How to Judge Your Ascent Rate Without a Dive Computer
by Andy Davis
Novice divers often express frustration about how to best conduct safe and controlled ascents from their scuba dives.
In particular, I see many new divers questioning the best technique for judging their ascent speed. The common use of dive computers has automated this skill – providing divers with a graphical illustration of their ascent speed – and warnings if they exceed a safe ascent rate.
However, many novice divers don’t have a dive computer… and dive computers occasionally do fail during dives. Therefore, every diver should be familiar with analogue methods for judging their ascent rate. Proper ascent control is one of the most critical scuba diving skills – as this plays an important role in preventing the occurrence of decompression illness.
Without a dive computer (all have ascent rate monitors/displays), it can be more tricky to monitor the speed of your ascent. During training, the instructor will have demonstrated an appropriate rate as part of scuba diving skills development, but I do understand that it can be hard to replicate this initially.
It will become an instinctive/unconscious skill as you practice it more and more.
The old scuba adage of “never ascend faster than your smallest bubbles” is practicable in emergencies, but for general diving, it is imprecise and far from foolproof. It should not be viewed as a routine technique – especially for deep and/or repetitive diving.
You should always bear in mind that the recommended ascent rate is the maximum speed. Ascending at a rate below this can be viewed as a prudent and conservative diving practice.
PADI recommend 18m/60ft per minute. Other training agencies recommend a much slower rate, typically around 8-10m/25-30ft per minute (in line with the ascent speeds used by modern dive computers).
The techniques taught to you on your Open Water course should go a long way towards ensuring that you can ascend safely – and also enable you to track and assess your ascents.
You should have a depth gauge and timing device for your dives.
When you decide to end your dive and make a direct ascent to the surface, it is helpful if you check the time and depth. This ensures that you know your dive’s bottom time (for dive logging and repetitive dive calculation) and also allows you to gauge your ascent through a simple speed-distance-time calculation. Simply divide the depth by the time (i.e. 40ft divided by 40 seconds = 1ft per second = 60ft per minute)
As you ascend from a dive, you can monitor your depth and time to ensure that your ascent speed is not excessive. To ensure a uniform and constant rate, break the ascent into smaller ‘levels’ and apply the speed-distance-time calculation to that.
The time taken to ascent each level allows you to determine your rate as you go. A 30ft per minute rate is ideal (half of the maximum speed recommended by PADI). For ease of monitoring, this means you are ascending 1ft every 2 seconds / 10ft every 20 seconds.
For instance, for a 60ft/18m ascent, you can sub-divide the ascent into six 10ft levels, each of which should take you 20 seconds. Your entire ascent to the surface would take 2 minutes.
To add a safety stop, simply pause at 20ft depth and hover for 3 minutes – meaning your total ascent time is 5 minutes. After the stop, recommence your ascent – allowing 40 seconds for the final two 10ft levels.
If you know how long your ascent should take, even a casual glance at your dive watch will inform you of whether you are on track or not.
Direct ascents to the surface, especially without a visual/tactile reference (shot line/reef wall/slope) can be very taxing for novice divers. Buoyancy control is essential to maintain a slow ascent speed – which, of course, is reliant upon proper weighting.
If over-weighted, the diver will suffer excessive and regular positive buoyancy from the expansion of a greater volume of air in their BCD. Proper weighting ensures that a minimal amount air is needed in the BCD. The smaller the amount of air, the smaller the proportional expansion of air…and the less likely you are to lose control.
For more details about how buoyancy and weighting impact on ascents, have a look at my article;
Scuba Buoyancy Masterclass 7of9 – Ascent, Descent and at the Bottom – Scuba Tech Philippines
Where possible, utilise a line for ascents. When boat diving, a shot (buoyed) or anchor line is often present. Ascend on a line hand-over-hand – that will give you a slow, regular ascent rate. If that isn’t available, then you could consider getting a Delayed Surface Marker Buoy (DSMB) and a spool.
Whilst this takes some practice to deploy safely, it is invaluable in providing a ready reference from which to gauge an ascent. If you are winding in a reel/spool, it is hard to ascend too quickly (I’ve found that winding in a finger spool tends to keep my ascent rate well below 12ft per minute).
Hope that helps!
For more information about scuba diving buoyancy control, see my article series on Buoyancy Control.
About the Author
Andy Davis is a RAID, PADI TecRec, ANDI, BSAC and SSI qualified independent technical diving instructor who specializes in teaching advanced sidemount, trimix and wreck exploration diving courses across South East Asia. Currently residing in ‘wreck diving heaven’ at Subic Bay, Philippines, he has amassed more than 9000 open circuit and CCR dives over 27 years of diving across the globe.
Andy has published many magazine articles on technical diving, has written course materials for dive training agency syllabus, tests and reviews diving gear for major manufacturers and consults with the Philippines Underwater Archaeology Society.
He is currently writing a series of books to be published on advanced diving topics. Prior to becoming a professional technical diving educator in 2006, Andy was a commissioned officer in the Royal Air Force and has served in Iraq, Afghanistan, Belize and Cyprus.
Originally posted 2018-11-05 06:10:26.