OMS vs Pst?

Please register or login

Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.

Benefits of registering include

  • Ability to post and comment on topics and discussions.
  • A Free photo gallery to share your dive photos with the world.
  • You can make this box go away

Joining is quick and easy. Log in or Register now!

Divesherpa

Contributor
Scuba Instructor
Messages
1,223
Reaction score
19
Location
Girdwood, Ak
Comparing PST 104's doubled and OMS 112's doubled. Any input would be helpful.
 
All stats are for one tank.

Physical Size:
Height and width are virtually the same

Weight on Dry Land:
PST 104 = 46 lbs.; OMS 112 = 41 lbs.

Buoyancy:
PST = -5.3 full, -2.5 empty
OMS = -8.0 full, -1.0 empty

Exterior Protection:
PST = hot dipped galvanizing process
OMS = some sort of spray galvanizing process
Hard to say which is better -- most opinions I've heard favor the PST process for better rust resistance. I have heard at least a few folks say they think the OMS tanks are more durable.....

See link below for tank stats. Hope this helps.

http://diverlink.com/gear/tanks.htm
 
Someone's math is a bit off.
If the PST 104 is negative 2.5 lb empty, then it's negative 11 full. Unless they mean 500 psi as empty, then it'd be negative 9.5 full.
Rick
 
This information came from http://www.huronscuba.com/

PST 104 0.7 empty w/valve -8.2 full w/valve
OMS 112 -1.0 empty w/valve -8.0 full w/valve

:tree:Bob
 
Air weighs .081 pounds per cubic foot (104 CF weighs about 8.5 pounds). When you put it in a tank, the tank's weight increases by the weight of the air you put in it. The only way you can have the buoyancy change less than the weight of the air you put in the tank is to have the tank expand to displace more water, and expanding to displace another 5.5 pounds (all but 3 of the 8.5 pounds that 104 CF of air weighs) is a bit much, don't you think? (For those of you who think of tank sizes in liters, for the figures given to be accurate, the tank would have to expand by two liters when you fill it - I don't think so)
Rick
 
Rick -- OK -- that makes sense to me.

What makes less sense to me is that the delta between the full/empty stats for the larger OMS tank (7 lbs) is less than that for the PST (8.9lbs).......hmmmm. What other variable is affecting this?
 
I suspect it's a difference in the definition of "empty." To many, that means 500 psi, while to others (me, for example) empty means empty.
In the case of a LP 112, for example, 500 psi leaves 21 CF in the tank, so the difference between "empty" and full is 91 CF, which weighs just a bit over 7 pounds - and that makes the numbers make sense.
Scuba is absolutely *filled* with "information" like this, and occasionally that info gets repeated so much it becomes common knowledge and "well known fact." Folks don't question it because they see it in print.
It's how one heavily advertised small compressor manufacturer manages to claim a 4.2 sCFM rating for their 3.5 sCFM compressor that's *really* a 3.4 sCFM... IOW they're claiming an air delivery rate that's over 20% above reality. They've been running the same add for years, and I have yet to see any dive publication take them to task for it.
Rick
 
Originally posted by Divemstr13
OMS tanks are actually made by Faber.
That is correct.
Rick
 
https://www.shearwater.com/products/peregrine/

Back
Top Bottom