Descent and Ascent Rates
By Dr. Sawatzky
I was recently diving off Vancouver Island with some very experienced diving friends (saw six gilled sharks on three of the four dives!) and the topic of descent and ascent rates came up. We had several long involved discussions and they suggested I write a column on the topic. It seems most divers do not have a clear understanding of this very important concept. Descent rate is simply the rate at which we descend from the surface to the bottom or maximum depth of the dive. You can either take your time or you can drop like a rock so that you have more time on the bottom. What difference does it make? Actually, a lot.
Remember that the two most important factors in determining your uptake of inert gas and therefore your risk of DCS are depth and time (blood flow through the lungs is also very important but will not be a factor in this discussion). Most decompression tables have a maximum rate of descent (a few assume instantaneous descent to maximum depth) and the table is calculated for a diver descending at that maximum rate. What does this mean? Let's look at an example.
A diver is going to do a dive to 120 fsw for 10 minutes, a dive that is right on the no-decompression limit for the DCIEM tables. These tables have a descent rate of 60 ft per minute and therefore they are calculated for a diver who takes two minutes to descend and spends the rest of the bottom time at 120 fsw. Therefore the diver will have spent two minutes at an average depth of 60 fsw and eight minutes at 120 fsw. If a diver is in a hurry and only takes one minute to descend from the surface to 120 ft, they will have spent one minute at an average depth of 60 ft and 9 minutes at 120 fsw. That extra minute on the bottom will result in the diver absorbing more inert gas than the table allows for. Therefore their risk of developing DCS will be elevated because their rapid descent resulted in them spending more time at depth.
What about the diver who descends very slowly? Let's look at someone who takes six minutes to descend to 120 fsw. They will spend six minutes at an average depth of 60 fsw and only four minutes at 120 fsw. The table assumes that they spend only two minutes at an average depth of 60 fsw and eight minutes at 120 fsw. The extra four minutes the diver spent at an average depth of 60 fsw rather than 120 fsw means that they will have absorbed significantly less inert gas than the table expects and therefore, their risk of DCS will be substantially reduced. Therefore, descending faster than the table is designed for increases your risk of DCS while descending slower will reduce your risk.
If that explanation left you confused, lets try again, visually. Figure one shows the expected dive profile, two minutes to descend to 120 fsw, eight minutes on the bottom. The amount of inert gas you will absorb is represented by the area above the line. If you descend faster, the area above the line is increased and you will absorb more inert gas during the dive and increase your risk of DCS. If you descend slower, the area above the line is reduced, the amount of inert gas you absorb is reduced and your risk of DCS is lower. If you are diving a computer, the computer will take into consideration the descent rate and make your no-D time longer or shorter as appropriate. Of course, this is a simplified discussion. The diver never makes a perfectly linear descent and the uptake of inert gas is also not linear.
How about ascent rates? This discussion is a bit more complex. All tables are calculated on a specified ascent rate. The DCIEM tables use 60 ft per minute. Therefore, on the 120/10 dive discussed previously, the diver is expected to make their way smoothly from the bottom to the surface in two minutes (plus the safety stop). What happens if they ascend off the bottom slower than 60 ft per minute? They will be spending more time deep, absorbing more inert gas/off gassing less inert gas than the table allows for and they will have an increased risk of DCS. This is the reason most divers leave the bottom a few minutes before their planned bottom time is up. It allows them to be a bit slower on the ascent and as long as they reach 30 fsw on time or early, their risk of DCS will not be increased.
How about the diver who ascends at 60 ft per minute until they reach 20 fsw and then spends three minutes looking at the fish (doing a safety stop). While they are at 20 fsw, they will be off gassing inert gas (from the fast tissues, the only tissues that absorbed much inert gas for this dive profile) and therefore they will finish the dive with less inert gas and have a lower risk of DCS than if they ascended directly to the surface. This is the rationale for doing safety stops on deep no-D and longer no-D dives. In fact, there are no decompression limits as long as you are shallower than 30 fsw (at sea level) and any time you spend shallower than 30 fsw at the end of a dive is going to reduce your risk of DCS. The previous statement is true for practical purposes in recreational diving. In saturation and extremely long dives, the true no decompression limit is somewhere between 21 and 26 fsw. Therefore, it would be better to spend your "safety stop" time between 10 and 20 fsw. How about ascending off the bottom faster than the table expects? When you are deep, this is generally a good idea as it reduces the amount of inert gas you absorb by reducing your time at depth. The problem arises when you get closer to the surface. The ratio between the partial pressure of inert gas in the tissue and the ambient (surrounding) pressure determines whether you form bubbles or not. While you are ascending, the fast tissues are off gassing and the partial pressure of inert gas in them is falling. If you ascend faster than the tables are designed for (60 ft per min for DCIEM), you might end up with an inert gas to ambient pressure ratio higher than is safe, develop bubbles, and thereby increase your risk of DCS.
Over the past couple of decades, decompression research has suggested that the depths of first stops on many tables are too shallow. This suggests that divers are actually starting the processes that can result in DCS before arrival at their first stop. Therefore, many newer tables and computers use slower ascent rates (30 ft per minute) to address this problem. This is an excellent idea and further supports the practice of leaving the bottom a few minutes early so that you can make a more leisurely ascent and still arrive at your deco or safety stop on time.
What do you do if you ascend too slowly and arrive at 30 fsw later than your dive plan? In general, you will have spent more time absorbing inert gas and therefore this extra inert gas must be dealt with. You should add the time of the delay to the bottom time of the dive and then determine if you have to do a decompression stop (for very deep dives with long delays at a relatively shallow depth, there are more complex, multilevel dive rules). For example, you plan a 120 fsw dive for 10 minutes. Your descent goes as planned and you leave the bottom at minute eight to give yourself a couple of extra minutes to get up to 30 fsw. On your ascent you have some equipment problems and you stop at 90 fsw for five minutes to sort them out and therefore arrive at 30 fsw at minute 14.5. Your original plan called for you to arrive at 30 fsw at minute 11.5 (10 minutes bottom time plus 90 seconds for the ascent to 30 fsw at 60 ft per minute). Therefore you are three minutes late getting to 30 fsw. These three minutes should be added to your planned bottom time of 10 minutes so that the dive is now 120 fsw for 13 minutes. On the DCIEM tables, this dive (120/15) requires a 10 minute stop at 10 fsw. If you simply continued to the surface without doing this decompression stop, you would have an increased risk of developing DCS.
If you are diving a computer, it will usually take into consideration slow ascents and warn you if your ascent rate is too fast. I routinely dive Aladin Pro computers. They use an ascent rate of 67 ft per minute deep and slow down to 23 ft per minute shallow. They continuously give a percentage reading comparing your ascent rate to the recommended ascent rate. If you ascend significantly faster than the computer is designed for, it will penalize you with more decompression time, a longer surface interval, or both for up to 72 hours after the dive in question. These computers are very intolerant of poor divers!
I hope the preceding discussion has helped you understand the importance of accurate, controlled descent and ascent rates. You must not descend faster than the tables you are using have been designed for. Slower descents are OK and will reduce your risk of developing DCS. When ascending, it is an excellent idea to leave the bottom a few minutes early so that if you have a delay on your ascent, you will still arrive at 30 fsw on time. Do not ascend faster than your tables have been designed for. Any delays on ascent to 30 fsw beyond the allowed time must be added to your bottom time or dealt with in some other acceptable way. Finally, any time spent shallower than 30 fsw at the end of the dive (doing safety stops) will reduce your risk of DCS. Therefore, it is an excellent idea to spend several minutes ascending from 30 fsw to the surface on all dives.