using the loop of a SCR is certainly possible, but the possibility of a problem with the loop should always be considered. If a loop problem occurs the diver has two options: bail out immediately to the OC gas, or stay on the loop in OC mode before switching over. Depending on the nature of the problem it may not be possible to stay on the loop, so sufficient bailout needs to be carried for the dive. It is also an idea to calculate the bailout gas required at a higher RMV than usual to take account of the raised stress levels in the event of a problem. For example if at 35m, with a stop of 2 min at 9m and 8 min at 6m, a diver has to bail out, and assuming an RMV of 18, the diver will need at least 572 litres of gas-191 bar out of a 3L cylinder. When it is considered that gas will be added to either the wing or drysuit on descent it becomes apparent that for decompression of any length a larger source of decompression gas is required.

In the UK a popular way of achieving this is to use a 10L cylinder with an H valve. This will provide enough gas for bailout and decompression obligations. In the example used above, the diver will require just over 57 bar of gas from the 10L to meet their decompression needs. The only disadvantage of a 10L from the redundancy point of view is that a catastrophic failure of the cylinder neck O ring or an O ring in the valve itself will deplete the gas the diver was depending on for decompression. Fortunately this is an extremely rare occurrence and one that is not likely to be encountered by the user.

A completely redundant source of decompression gas, such as a sidemounted stage is another way of diving beyond no decompression limits. A 232 bar, 7L stage will provide over 1600 litres of gas to the diver. While it is possible to plumb this in and provide drysuit inflation, it can be kept totally separate and for decompression only. In the dive above, using a 7L will cause a drop in tank pressure of 82 bar, so it is perfectly feasible to dive the tank for two dives and still have a healthy reserve.

Another point to consider is the step up to the decompression gas fO2 from the loop fO2. If a 10L is being used for both loop and decompression, the gas required to achieve the desired loop fO2 may not provide much of a decompression advantage over staying on the loop.

For a dive to 36m for 35 min using a cylinder fO2 of 40% and a flow rate of 5.8 L/min, a diver with a vO2 of 1L/min would have the following dive profiles, depending on which gas was used for decompression.

Note that in these calculations the safety factor of planning for a loop fO2 2% below the calculated loop fO2 has NOT been used, and the calculations are based on a loop fO2 of 28%.



So it can be seen that using the option of a higher fO2 supply can reduce the dive time in this example by up to 16 minutes, especially important when diving in cold water. Of course the down side of this is the pO2 of the deco gas is not useable as bailout at depth. OC divers use this method for decompressing, and their bailout is that their twinset [two tanks manifolded together] features valves that can isolate a failed regulator, preserving as much gas as possible for an ascent where a switch to a deco mix can take place. The rebreather diver does not have this option in the event of a catastrophic loop failure, which although rare must be planned for. However it is preferable to take a breath from a too rich mix gas at depth and have the possibility of an O2 problem than it is to drown for certain.



Abbreviations used in this article.

CCR Closed Circuit Rebreather. Mechanically [mCCR] or electronically [eCCR] controls a pure oxygen supply into the loop. Two cylinders are required, one for a dilutent [also called diluent] gas, which will be air or trimix, and the other 100% O2.

CMF Constant Mass Flow. Most semi closed rebreathers are of this type, where there is a continual metered flow of oxygen rich gas into the loop.

fO2 Fraction of oxygen. The measure of how much oxygen is in a gas, can be writted as a percentage [ie 21%] or a decimal [ie 0.21]. IfO2 is used for inspired fraction of oxygen elsewhere, which this article refers to as loop fO2.

OC Open Circuit. Conventional SCUBA equipment.

OPV Over Pressure Valve. Usually found in the exhale counterlung, and vents the excess gas from the loop.

pN2 Partial Pressure of Nitrogen. May also be written as ppN2, the pressure of nitrogen within a gas in Bar.

pO2 Partial Pressure of Oxygen. May also be written as ppO2, the pressure of oxygen within a gas in Bar. Normal diving is limited to 1.4 bar as the ‘working’ pO2 and 1.6 bar for decompression.

RMV Respiratory Minute Volume. The amount of gas consumed by a diver while on open circuit, in litres per minute at the surface. Also known as SAC [Surface Air Consumption].

SCR Semi Closed Circuit Rebreather. One where a flow of nitrox is supplied into the loop, usually from a single cylinder. Some recent SCRs have two tanks that can have different mixes, usually one for the bottom mix and one for decompression.

vO2 The measure of how much oxygen the diver consumes, in litres per minute [L/min].

__________________
David.

Currently owner of two differently sized ankles.

Freef

Bumpity bump.

__________________
David.

Currently owner of two differently sized ankles.

pirateeye

Let me just say Thanks for all the info... On behalf of all of us Draeger user who like to exten d things and do it with an educated mindset....

__________________
Why Buy It When You Can Spend More to Build It

Freef

You're welcome, feel free to ask if you have any questions.

__________________
David.

Currently owner of two differently sized ankles.

OTHMAN

Hi Freef

Wat a super detail thanks
By looking at it, you seen to be a very experience diver in Dolphin

Need to ask you .. I need to purchase the side tank holder .. do you have any detail on it . and where can i get it


OTHMAN
SINGAPORE
DOLPHIN REBREATHER

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Your skills is not on the equipment
but your skills is on the person

Freef

Hi Othman,

Do you mean the one that holds the 3L bail out cylinder? I bought my Dolphin new, and it came as standard. I'll have a look tomorrow and see if it is attached to the wing or seperate.

__________________
David.

Currently owner of two differently sized ankles.

mathias

hi othman,

is it this one you are looking for ?
i have e new one for sale: 40 euros plus shipment.

mathias

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Plus près du fond, plus loin des cons !