BM vs SM weighting discrepancies

[guruboy]

I often see people post that they drop an irrational amount of lead when going from BM to SM. Is it just a mental thing where they were overweighted in BM? Am I missing something else?

 

[guruboy]

Here is an example. I am not using this person’s posted configuration for any particular reason. It is simply the first example I found to copy.

Single tank BM:
HP80
BP/W
20 lbs lead total (includes SS BP, STA, etc)

SM:
2 x HP80s
Hollis SMS75
NO lead

 

[guruboy]

In the example above, the differences in gear are-

BP/W vs SMS75
1 HP80 vs 2 HP80
1st + 2 2nd + SPG vs 2 1st + 2 2nd + 2 SPG
20# vs 0#


HP80: 1.75# empty
500psi: 1#
Valve : 1.75#
1st: 2#
SPG: 1#
Bolt snap: 1#
——————-
Total: 8.5#

Unless the SMS75 is almost 12# negative, what am I missing?

 

[RainPilot]

That example does seem extreme. I have noticed that few SM divers seem to remember to do their weighting with 2x empty tanks. I suspect they are doing a normal dive then at the end doing a check at the SS. In that case they will have, basically, a full second tank as ballast.

I also suspect that the original 28lbs will be quite overweighted and the 0lbs probably underweighted so....

That being said, in BM with an AL80 and a steel BP, I use 4lbs of lead. In SM, with 2 empty AL 80 (additional 3lbs positive) and no BP (another 3lbs positive) I only need 2 lbs.

SM seems in my case to require 8lbs less ballast all round and I honestly have no idea why.

 

[vovanx]

I think its all these metal carabiners and bolt snaps add that extra weight. When I dove BM I used 3kg of lead to compensate a near empty (~70bar) AL tank and with SM I use only 1kg to compensate two near empty AL tanks.

 

[tbone1004]

in most circumstances if coming from a SS BP/W you have to add a bit of lead when you switch to sidemount. Anyone who says they dropped weight was just overweighted assuming the tanks have similar buoyancy characteristics. I.e. not going from AL80 in backmount to HP steels in sidemount


@vovanx the bolt snaps and other attachment points are typically more than offset by SS cam bands on a backplate and certainly will not outweigh the backplate.

 

[Marie13]

Here is an example. I am not using this person’s posted configuration for any particular reason. It is simply the first example I found to copy.

Single tank BM:
HP80
BP/W
28 lbs lead total (includes SS BP, STA, etc)

SM:
2 x HP80s
Hollis SMS75
NO lead

Gee, thanks.

You neglected to mention the above weighting is diving dry.

Diving wet, I need 8lbs additional (on top of SS BP and weighted STA) with 5mm. SM, no additional weight with 5mm. Seriously overweighted with 3mm in SM. Didn’t try 7mm in SM.

Lynne (dearly departed TS&M) once posted she needed nearly 30lbs. I’m assuming this was with doubles. But maybe not. Some guy online said there was no way she needed that much weight until he actually saw her diving and that she actually did need that much lead.

You seriously think I wouldn’t drop all that weight if I could? My knees would love it.

ETA: I might add a couple of lbs next time I dive SM dry as I’ll probably have heavier undies. I only had a thin base layer the two times I’ve dived SM dry. I’m still shaking everything out.

 

[Beau640]

Reviewing some of the posts you’ve made recently, you should consider paying for a day of coaching from a tech instructor. They will help you sort out a lot of the trim and weighting issues so much faster and save a lot of frustration.

Can you do it all on your own? Yes and I’m sure you’re doing fine. But having an extra pair of eyes will fast track you and keep you from wanting to rip your hair out when you feel like you’ve tried everything and things just aren’t quite right

 

[Marie13]

Had just that Saturday afternoon, but there was also another diver in the session. Scheduling can be an issue. I’m also going to be working with a more experienced friend or two once the Great Lakes season is over at a regional quarry.

 

[sigxbill]

Single tank BM:
HP80
BP/W
28 lbs lead total (includes SS BP, STA, etc)

This person is most likely considerably overweighted! And the SM example after it is likely underweighted. My experience has been similar to @tbone1004 - it's usually the other way around - SM usually adds weight ...

I would disregard the scenarios you presented ...