Urban Legend? HP104 vs. HP130?

Sounder · Post by **Sounder** » Thu Sep 28, 2006 8:20 am

Was talking with a diver who swore up and down that a 104 will actually hold more gas than a 130. It sounds like an urban legend to me but I'd like to know if it's true... and if not, why people believe it to be true.

Lamont? Captain? GD? BDub? JohnR? Anyone else know... I mean, REALLY know if this is true? Research project!!!

Also: my 130's are rated to 3442psi but everyone talks about them going to 3500. Is this a concern?

cardiver · Post by **cardiver** » Thu Sep 28, 2006 8:55 am

If I have an hour or two to wait at the dive shop, I'll get a 4000 psi fill on my x-8 130 tank. As for which holds more air...If both tanks are hp then the 130 is always going to hold more air than the 104.

CaptnJack · Post by **CaptnJack** » Thu Sep 28, 2006 9:12 am

For all practical purposes they have the same interior volume and hold the same amount of gas when pressurized the same.

Its hard to find water capacities on the web - I tried all the usual suspects.

lamont · Post by **lamont** » Fri Sep 29, 2006 1:10 pm

The only way to settle this one is to measure the actual water volume of the two tanks.

If you use linear interpolation on the published stats you will find that a 104 should hold slightly more than 130 when filled to 3500 psi. I've always read that this was due to nonlinear incompressability and/or inaccuracies in the manufacturer's numbers.

Sounder · Post by **Sounder** » Fri Sep 29, 2006 1:24 pm

That's really interesting - someday I'd like to do the water test.

Calling it a 104 and having it hold 130+cu' of gas isn't a "slight miscalculation." Buyer Beware (pays to do your research): I would hate for someone to do their calculations for gas consumption based on a 104 at 3500psi and then go on vacation where they're diving an 80cu' tank and plan their dive according to their 104 numbers.

This would mean that someone was actually breathing roughly 130cu' (or more) of gas but only THOUGHT they were breathing 104cu'! That's a 20% difference which means that planning a dive with an 80cu' would leave you roughly 20% shorter than you'd planned... which sucks.

Is this correct? I hope I'm wrong.

dsteding · Post by **dsteding** » Fri Sep 29, 2006 3:24 pm

I think that the issue is the oddity of the fact that if you calculate a tank factor (in cubic feet per 100 psi) for a 104 (at 2640 psi) and a 130 (at 3442/3500 psi) you get a different factor for each, despite them being dimensionally similar, if not the same.

I think, IIRC, a 104 has a factor of about 4.1 (3.9?), the 130 has a factor of 3.7, so this may be the source of the statement that a 104 holds more gas, at least on a per 100 psi basis . . .

CaptnJack · Post by **CaptnJack** » Fri Sep 29, 2006 3:49 pm

Part of this has to do with compressability. 0 to 100 psi is not exactly the same amount as 3400 to 3500 psi.

I have always heard the bulk of the discrepancies are due to marketing inaccuracies. ALA an AL80 really holding 77cf.

My double hp100s have a tank factor of 5.7 and lp85s 6.4 but I just use 6 for both and can't tell any difference.

Life in the US would be easier if we used metric.

lamont · Post by **lamont** » Fri Sep 29, 2006 4:02 pm

yeah the discrepancy is in jacking up the LP104s to 3448 psi:

104 cuft * 3448 psi / 2640 psi = 135.8 cu ft

given rounding errors, manufacturer uncertainty and possible incompressibility i would consider that identical to a 130 -- heck, i usually dive 119s with the same gas plans as 130s -- you shouldn't be cutting gas plans that close.

Sounder · Post by **Sounder** » Sat Sep 30, 2006 6:47 am

That's a great point - with proper planning, it should't be an issue regardless... SHOULDN'T being the key. I feel very fortunate to have so much experience to learn from at my fingertips.