Why - sms75 and sms100 rated for the same size (liters) tanks?

[Reku]

I was reading the manuals for these two systems and it seems that even though the sms100 has 52lbs of lift and the sms75 has 40lbs of lift they are both rated for 1x 20L cylinder or 2x 15L cylinders or smaller. Why is this?

Maybe since the sms75 has to lift less of itself - because it is smaller so it evens out vs the extra 12lbs of the sms 100. the sms75 weighs ~7lbs and the sms100 weighs ~10lbs. Maybe I'm just being picky lol I was just wondering.

SMS 100 - Page 4

https://www.hollis.com/media/wysiwyg/HO/manuals/SMS100HarnessGuide_12-4056.pdf

SMS 75 - Page 4

https://www.hollis.com/media/wysiwyg/HO/manuals/SMS75-12-4163-r01_5-1-14.pdf

 

[XRedX]

In reality, every test I've seen on the SMS75 has shown more than 40 pounds of lift, one test showed 42 pounds for the small, 46 pounds for the Lg/XL, and 48 pounds I believe for the XXL. I haven't scientifically tested mine, but I would assume more than 40 pounds lift (Lg/XL). These BC's seem to be consistently and significantly under-rated, I have seen SMS100's rigged with 4-6 cylinders for a trimix class, and SMS75 rigged for 3-4 tanks, neither setup seemed to have any problems so long as weighting was correct. I am no "professional" but the exact amount of lift doesn't seem to be the biggest factor, the relative lift to weight ratio of the diver compared to the amount of lift available seems more appropriate in most cases. Consider that some of the largest available backplate/doubles wings are only around 60 pounds lift but technical divers regularly dive twin backmount + 2-4 stage tanks on them without problems.

In my opinion, that warning is merely Hollis' way of stating that if you use more than the stated amount of tank, "you're on your own"... Anyway, if you were running more tank than that and encountered an overweight issue, you could strategically ditch some weight or even a cylinder or two (last ditch effort only, of course) to regain buoyancy control.

Also, the "weight" of the BC listed does not necessarily correspond to the "buoyancy" of the BC. For example, I have a nearly 10 pound Oceanic Excursion 2 (backmount) BC which although it weighs about 9 pounds dry and on land, considering the neoprene, fabric, and hollow plastic parts in the BC, still takes about 2 pounds of lead to make neutral (with zero air in the bladder), so as you can see a 9 pound BC does not need 9 pounds worth of lift to make it neutral or even positive. Relative lift.

 

[DA Aquamaster]

There's a lot more to involved than just wing size, you have to look at the over all design, where that lift is located and how the rest of the system effects it.

For example, the Nomad XT has 50 pounds of lift in it's standard form, but it needs a 60 pound wing to achieve it, given how the wing is actually constrained when loaded. And, that lift isn't ideally placed as it carries too much over the upper back, so you'll often need 4-6 pounds of trim weights if you're diving it wet. That turns that 50 pounds of lift into 44 or 46 pounds of lift, despite having a 60 pound wing. The XT will do a lot of things, including back mount doubles, but it's not horribly efficient at producing usable lift in sidemount.

At the opposite extreme, the Manta side mount system has a 50 pound wing on a very clean razor style harness and it is very efficient at producing usable lift - but with minimal constraints on the wing, you'll start to get some turtle shelling as the wing gets full. Some people are deeply offended by turtle shelling, but IMHO, the shelling on the Manta results in less drag in general and less bulk in particular compared to all the methods DR uses to constrain the wing on the Nomad, and in restrictions the Manta is about an order of magnitude cleaner than the Nomad XT.

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With the SMS 100 you see some of the same issues you see on the Nomad XT. The wing is constrained with a bungee system and there is a lot more lift over upper back that is largely useless. In contrast the SMS 75 carries the lift lower, there are no bungees needed to control location of the lift, and the loop bungee for the tank isn't costing any significant necessary lift.

So it's more than just Hollis underrating the lift (probably to appeal to a market that would see anything more than 40 pounds as excessive), it the SMS being much more efficiently designed and having as much usable lift as the larger SMS 100.

 

[Reku]

Huh...well that sure is interesting. Thanks for the replies!

 

[tbone1004]

15l's are LP95's, FX117's, E8-119's etc etc. Total lift required for these bottles is 19lbs for the gas/bottle/valve, and another 5-6lbs for regulator, hardware etc. Total lift for a pair of them is 25lbs. Add a manifold, bands, etc, so call it 35lbs if you're diving doubles as a worst case, and you're good to go. If you dive thick wetsuits, you need to do the math on what you actually need for lift requirements, but only you can do that. Don't take mfg recommendations for bottle size, you HAVE to do the math on what you need for lift requirement on your specific rig. I've seen the SMS75 handle a pair of 19l bottles *LP121's*, and 4 al80 stage bottles, but it was in a drysuit with a good diver. The bottle recommendations are a CYOA policy for the manufacturers, nothing more