The Good, The Bad, The Ugly of Side Mount

[Dive-aholic]

FWIW, I just spent 3 days this week diving backmount. It was no big deal. One thing I do notice when going from backmount to sidemount is that, for me anyway, it is so much easier to stay neutral and horizontal in sidemount than in backmount. In sidemount it's effortless. In backmount I have to work at it a little. While it's true that a majority of my diving is in sidemount I still dive fairly regularly in backmount as well, so it's not a matter of inexperience in backmount. It's completely related to the configuration. Both configurations have pros and cons. Which one you decide to go with has to be based on your own personal choices. I have bad knees and ankles so walking around with 100 lbs of steel on my back isn't good for me from the orthopedic standpoint. I also do a lot of dives in which sidemount is the only configuration I can use. There are some people who say you should only dive sidemount when you are using the configuration for low passages. I disagree wholeheartedly. You need to be extremely familiar with your gear in those types of passages, especially since it is highly likely you will be exiting in zero visibility. You need to build up that familiarity and muscle memory somehow. The time to do that is not in low passages, it's in larger passages, or better yet, in open water. I know exactly where everything on my rig is located. This is something I need especially in low passages where I might even be stuck. Whatever configuration you choose, dive it often so you build up that familiarity that can be a lifesaver.

 

[dwhthediver]

Warning: Major Drawback Spoiler Alert! Go no farther if you're a die hard sidemounter.

A reg failure (diaphragm failure, hose rupture, o ring failure, etc) results in losing half your remaining gas. This is bad. It can be very bad. Reg failures are much more likely to happen than manifold failures, and with manifolded doubles, you still have access to all your remaining gas. Even a modest delay ascent or exit can result in you either a)drowning or b) ending up on a long hose (if your buddy has one). Bad.

You'll hear people talk about 'feathering the valve' (which only works for certain failure modes, and regs real interesting when you need two hands) and swapping regs (which could easily end in tears).

There's also a question of which hose to donate. Typically, with BM doubles, you just give up the one in your mouth. Certain SM configs do no then themselves to this. Also, I see lots of SM guys putting the longhose on the right tank. This is the tank that rolls OFF with contact in the bottom (valve knobs out). There are multiple accounts of SM rolloffs on CaveDiver.net. This could be bad, as well, especially if you're taking SM into smaller areas.

There IS increased complexity to the dive with gas switching, which takes up some mental bandwidth. If you don't need to add this complexity to your dive (small spaces), why add it? Dress up/ cool kid factor?

And run kicking and screaming from the UDT Z-System. SM can be done reasonably, and the Z-system is horrifying goobery.

Use left & right handed valves solve the roll off problems, I don't cave dive, so not going to comment on what caverns run into. I do dive wrecks, and appreciate side mount for tight places.

 

[buiszie]

I heard razor system are the best sidemounting systems, are there alternatives? and how much?

 

[JamesK]

I heard razor system are the best sidemounting systems, are there alternatives? and how much?

Well, to say it's the best is simply someone's opinion. It all depends on what is best for them. I have never heard anyone try it and say they did not like it. I dive the UTD setup personally. It is a harness setup with a small wing. They offer three different wings. Then you have Hollis with the SMS100 and SMS50. Then Dive Rite, Manta, etc etc etc. You have many options when it comes to sidemount, it all depends on what you want out of sidemount.

 

[mdax]

One problem I've had is the need to purchase two sets of regs. I don't like moving hoses around when we switch configurations and it get's expensive. I also have two different lights, LM hand mounted LED for SM and canister for BM.

A definite con is diving mayan blue, the mosquitos eat you alive and making two trips seriously sucked. We were pretty much back in tulum before taking off our hoods on the way back as the critters had followed us into the car. Oh and Temple of Doom is better in backmount as well, it's a long ladder climb.
Diving Ginnie in the summer/lord of the flies season is a pain because you have to walk your tanks to the deck and not leave them in the water or they'll get stolen. Back mount is better when you are surrounded by drunk thieves as you only need one trip.

However SM rocks for wriggling into small spaces, and for me, solo diving. However everything else is done in BM. Like someone else posted, it's a tool used for specific diving. SM vs. BM threads are silly as each have advantages/disadvantages and should be leveraged in situations where the advantages outweigh the disadvantages....however like I initially posted, it's a financial pain to have two sets of gear.

 

[mainedvr]

One problem I've had is the need to purchase two sets of regs. I don't like moving hoses around when we switch configurations and it get's expensive. I also have two different lights, LM hand mounted LED for SM and canister for BM.

A definite con is diving mayan blue, the mosquitos eat you alive and making two trips seriously sucked. We were pretty much back in tulum before taking off our hoods on the way back as the critters had followed us into the car. Oh and Temple of Doom is better in backmount as well, it's a long ladder climb.
Diving Ginnie in the summer/lord of the flies season is a pain because you have to walk your tanks to the deck and not leave them in the water or they'll get stolen. Back mount is better when you are surrounded by drunk thieves as you only need one trip.

However SM rocks for wriggling into small spaces, and for me, solo diving. However everything else is done in BM. Like someone else posted, it's a tool used for specific diving. SM vs. BM threads are silly as each have advantages/disadvantages and should be leveraged in situations where the advantages outweigh the disadvantages....however like I initially posted, it's a financial pain to have two sets of gear.

I was not clear in my OP as I did not mean to find out the pro's and cons to convince me which way to go. Just wanted to find out what they were as I know most things have the good and bad, it was more for informational purposes. Sorry for any confusion.

 

[halocline]

Worst case scenarios (all pretty unlikely), a manifold failure, or an inability to shut down the correct tank with a problem in manifolded doubles and you could conceivably lose all your gas. Curt Bowen did an article in ADM some years back with charts about how fast you could drain a tank with a blown hose at various depths that was pretty frightening. If you follow the rule of thirds in sidemount, you've (theoretically anyway) got enough gas if you lose one cylinder completely. I think the odds of not being able to do a valve shut down are infinitely greater than having a manifold failure. Backmounted doubles are a good safe system and most people dive them. I personally haven't found any cons to sidemount, and the pros for cave and wreck diving are obvious.

I'm not sure the depth matters in terms of how quickly gas escapes from cylinders; if anything, it would flow slightly slower at increasing depth because the ambient pressure that the gas is flowing into is higher. Certainly the deeper you are, the more you need to get out, maybe that's what the article was based on.

The idea that independent doubles, BM or SM, is somehow inherently 'safer' than manifolded doubles is simply not true, and is an assumption based on overly simple reasoning. (I realize you were not making this assumption in your post) First, you need two consecutive failures, a tank or reg related and the iso valve, or both tanks/regs, to lose all gas in manifolded doubles. In independent doubles, you also need two consecutive failures; each tank and/or reg. The iso valve is extremely simple and backed up with multiple o-rings that must fail in series; to my knowledge there has never been reported an actual iso valve failure that resulted in catastrophic gas loss. Two consecutive regulator failures, in either BM or SM, is undoubtedly more likely.

Having easy visual access to the valves does simplify things I suppose, but having to switch regs adds a bit of complexity, so maybe that's overall a wash. It is true that a sudden leak in manifolded doubles requires quick action and good practice.

Like any gear configuration, there are lots of reasons other than assumed safety increase to consider. If divers feel better and enjoy using SM more, that's great. But 'feeling' safer is not the same as actually 'being' safer, and in most situations, actual safety has less to do with gear configuration and more to do with diver judgement and skill.

 

[DaleC]

I would counter you're argument by saying that redundancy with an isolation manifold requires action during the failure. Redundancy for independent doubles requires action before a failure. Premeditation vs reaction. The only time manifolded doubles have an advantage is when someone is doing a dive so committed that they are gas limited by the number of bottles they can carry. Then, the ability to preserve more gas is there.

Using hypothetical 99cuft tanks for simplicity, let's look at the worst case scenario; a failure at max penetration/depth using the rule of thirds:

SM or ID: you will have breathed 1/3 from each tank (66cuft) and then lost 2/3 gas in one. You now have 66cuft to return.. not good.

Manifolded: you have breathed 66cuft. from your tank. Assuming you isolate quickly you potentially have 132cuft of gas to return.. better. Assuming assuming assuming.

So, for committed diving the isolation manifold is better (gas volume wise) but for OW diving (which is what sm seems to be used for more and more these days) the rule of thirds don't apply in the same way (far too conservative). Instead a rock bottom calculation can be made and that volume kept in each tank for ascent. When this is done the whole debate about preserving more gas is mute.

I won't argue one is better than the other but I would argue that one is not worse than the other.

 

[halocline]

I would counter you're argument by saying that redundancy with an isolation manifold requires action during the failure. Redundancy for independent doubles requires action before a failure. Premeditation vs reaction. The only time manifolded doubles have an advantage is when someone is doing a dive so committed that they are gas limited by the number of bottles they can carry. Then, the ability to preserve more gas is there.

The big advantage with the manifold is that gas from both tanks is easily available in the event of a regulator failure. You simply shut down that post. Considering how much more likely a regulator (or tank valve downstream of the seat) issue is than a tank burst disc or neck o-ring, that's a pretty big advantage IMO. Think about all the things that can and do go wrong with regulators; dozens of o-rings, hoses, 2nd stage flooding, diaphragm or exhaust valve leaking, IP creep, etc...and on a device that is normally not pressurized, and opening/closing with every breath. Now consider what needs to happen for a tank to lose gas upstream of the valve seat; a tank neck o-ring, a burst disc, or manifold crossbar o-rings, typically 3 in a row that would need to sequentially fail. All of these things are static and normally pressurized (unless the tank is empty) meaning that failure is more likely while the tank is on land, unless the tank spends most of its life in use.

And, in the exceedingly rare occurrence of one of those tank-related failures in water, the iso valve would also have to fail for a total gas loss. So, some folks might argue, as rare as that combination of events is, it's still a possibility, so better to go with two completely independent tanks. The problem with this (in terms of increased safety) is that its just as likely (meaning still very unlikely) to lose both burst discs, or tank neck o-rings, or one tank and one regulator, etc....as it is to lose one and the iso valve on the manifold.

As mentioned, what's much MORE likely is the loss of one regulator. One might even consider that a significant possibility given enough dives. In that case, the immediate loss of gas in one cylinder is, IMO, a major disadvantage. If you needed access to that gas to get out of whatever jam you're in while this bad stuff is happening, then the idea of "premeditation vs reaction" pretty much goes out the window as you're trying to figure out how to get gas out of a cylinder with a non-functioning regulator. Sure, it's theoretically possible, but comparing the difficulty of that vs closing an iso valve or maybe a tank post....

I do think that the iso valve design could be improved; maybe a simple ball valve with a straight lever that points parallel with the crossbar for open, perpendicular for closed, no confusion about which way to turn, very easy to close in a second or two. Paying attention, halcyon and thermo?

I also won't argue that BM manifold or SM is "better", there are so many factors other than redundancy. But in terms of redundancy, it's very difficult to successfully argue against the real-world utility of the manifold, once you consider all the factors.

 

[ianr33]

Here is a link to the tests that Curt Bowen did.
The Deco Stop

Experimentally a freeflowing reg did empty a tank faster at depth.

I used to dive backmount but now dive sidemount almost exclusively,even when I don't need to. Why?
1) I'm too lazy to switch hoses etc around all the time and don't want to buy even more gear!
2)Sidemount is what I use on "big" dives. Makes sense to me to use it on "practice' dives as well

As to which is safer,backmount or sidemount, that could be argued for ever more (and probably will be) I do know I FEEL safer in sidemount,especially in the following 2 situations:
1) Solo. With sensible gas management I don't have to do anything in the event of a failure except get out of dodge. With backmount you need to act fast,and act correctly ,otherwise you die. Imagine a burst disc/blown hose solo in a cave.Bubbles noise and silt everywhere. Would I act correctly? Maybe . Could I guarantee that? No. (Maybe I need to take Fundies or something. )
2) Small(ish) caves. Exiting Ginnie or Jackson Blue down the goldline sharing air off a longhose should be straightforward enough. Exiting a tight(ish) cave with assorted restrictions? not so much.