Without a dive computer (all have ascent rate monitors/displays), it can be more tricky to monitor the speed of your ascent. Your instructor will have shown you an appropriate rate, 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.
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.
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 insures 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!
