‘Plan the dive and dive the plan' has long been the mantra employed in all areas of diving. Technical divers, in particular, spend more time planning their dives than many recreational divers. This is due to a number of factors including increased risks, greater depths, high gas usage at depth, increased decompression obligations, increased oxygen toxicity loading, and a host of other reasons. For many recreational divers, dive planning has become a lost art, but technical divers still place a large emphasis on the value of dive planning. Despite this, the methods of dive planning have changed to take advantage of changes in technology and equipment. In this article, we will look at how dive planning for technical divers has evolved, and how we can best make use of modern technology while still maintaining safety. We will consider how understanding the functions of your dive computer can provide additional information to help you dynamically plan your dive.
In the olden days…
In the early days of technical diving, there were no PC planning tools or dive computers suitable for technical dive planning. The only option for planning a dive was to look up a decompression schedule using pre-generated tables. Initially, not even the pre-generated tables were publicly available and the very earliest technical divers had to use commercial diving tables or work directly with decompression researchers if they wanted to obtain a set of trimix tables. The decompression schedule would be copied onto a dive slate with fixed decompression stops and run times. CNS and OTU loading would be calculated by hand and gas usage would be calculated for each phase of the dive, and the rule of thirds used to add in a safety reserve. The dive would then be executed by following the dive plan run times written on the slate with depth and time being monitored using a bottom timer.
Backup plans would also be prepared just in case the diver goes slightly deeper, stays slightly longer or, in the worst case, goes both deeper and stays longer. With pre-prepared decompression tables, “slightly deeper” was usually taken to mean the next depth increment, which on many tables was 3m or 10ft deeper. “Slightly longer” would be taken to mean anything from 3 to 5 minutes longer. Finally, a backup plan would also be prepared showing the decompression schedule if the diver loses their decompression gas and has to complete their deco using back gas.
With the increased availability of personal computers, it became feasible to generate custom tables using a PC planning tool. This allowed divers to use a number of different gases, decompression models, and conservatism settings. The overall process of planning a dive remained the same, just using a planning tool instead of tables. The planning tool would generate the decompression schedule, CNS and OTU loadings, as well as gas requirements. The only difference would be that the PC planning tool would do the laborious arithmetic required to calculate gas requirements, CNS loading, etc, rather than the diver doing it by hand. When used correctly, these PC planning tools removed the risk of the diver making a silly mathematical error. The computer-generated schedule would then be transferred to a slate just as when the plan is generated by hand. In the water, the dive would be executed in exactly the same way with the diver using their bottom timer to monitor the run times written on the slate.
In time, personal dive computers became available that could handle decompression diving, trimix or rebreathers, but they were still expensive and often unreliable. As a result, it was common to use a written plan on a slate with a computer as a backup in case of going off the plan or in case of an emergency.
This was not an ideal situation as divers would have to spend a significant amount of money on a dive computer without being able to make full use of it. This led to the difficult situation where the diver would have to forego the flexibility offered by the dive computer and stick to a fixed depth and time in order to be able to fall back to their written backup plan in the case of a computer failure. This difficult decision made many divers and agencies question the suitability of dive computers for technical diving.
A new dawn…
However, as computers become more common, reliable, and affordable, this gradually changed. Divers would still use a planning tool to generate a deco schedule to write on their slate just as before. The change was that this schedule was now used as a backup to the computer which became the primary method of running the dive. Despite this, the plan would still primarily be predetermined in terms of a fixed bottom time, in order to still be able to fall back to the written plan. However, the actual ascent time would now be determined by the deco schedule on the computer.
Now computers are much more available and reliable. In addition, the costs have reduced so much that many people have backup computers. The flexibility offered by the computer is in contrast to the rigid nature of tables. Unfortunately, when your backup is based on written tables, you can't make full use of this flexibility. However, when you have a backup computer, suddenly this flexibility comes into its own and this is where significant changes to planning styles started to be adopted.
This is a real mindset shift for many divers. There is still an impression that we should always use tables or that tables are somehow safer than using a dive computer. In reality, a dive computer gives a much more flexible tool for managing the dive. However, many divers keep the tables mindset even when using a very reliable and flexible planning tool. It is important to understand the features incorporated in your dive computer as they can provide additional information that can be used to manage the situation.
When you have a fixed deco schedule, working out the gas usage for that schedule is relatively straightforward. The disadvantage of having flexibility in the deco schedule is that it now becomes impossible to calculate exactly how much gas will be required in advance. This is where a shift in the approach is required. If we think about the point of gas planning, it is to ensure you don't run out of gas, even in an emergency situation. Specifically, you want enough gas to get yourself and your buddy to the surface, or to the next breathable gas source even in a stressful situation. This is known as minimum gas. You can calculate your minimum gas in advance for your maximum planned depth. This is based on combining the breathing rates of you and your buddy, then doubling this figure to take into account the stress of an out of air emergency. This is then multiplied by the total time involved in dealing with an issue on the bottom combined with the time required to ascend to the first gas switch stop. You can then multiply this by a figure to account for the increased pressure at depth to give the total volume of gas required in litres. Finally, convert this into a bar pressure by dividing by the size of your cylinders. Let's say that after performing this calculation you know that your minimum gas is 70 bar. This means that at any point in the dive, as long as you have at least 70 bar, you know you have enough gas to get to the next source of breathable gas, even if your buddy has a catastrophic gas loss. Once either of you reaches 70 bar, you must start the ascent. Using minimum gas rather than fixed usage gives you the flexibility in back gas planning to match the flexibility in deco schedules provided by the dive computer.
Minimum gas calculations will cover the gas required to get to the first gas switch but what about the gas required for the deco stops? The traditional approach has been to work out exactly what is required and see how much is available to ensure that the amount required, plus a contingency, is less than the amount available. The alternative is to use a planning tool to find the maximum amount of deco that can be done on the gas available, without exceeding the safety reserve. You now know that you can do this amount of deco, and this can be converted to a total time to surface (TTS). Again, you know that this time to surface can be done within the gas available. This means that as long as the total time to surface is less than this maximum amount, you know you have enough gas available.
Putting these two concepts together, the procedure is to first calculate the longest dive that can be done at the target depth within the deco gas limits. This can be used to find the maximum TTS. You then calculate the minimum gas required to get you and your buddy up to your first gas switch. Provided the dive is around the target depth, you just need to monitor your available gas and your time to surface. The actual bottom time becomes less important. The dive is terminated when either of these limits is reached; either the available gas reaches the minimum gas limits or the total TTS reaches the maximum amount.
On the Shearwater computer range, the TTS is shown on the display so that you can instantly relate your current TTS with the maximum TTS that you have calculated. It doesn’t matter what depth you have been at or what your total dive time has been. You know that as long as your TTS is less than your predetermined maximum TTS, you have enough gas to complete your decompression.
If you dive with a regular buddy and always use the same size cylinders and the same gas mixtures, then this means that the minimum gas and time to surface will always be the same for each dive at that depth. As a result, you only need to calculate these numbers once for any given dive depth. With a PC planning tool, it is very easy to calculate these two numbers for a range of dive depths. This can be turned into a table in your wet notes that contains all the required information you need for dive planning. With modern dive computers, you don’t even need to use a PC planning tool. Your dive computer can do all of the gas calculations for you.
|Depth (m)||Min Gas||TTS|
The discussion so far has mainly been concerned with open circuit diving, but CCR diving has progressed along a similar path. Modern rebreathers almost always have a built-in decompression computer integrated into the handset and most divers have a backup computer. However, gas planning is very different on a rebreather compared to open circuit. A CCR has almost unlimited gas and, if nothing goes wrong with the CCR, it is likely to be scrubber duration or CNS limits that will determine the maximum length of the dive. The only time that gas usage becomes an issue is in the case of a bailout where gas availability becomes critical. In reality, it is the bailout scenario that will normally be the limiting factor for most CCR dives. This means that bailout planning will determine the limits for TTS. This is done by using a planning tool to calculate the maximum CCR bottom time that can be done without then exceeding the available bailout gas when the diver bails out at the end of the planned CCR bottom time. The CCR TTS at this point becomes the endpoint of this dive as we know that as long as we stay within this CCR TTS, the corresponding bailout ascent is achievable with the bailout available. For most dives, it will be gas usage, either back gas, deco gas or in the case of CCR, bailout gas, that will determine the limits of the time. Other factors such as CNS should also be considered, but when the dive plan is generated using the PC planning tool or your dive computer, the CNS can be reviewed and, provided it is well within safe limits, can be considered as a secondary consideration to the real limiting factor.
Tech training tends to follow the evolution above with new divers starting with written plans, generated from pre-printed tables or pc planning tools. This ensures that the diver understands the principles behind decompression schedules and gas planning. It also ensures that the diver can manage ascent rates and display the discipline required to follow the dive plan on the computer accurately. They then move on to using dive computers with tables on a slate as a backup before eventually planning using the TTS and minimum gas approach.
It must be remembered that overhead environment diving also introduces a number of other factors when considering dive planning. For cave and wreck penetration, the minimum gas and time to surface calculations will have to include the time required to exit the overhead environment as well as the time to ascend, and so the planning becomes more complicated. The TTS setting does not include time to exit a wreck or cave, and so cannot be applied as easily in an overhead environment setting.
Real Time Management of Risk
On the Shearwater computer range, the NDL is shown on the display and counts down the time available until it reaches 0. Once the diver goes into deco, this field can be configured to show a number of other pieces of information. Any one of these can be selected to be shown in the NDL space once the NDL reaches zero. Alternatively, all of the following options can be viewed together by stepping through the display options.
The @+5 option is particularly useful. It shows what the TTS will be in 5 minutes, assuming the diver stays at the same depth. This can be used for looking ahead. If you know your maximum TTS, then you can compare this against your current TTS to see if you have reached your limit, but the @+5 setting allows you to look ahead 5 minutes and see what your TTS will be in the future. You can use this to decide whether you have time to look at another piece of the wreck or whether you must turn around and head back to the shot-line. This is particularly important at deeper depths where the rate at which decompression is built up is much faster, and a large amount of deco can be incurred in a relatively short period of time.
The Δ +5 option shows the difference (the delta or Δ) between your TTS right now and what your TTS will be in 5 minutes. For example, if your TTS is 20 mins and your @+5 is 30 minutes then the Δ +5 would be 10 minutes (30 – 20 = 10). In other words, in 5 minutes time, you will have incurred an additional 10 minutes of deco more than you have right now. This could be done manually, but in some situations, it is nice to be able to see the delta without having to constantly make that calculation. The size and magnitude of this figure can also be used to tell the current state of your decompression. If the Δ +5 is positive, this means that you are on-gassing and will have more decompression in 5 minutes than you have right now. If Δ +5 is 0, then you are neither ongassing or offgassing and you will have the same amount of decompression in 5 minutes as you have right now. Finally, if the number is negative then you are offgassing and you will have less decompression in 5 minutes than you have right now. This is particularly useful for multi-level dives. Let’s assume you are on a deep reef and you notice that your TTS is approaching your maximum TTS. You ascend a few metres and you notice that your Δ +5 is now +1. This means that you are still incurring additional decompression, although at a much slower rate, and so your TTS will continue to increase. If you come up a few metres more, you can now see that your Δ +5 is zero. This means that you are neither ongassing or offgassing and you can stay at this depth without increasing your TTS. If you ascend slightly shallower and your Δ +5 changes to -1 then you can see that you are now offgassing and you can stay at this depth almost indefinitely as your TTS will slowly reduce.
The settings above can be used to proactively manage the dive and can be used on any dive. There are several other options that would primarily be used in an emergency to change some of the dive parameters on the fly.
The CEIL option shows the raw decompression ceiling. Once the diver goes into decompression, they can no longer ascend directly to the surface, and there is a depth at which the supersaturation would exceed the maximum allowed. The decompression ceiling is the exact depth at which this would occur. This is slightly different from the decompression stops shown on the computer as the deco stops are rounded to the nearest 3m increment below the actual decompression ceiling. The actual value of the ceiling will slowly get shallower during the decompression, but the decompression stops will stay at the 3m increment until the ceiling reaches the next 3m increment. At this point, the decompressions stop will jump up to the next 3m increment. By comparing the decompression stop and the CEIL value you can see how much margin for error you have at that stop or how close you are to the end of the decompression stop. If your computer shows a 9m stop and your CEIL is 8.9m then you can see that the ceiling is only slightly above the current decompression stop and so there is very little margin for error in your position in the water column, and you also know that you will be at 9m for some time to come. As the CEIL moves up and gets to 8m then 7m and then 6.5m you know that your decompression stop is coming to an end. This can be useful to know if, for example, you are decompressing on a line at 9m along with a number of other divers. If it is getting crowded on the line at the 9m stop, but you know your CEIL is showing 6.5m then you can move up to 8m or 7m without breaking your ceiling. Your computer will alert you that you are above your decompression stop, and if you stay at that depth, it will give you a MISSED DECO alarm but you know that despite this you are in fact below your decompression ceiling.
The next setting that it is possible to select in the NDL space is the GF99 setting. This is useful information to know as it shows the current GF, in other words, how close you are to the M-Value which corresponds to a gradient factor of 99. Whether a diver selects his own gradient factor settings or makes the decision to use the default settings, the computer will display the ceiling, decompression stops, as well as the time to surface, based on these gradient factors. If the diver is using 30/80 gradient factors, then during the ascent up to the first stop the GF99 should be approaching 30, as the first stop is calculated as being at the point where the GF is at 30% of the M-Value. At the surface, the GF99 will be 80, as the high GF determines how close the diver is to the M-Value on surfacing so a GF Hi of 80 means the diver should be at 80% of the M-Value as they surface. For intermediate decompression stops, the GF99 will slowly increase from 30 on arrival at each subsequent stop. During each deco stop, the GF99 should slowly decrease as the tissues offgass and the ceiling increases. Once the stop clears and the diver moves up to the next stop, the GF99 will again increase. This allows the diver to “see” the offgassing taking place as it shows that as they offgass, the level of supersaturation is dropping, and they are moving further away from the M-Value.
If the GF99 is much lower than 30 on the initial part of the ascent or does not slowly increase on the ascent up to each subsequent stop, then the diver is ascending slower than intended. The TTS shown assumes the diver will be ascending at the prescribed ascent rate. If the diver is ascending slower than the correct ascent rate or stops below the decompression stops, then they are, in effect, lagging behind the calculated decompression schedule. The result of this is that the diver is not offgassing as quickly as the model has assumed, and so the diver will take longer to decompress. In extreme cases, the diver may still be ongassing in some tissues, and the slow ascent may actually increase the decompression requirement. As a result, the actual ascent time may be considerably longer than the calculated TTS. If the diver is using the calculated TTS to manage their dive as described above, this can cause a problem as the gas planning assumed that they would be following the calculated decompression schedule. By causing additional decompression time, they will end up requiring additional gas for this extra time.
If the diver ascends above the deco stop, the computer will give a warning. As we have already seen, you can ascend above this deco stop, but still, stay below your decompression ceiling as shown with the CEIL display. If you ascend even further beyond the CEIL depth, the GF99 can be used to provide some additional information. For example, if the diver has set a GF Lo of 30% and ascends above their initial deco ceiling, the computer will give a warning. The GF99 may still show that they are only at 40% GF which, although it is beyond both their deco stop and deco ceiling, is still well within the M-Value. Similarly, for the later stops, if the diver has set a conservative GF Hi of 70% and ascends above their deco stop, the computer will give a warning. The GF99 may still show that they are at 80% GF which is still well within the M-Value. However, if the GF99 shows more than 100%, the diver is now well over their M-value and is in a much riskier position.
The same goal can partly be achieved in the Dive Settings menu where it is possible to change the high gradient factor during the dive. By changing the high gradient factor from, say 70 to 80, you would reduce the rest of the remaining decompression. Although it is possible to change the high gradient factor in this way, it is not possible to change the low gradient factor, and so the initial stops would be unchanged.
This functionality is not intended to be used on a regular basis, and the diver should stay within the ceilings indicated. However, in an emergency, this functionality may be very helpful. For example, assume that a diver on a decompression dive is running low on gas. Their computer tells them that they have another 5 minutes of decompression to do before they can move up to the next decompression stop where there is more gas available. They could edge up from the current stop to the next stop while watching their GF99 setting. Even though they are breaking their decompression ceiling they can use the GF99 display to show them how close they are getting to their M-Value, and can then make an educated decision on what is the more important risk.
These last few options may seem worrying, or even dangerous, but remember that the stops are determined by the gradient factor settings. If you are using a GF of 70 then you may have a deco stop which would not be present if you had selected a GF of 80. So, missing a deco stop using a 70 GF, but still staying less than 80 on the GF99 display is equivalent to staying within the deco stops on a GF setting of 80%. In fact, you may have deco stops, while the underlying Buhlmann model, which is based on a maximum gradient factor of 99, may indicate that it is within the No Decompression Limit. This is completely normal for the first few minutes of going into deco. If you have your GF settings set to anything less than the maximum value of 99%, then a GF profile will always go into deco before the underlying Buhlmann NDL limit is reached.
The same approach could be adopted with ascending all the way to the surface. In a critical emergency, the diver could edge up towards the surface watching their GF99 display and making sure that they stay close to, but not exceeding, their M-Value. However, this case can be managed more effectively using the Surfacing GF display feature. This is a newer feature, and may not be available on your computer unless you have updated the software recently. The Surfacing GF displays the GF that you would get if you were to ascend directly to the surface right now, without doing any stops.
If the SurfGF display shows 50, this means that if you were to ascend to the surface directly, your maximum tissue saturation would be 50% of the M-Value. I.e. well within your M-Value limit with almost no chance of DCS. If your SurfGF shows 150%, this means that a direct ascent to the surface would put you at 150% of your limit, and well over the M-Value limit with a very high chance of DCS. Finally, if your SurfGF shows 99, then a direct ascent to the surface would put you right on your M-Value limit and is equivalent to the NDL limit of a straight Buhlmann model. Interestingly, you can be in deco but still have a SurfGF of less than 99. Remember that the deco stops are based on your selected GFs. If you have the default GF setting of 30/70, then you will start to get deco stops well before you reach the underlying NDL limit. So, if you have 5 minutes of deco shown on your computer but your SurfGF is 90 this means that you have 5 minutes of “GF Deco”, but you have not yet reached the NDL of the underlying Buhlmann model. This means that, in an emergency, you could still go straight to the surface without breaking the Buhlmann decompression schedule. This is very different from the situation where you have 15 minutes of deco and your SurfGF display shows 120. In this case, you have “GF Deco” as well as “Buhlmann Deco”. If you were to go straight to the surface, you would not only miss the deco stops indicated on the computer, but would also end up being over your M-Value on the surface and have a significant risk of DCI.
The SurfGF feature can be used at any point of the dive, not just at the start of the ascent. For example, you can track your SurfGF during your ascent and deco. Once your SurfGF drops below 99, you know that from that point onwards if there is an emergency, you could go to the surface and still be within the Buhlmann limits. Equally, you can use it the other way around. After your deco stops have cleared, you can monitor the SurfGF to see your updated SurfGF. One technique that can be used is to have a slightly more aggressive high GF such as 80 or 90 to reduce the mandatory decompression stops but then wait until the SurfGF has dropped to a lower level as a “safety stop”.
As the tools available to divers continue to change and improve, it is inevitable that the techniques used must also change to make the most of the available tools. This article is intended to show that, far from removing the need to plan a dive, the sophisticated dive computers available today can help to improve the planning process. They can be used to provide a more realistic and more flexible planning tool. They can also be used to adapt the plan when the situation changes. This is only possible if the diver understands the tools they have at their disposal and practices using them. After reading and digesting the information contained in this article, I would encourage you to make sure you know where to find the various display options on your computer. On your next dive look at the SurfGF value during the dive and watch the relationship between it and the NDL value. During the NDL ascent, look at the GF99 and SurfGF values. Then on a decompression dive, compare the CEIL and Stop Depth values as well as comparing the CEIL, GF99, and SurfGF values. It is essential that you understand all of the information in this article and practice it before using it to plan your dive or modify your dive plan. Like any tool, you must practice before using them for real. However, a bit of investment in time and practice will give you the ability to manage your ascent in a much more intelligent way than blindly following your computer or a fixed set of deco tables.
About the author
Mark Powell has been diving since 1987, and became an Instructor in 1994. In 2002, he set up Dive-Tech, a dedicated technical diving facility. He has been a full-time Diving Instructor since then. He is a TDI Instructor Trainer, and a member of TDI’s Global Training Advisor Panel. He is a TDI/SDI International Business Manager, and supports Regional Managers and divers across the world. Mark represents TDI on the British Diving Safety Group and the HSE Recreational Diving Industry Committee. He is also a member of the Diver Training and Breathing Apparatus committees at the British Standards Institute. Mark is an author, and his books “Deco for Divers” and “Technical Diving – An Introduction” are both highly recommended readings by a number of the technical diving agencies.
Photo credit: Trish Stovel and Mark Powell