buoyancy issue

[Fun Diver]

"I distinctly remembered to always deflate my BCD on the way down till hovering but someways toward the middle and end of my dives, i kept floating away and when I deflated I was surprised to find that there was so much air inside."

if there was still air in the BC as stated and no other equipment changes, then sounds like perhaps air was trapped in the BC or the hose was not able to be fully extended. So also to be sure to roll slightly to the right side to help force any air to the left side where you can dump it (assuming of course the hose is on the left) and that the hose is fully extended. Also if air is trapped in the BC and you are going feet up, that's what the dump valve is for on the BC on the lower back side (on the left usually).

 

[Charlie99]

......and when I deflated I was surprised to find that there was so much air inside.

I know that I didnt accidently keep inflating anytime. Nor was my BCD hose valve leaking inside, so where did this exta air come from ?

How do you know that your inflator valve wasn't leaking and slowly filling the BCD over a period of minutes? This is a fairly common failure. A simple way to test for this problem is to set up your gear normally, turn on the tank valve, and the leave the gear sitting there for a couple hours. Then check and see if the BCD has been inflated.

Ignore the questions about aluminum tanks. Air weighs the same, whether it is inside an aluminum tank or a steel tank. You'll be lighter at the end of the dive by the weight of the air you have used. That won't put air inside the BC, though.

Small amounts of air left in the BC when you partially empty it will expand as you rise, but only by a factor of 2 or so. For example 1 liter of air at 100'/4ata becomes 2 liters at 33'/2ata and will triple in volume to 3 liters at 11'. You'll have to judge as to whether or not this could account for the excess air you discovered in your BCD.

My bets are on a slowly leaking inflator that slowly fills the BCD during the dive, with you only noticing it during the ascent.

 

[countryboy]

When you descend, you inflate your BC to become neutral. (Compensate for loss of buoyancy from exposure suit compressing, etc.)

When you ascend, you deflate your BC to prevent excessive lift (air expands, exposure suit un-compresses) and remain neutral throughout your dive.

Your description is backwards...

<quote>
I distinctly remembered to always deflate my BCD on the way down till hovering but someways toward the middle and end of my dives, i kept floating away and when I deflated I was surprised to find that there was so much air inside.
</quote>

The air in the bladder will expand as you ascend... so yes, it will seem like there is "a lot of air in there.." Perfectly normal..

 

[countryboy]

If the inflator was leaking it would be noticeable throughout the dive, not just at the end when you ascend..

 

[Charlie99]

If the inflator was leaking it would be noticeable throughout the dive, not just at the end when you ascend..

Not always true for a slow leak.

I've had a couple of buddies with this sort of problem. In one case, he just dumped air a couple times during the dive without really thinking about it during the dive. Only during the SI did he get to thinking it was a bit strange. During the second dive, only because he specifically paid attention to it did he realize that he had a slow leak through the power inflator.

In another case, the buddy didn't notice anything except that he had to dump more air during the ascent than is normal. He didn't really realize that he had a leak until just before the 2nd dive, when he noticed that his BCD had partially inflated during the SI.