Thursday, September 29, 2005

Ascent Rates

Recently I wrote about a fellow instructor who is considering becoming a commercial diver. He mentioned that the ascent rate that they had to keep to during commercial diver training was 30 feet per minute or less. Some training agencies teach recreational divers to ascend at a rate no faster than 60 feet a minute.

With commercial diving, making a slow ascent after every dive is critical, since diving is one'’s source of income and you would not want to run the risk of decompression illness. The advantage that commercial divers have over recreational divers is that they are usually supplied with air from the surface. Slow ascents and elongated safety stop are therefore not influenced by remaining air supply as with recreational divers.

All this talk of various ascent rates might seem a little esoteric and theoretical. However, I would contend that safety should always be in the mind of the recreational diver. Even though we are not required to do safety stops for certain dives, a responsible diver will make a safety stop as a matter of habit. The same could be said for a slower ascent rate.

The real issue with ascent rates is the amount of nitrogen dissolved in the diver'’s blood. A slow ascent rate will give the diver'’s body the time necessary to "off gas"”. As the ambient water pressure decreases, so the excess dissolved nitrogen will be breathed out by the diver. The advantage of a slow ascent rate (and a safety stop) is that you are giving your body enough time to get rid of the excess nitrogen safely.

Divers have come up with all kinds of techniques to ensure a proper ascent rate. For example one technique that I read about is breaking the dive up into 10 foot segments. If your ascent rate is 30 feet per minute, then you should take at least 20 seconds to ascend 10 feet. If you exceed your set rate, stop at the 10 foot mark and wait until the time "catches up" to you. Another technique is obvious - counting slowly in your mind as you watch the depth gauge. Each foot increment should take 2 seconds (one thousand and one, one thousand and two). With practice one should be able to keep to a fairly consistent and safe ascent rate.

However, most divers, myself included, use their dive computers as a rate guide. My dive computer starts beeping loudly if I ascend too fast and I find this a useful feature when working with students, since I cannot watch the student and my computer at the same time. Additionally most computers will also give you a visual warning if the ascent rate is fast.

Ascents do not have to be boring. Since I am one of those people who really enjoy being in the water, I love every part of the dive. The ascents give me the time to practice buoyancy skills by releasing just the right amount of air from the BCD to slow down the ascent rate. The ascent also gives me time to do a little more sightseeing and take stock of what I saw on the dive.

A slow ascent is vital to safe diving and should not be rushed (unless you are running out of air or have some kind of medical emergency). Learn to enjoy every part of the dive and you will never find slow ascents and safety stops boring.


Willy Volk said...

That's interesting. I thought that 30 feet per minute was considered the standard ascent rate for rec divers, too (

BWRAF said...

Thanks for the heads-up Willy. I did a little research after your post and noticed that most other agencies recommend 30 feet per minute. At this time PADI is still at 60 feet per minute (max).