SMB as a backup depth gauge?

I carry a small open bottom SMB and finger spool in my pocket on every OW dive. Recently, I was diving with a bottom timer and I noticed it was screwy... I was on a 60' reef and it said I was 16' down.

It was a group dive so I hadn't bothered with a backup BT (a mistake I will not repeat). In any event, when it came time to ascend I sent the bag up as usual and estimated my 30', 20', and 10' stops by looking at the length of line out and comparing it to my memories of doing the exact same stops in the past. I was probably off a bit, but I reasoned I was being crazy conservative compared to doing a single 15' safety stop or no stop at all, so no biggie...



Any ways, I'm now wondering if it makes sense to mark the line in 10' increments for future reference. While this can't tell me how deep I am on a dive, it can serve as a backup for doing my stops when I'm done.

Any thoughts on this as a backup technique?

if it is reasonable, what kind of waterproof marker or other substance can I use to mark the line? Would knots tangle the line on the spool?

Thanks in advance...

A sharpie will mark the line and I don't think a knot would cause a problem.

It's a very reasonable thing to do. Most of us knot the line. There are a variety of approaches for doing this, because it's kind of a PITA. Almost all of my spools are knotted -- it doesn't cause any tangling issues.

Nice pic, BTW!

Current can have a big effect on the amount of line that is let out and should probably remain at it's intended design, a surface marker. While I wouldn't condone continuing a dive without a functional BT, I'll assume there are certain ways to continue the dive safely. If you have prior knowledge of the depth of the reef and can calc the NDL's. If you have a good team who has descended along with you and has not changed elevation within +/- a few feet of you. If you can trust a teammate to call out a standard minimum deco....ect...ect....

Why not? Would have helped this time.

You could code the marks, 4 close marks for 40 feet, 3 for 30, so on.

ScubaInChicago:

Current can have a big effect on the amount of line that is let out and should probably remain at it's intended design, a surface marker. While I wouldn't condone continuing a dive without a functional BT, I'll assume there are certain ways to continue the dive safely. If you have prior knowledge of the depth of the reef and can calc the NDL's. If you have a good team who has descended along with you and has not changed elevation within +/- a few feet of you. If you can trust a teammate to call out a standard minimum deco....ect...ect....

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Let's assume I notice my BT has failed. I estimate my average depth and time based on knowledge of the terrain, gas consumption, and the last known reading I trust. So I thumb the dive and want to do my stops on the way up.

That's kind of what I had in mind, and only in a situation where I am not carrying a backup bottom timer. As you point out, current has a big effect on this, I might let out 40' of line but if there's a 45 degree angle in the line, I'm much shallower than the line would indicate, and that's not good.

TSandM:

Nice pic, BTW!

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Marc Blackwood took the pic of me. He caught the exact moment the rising SMB hit the surface. On this dive I was up to my usual air hog ways and when combined with a rather conservative approach to gas planning, I bailed early. Visibility was amazing and the boat would pick me up, so the team abandoned me as shark food, continuing drifting along a shallow reef while I floated above them doing my stops and looking at turtles and big fish cruising the neighborhood. Marc hung back to grab the shot and also to keep an eye on me in case I tried to demonstrate the many ways you can become entangled in your "safety" equipment.

I like knots and marks on a line. Knots you can feel, also I've used my spool to measure with so marking the line seems a no-brainer.

The currrent will have absolutely zero effect on the amount of line the smb will use or the angle of it (as long as the current is uniform from the surface to your depth). The wind can have some effect however. For trying to measure deco depth, if you put a lot of air in the SMB and get pretty negative and hand down on it "hard", the float line should be prety close to vertical

If you have cross currents, the line can bow out in multiple directions, which will certainly skew your markings. I have personally shot a bag from 70ft with a (full) 100ft spool and ran out of line. As you get closer to the surface, it will be less of an issue. As DumpterDiver noted, the wind can play a roll, as well.

A depth gauge is better at being a depth gauge than a SMB ever will be. If your depth gauge ends the dive before you do, get with your buddy and use his for the ascent.

Being proudly DIW, I have always used a computer. Eventually I added a bottom timer as a backup. However, I did muse about knotting my SMB line every 10 feet but never did do it. It is reassuring that better divers have had the same idea.

I have since transitioned to two computers rather than one computer and one timer. I know that this is a bit off-topic, but hey - if new SB members can post questions about BCs and have to read at length about how BP/W is superior, I thought that I could offer the opinion that two computers are better than none.

Cheers!