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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.