What is the correct pressure when PST 120 filled hold 120 cu ft?

[ScubaTexas]

What is the proper pressure when a PST 120 is filled to hold 120 cu ft. (it's rated capacity)? I read somewhere that is it 3442 lbs. sq. in. and that if filled to 3000 it is about 108 cu ft. Is that correct? If it is suppose to be 3442 lbs. sq. in. when filled to 120 cu ft, can I still use a Yoke connection if the tank has the combination Yoke/Din valve or will that pressure be too high for a Yoke?

 

[The Chairman]

You can use a yoke up to 3442, according to the DOT who regulates such things. PST reduced the service pressure on their HP tanks from 3500 to 3442 so they could legally ship the tanks with the yoke/din valves. They felt that since their LP tanks were seeing 3500 on a regular basis that they could no longer sell them as such. Great move on their part.

 

[LUBOLD8431]

What is the proper pressure when a PST 120 is filled to hold 120 cu ft. (it's rated capacity)? I read somewhere that is it 3442 lbs. sq. in. and that if filled to 3000 it is about 108 cu ft. Is that correct? If it is suppose to be 3442 lbs. sq. in. when filled to 120 cu ft, can I still use a Yoke connection if the tank has the combination Yoke/Din valve or will that pressure be too high for a Yoke?

Well, Pete took care of this pretty well. I would just like to add that when you fill an E7-120 to 3000 psi, it will only have 104.6 cu ft in it.
Yes, you can use a yoke reg with these cylinders as long as your reg can handle it (check the service rating of the yoke on your reg).

 

[padiscubapro]

You can use a yoke up to 3442, according to the DOT who regulates such things. PST reduced the service pressure on their HP tanks from 3500 to 3442 so they could legally ship the tanks with the yoke/din valves. They felt that since their LP tanks were seeing 3500 on a regular basis that they could no longer sell them as such. Great move on their part.

Netdoc,

Its not a blanked endorsement.. The diver must check the service rating of the Yoke.. Many older yokes have a service rating of only 3000 (and some of these thats pushing it) or not specified (remember 20 years ago steel 72s were extremely common and the regs only needed to work at ~2500 psi)... When 3300psi al cylinders started gaining popularity in the 90s many manufacturers beeferd up the yokes so a 3300psi rating is the current norm...

Before anyone uses a reg on a "3342" psi tank check the limits of the reg (some regs stamp it right on the yoke), there are some yoke adapters that have a higher rating...

 

[DennisS]

Here's a link to a site that has the different capacities at different pressures

http://www.lloydbaileysscuba.com/PST E Series Tanks.htm

 

[Otter]

How did he get 104.6 cu ft? 3000/3442 * 120 = 104.59.....no magic. The interesting question is...is an PST-120 120 cu ft at 3442 or 3500...in other words, did they increase the volume of the tank when the decreased the working pressure to accomodate yoke valves? If not, then 3000/3500 * 120 = 102.86 -- not necessarily a meaningful difference.

 

[The Chairman]

Regs should always be checked to make sure that they can handle the pressure. But the current DOT specifications limit yoke pressures to 232 bar.

 

[LUBOLD8431]

How did he get 104.6 cu ft? 3000/3442 * 120 = 104.59.....no magic. The interesting question is...is an PST-120 120 cu ft at 3442 or 3500...in other words, did they increase the volume of the tank when the decreased the working pressure to accomodate yoke valves? If not, then 3000/3500 * 120 = 102.86 -- not necessarily a meaningful difference.

Yes, they did slightly increase the size of the E7-120 compared to the HP-120. It holds 120 cu ft of gas at 3442 psi...

Actually, I got 104.6 by: (120/3442) * 3000 = 104.6
What the cylinder holds at rated pressure divided by service pressure (which is the baseline for that cylinder) times a desired pressure in the cylinder will give you the cu. ft. capacity for that cylinder at the desired pressure...

 

[The Chairman]

Did you guys add 14.7psi to each value to make them absolute pressures?

 

[in_cavediver]

Yes, they did slightly increase the size of the E7-120 compared to the HP-120. It holds 120 cu ft of gas at 3442 psi...

Actually, I got 104.6 by: (120/3442) * 3000 = 104.6
What the cylinder holds at rated pressure divided by service pressure (which is the baseline for that cylinder) times a desired pressure in the cylinder will give you the cu. ft. capacity for that cylinder at the desired pressure...

Yes, but you realize your calculation is no different then his? Multiplication and division are distributive. (3000/3442)*120 is the same as 120/3442*3000 and any other ordering. The only real difference people will see is error from a calculator. That is limitation of the hardware used not the physics or the math.

Mike