Sizing Tank Height for Sidemount

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MrBlenny

Contributor
Messages
146
Reaction score
59
Location
Washington State, USA
# of dives
200 - 499
Hi folks,

I've posed this question to my instructor as well, but I have the chance to pick up some used tanks (pairs of LP95's and 108's) and as I have not gone through training on the xdeep stealth yet, I don't want get excited and buy tanks that won't trim well for me. Is there a way I could measure my torso that would tell me the maximum tank height I would be comfortable diving? Or am I just better off being patient and waiting until I'm back from training to buy tanks? Apart from itching to have tanks to call my own, my local shop doesn't bank nitrox and only has one blender, so it takes a fair bit of planning to get rental tanks filled with EAN32. Having my own would make all of this much simpler.

Thanks in advance!
 
the quick and easy is the distance from your armpit to the top of your knee as a rough number. You need to at least be from your armpit to the bottom or your butt for minimum length. @DA Aquamaster dives 95's, but you have to be pretty short to dive them comfortable. Frankly I would look for HP100's if you can. LP tanks are really floaty and really inefficient if you can't get them cave filled, and the 8" bottles are not as comfortable as 7.25" tanks.
 
Are in WA State? What kind of dives are you trying to do? There's a whole group of xdeep/razor divers up here using al80s for long recreational dives.

hp100s in the xdeep are a challenge. They hang quite heavy in any SM BC with no butt plate. With dry gloves its pretty hard to get the neck bungies tight enough to roll the weight of a hp100 "up"

lp85s are a better choice but most WA shops wont fill them adequately and in salt water you end up needing to front clip them to hold them down like an al80 anyway.
 
the quick and easy is the distance from your armpit to the top of your knee as a rough number. You need to at least be from your armpit to the bottom or your butt for minimum length. @DA Aquamaster dives 95's, but you have to be pretty short to dive them comfortable. Frankly I would look for HP100's if you can. LP tanks are really floaty and really inefficient if you can't get them cave filled, and the 8" bottles are not as comfortable as 7.25" tanks.

That's simple enough, I get 31" for me, so now I have a reference point, which is really helpful. I'm also pretty sure my shop won't do cave fills unless it's the owner, which I can't count on being available. Also, I should have mentioned in the original post that I will be doing some deco, not crazy deep (130ish), but gas is definitely my friend. Do you find the 8" diameter cylinders to be just mildly annoying, or outright irritating to dive with? 133's would give me a ton of gas on a proper fill, and I when I go to Canada and get 3000psi hot fills I won't be hurting as much. HP120 seem like a stretch (literally) for me, but have the 7.25" diameter.
 
@MrBlenny my primary sidemount tanks are LP121's, but that is for cave diving and is a balance between dealing with the big tanks and having to carry extra bottles. I am also quite tall, that measurement for me is 36". Also keep in mind that measurement takes in the total length of the tank including valve, so make sure to remove about 4" from that measurement when looking at tank dimensions.

Frankly if you're doing OW stuff up there and @rjack321 is saying the HP100's don't work great with the xDeep *I don't dive one, but I do dive my LP120's on hip d-rings instead of a butt plate, but they act more like an AL80*, I'd just as soon stick with AL80's. In OW you aren't doing terribly long exposures so the gas volume with 2 tanks really isn't going to be a limiting factor. HP120's work great if you're tall enough, but if you're saying that 31" length is right, then they're going to be way too long for you. Those tanks are borderline too long for me. The image below is me diving PST HP120's with a Hollis Katana. If that dimension is 5" shorter for you, then the tanks will stick out beyond your knees. Not a huge problem, but it will be quite annoying.

13521843_10157082437055134_1904598610029687808_n.jpg
 
Are in WA State? What kind of dives are you trying to do? There's a whole group of xdeep/razor divers up here using al80s for long recreational dives.

hp100s in the xdeep are a challenge. They hang quite heavy in any SM BC with no butt plate. With dry gloves its pretty hard to get the neck bungies tight enough to roll the weight of a hp100 "up"

lp85s are a better choice but most WA shops wont fill them adequately and in salt water you end up needing to front clip them to hold them down like an al80 anyway.

Yep, I'm in WA state and dive the Puget Sound and Hood Canal :) I'm on the south end near Portland, so you being in Port Orchard makes me all kinds of jealous...

Barring some type of surprise during training, I'll be diving with the add-on rails in cold water. Just curious, wouldn't a drysuit with toasty thermals and pair of AL80's make for a big buoyancy swing during the dive? So very much for me to learn...especially training in Tulum with 75F water, then 50F water here.

Edit: Re-read you post and realized I didn't mention anything about dive profiles. I'm a photographer and want to spend some quality with the cloud sponges, wrecks, and other neat stuff we have with worrying about running out of gas or staying out of deco. Once I have a good number of dives in sidemount, plus AN/DP, I'm planning a trip to Chuuk to see the wrecks.
 
@MrBlenny my primary sidemount tanks are LP121's, but that is for cave diving and is a balance between dealing with the big tanks and having to carry extra bottles. I am also quite tall, that measurement for me is 36". Also keep in mind that measurement takes in the total length of the tank including valve, so make sure to remove about 4" from that measurement when looking at tank dimensions.

Frankly if you're doing OW stuff up there and @rjack321 is saying the HP100's don't work great with the xDeep *I don't dive one, but I do dive my LP120's on hip d-rings instead of a butt plate, but they act more like an AL80*, I'd just as soon stick with AL80's. In OW you aren't doing terribly long exposures so the gas volume with 2 tanks really isn't going to be a limiting factor. HP120's work great if you're tall enough, but if you're saying that 31" length is right, then they're going to be way too long for you. Those tanks are borderline too long for me. The image below is me diving PST HP120's with a Hollis Katana. If that dimension is 5" shorter for you, then the tanks will stick out beyond your knees. Not a huge problem, but it will be quite annoying.

View attachment 541052

Thanks, the picture really helps with perspective. Also, AL80's are cheap and I'll need a few for stage bottles anyway, so no money lost even if I find I want to dive something bigger sometime down the road.

Also, nice trim!
 
@MrBlenny the buoyancy swing is a function of gas volume and has nothing to do with the tank. I.e. 80cf of air has a mass of 6.4lbs, so from full to empty with a pair of them there is a swing of 12.8lbs. That holds true for any other tank that holds 80cf, whether it is an overfilled LP50, or an underfilled LP121. 80cf of air has a swing of 12.8lbs regardless of what it is put in.
Now, an AL80 becomes positively buoyant when empty, but you have to factor that in regardless. With the AL80 you will have more lead on your rig than you would with a steel tank, but the buoyancy shift is a function of gas volume consumed.

I will emphasize that if I could do the dives that I'm doing on AL80's practically, then I would in a heart beat as they make sidemount so easy. All of those videos that you see of bottle handling on youtube are done with AL80's for a reason.....
 
Yep, I'm in WA state and dive the Puget Sound and Hood Canal :) I'm on the south end near Portland, so you being in Port Orchard makes me all kinds of jealous...

Barring some type of surprise during training, I'll be diving with the add-on rails in cold water. Just curious, wouldn't a drysuit with toasty thermals and pair of AL80's make for a big buoyancy swing during the dive? So very much for me to learn...especially training in Tulum with 75F water, then 50F water here.

al80s have a ~10lb buoyancy swing no matter what your exposure protection :) That's how much gas is in them. There are quite a few people using al80s here simply because they work well with that kind of BC - minus any rails or butt plate. (12lbs if you run them dry)

Hp100s are doable, they are just about the priceiest most popular tank here and also tend to hang heavy.

If you are training in Tulum I would "dive what you know" up here for a good long time before worrying about a different tank, especially anything that is 8" in diameter.
 
That's simple enough, I get 31" for me, so now I have a reference point, which is really helpful. I'm also pretty sure my shop won't do cave fills unless it's the owner, which I can't count on being available. Also, I should have mentioned in the original post that I will be doing some deco, not crazy deep (130ish), but gas is definitely my friend. Do you find the 8" diameter cylinders to be just mildly annoying, or outright irritating to dive with? 133's would give me a ton of gas on a proper fill, and I when I go to Canada and get 3000psi hot fills I won't be hurting as much. HP120 seem like a stretch (literally) for me, but have the 7.25" diameter.

8" are massively bigger IMHO at least in how they feel in your armpit area for me.
I dive hp130s (3442 tanks) in SM, but only with trimix otherwise they feel like giant bricks.

My favorites are lp45s and lp72s but I have a hollis sms75.
 
https://www.shearwater.com/products/perdix-ai/

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