Filled Tank?

[icechip]

Ok, probably a dumb question so go easy, but what kind of drop (if any) should I expect of the PSI in a tank after I fill it with my compressor? I am fine with my new little SCU100 compressor going slow and chugging along, not over-filling or doing 'hot' fills or anything like that. Still, I would like my ready-for-job tanks to have a set amount (say 3000psi). I was curious if the amount just after a fill is appreciably different than after the filled tank has sat for a while.

 

[JackOfDiamonds]

As per Boyle’s law, any change in the volume occupied by a gas (at constant quantity and temperature) will result in a change in the pressure exerted by it. In other words, the product of the initial pressure and the initial volume of a gas is equal to the product of its final pressure and final volume (at constant temperature and number of moles). This law can be expressed mathematically as follows:

P1V1 = P2V2

Where,

• P1 is the initial pressure exerted by the gas
• V1 is the initial volume occupied by the gas
• P2 is the final pressure exerted by the gas
• V2 is the final volume occupied by the gas

This expression can be obtained from the pressure-volume relationship suggested by Boyle’s law. For a fixed amount of gas kept at a constant temperature, PV = k. Therefore,

P1V1 = k (initial pressure * initial volume)

P2V2 = k (final pressure * final volume)

∴ P1V1 = P2V2

This equation can be used to predict the increase in the pressure exerted by a gas on the walls of its container when the volume of its container is decreased (and its quantity and absolute temperature remain unchanged).

Boyle’s Law - Statement, Detailed Explanation, and Examples

 

[RayfromTX]

We can't give you an accurate answer because there are too many variables including the temperature of the air entering the tank which will be affected by the size of the filtration, the duration of the pumping cycle and the temperature of the air coming off the final stage of the compressor. There is also the size of the receiving tank. The smaller it is the faster it fills and the warmer it will get. To a lesser extent the material of the tank and the room temperature have some effect. You will have to let experience be your guide. Pump to 3200 and see where it settles to when it cools. Top off as needed. Alternatively, pump to 3,000 and see how far it drops after cooled. Adjust as needed.

 

[BoltSnap]

As per Boyle’s law, any change in the volume occupied by a gas (at constant quantity and temperature) will result in a change in the pressure exerted by it. In other words, the product of the initial pressure and the initial volume of a gas is equal to the product of its final pressure and final volume (at constant temperature and number of moles). This law can be expressed mathematically as follows:

P1V1 = P2V2

Where,

• P1 is the initial pressure exerted by the gas
• V1 is the initial volume occupied by the gas
• P2 is the final pressure exerted by the gas
• V2 is the final volume occupied by the gas

This expression can be obtained from the pressure-volume relationship suggested by Boyle’s law. For a fixed amount of gas kept at a constant temperature, PV = k. Therefore,

P1V1 = k (initial pressure * initial volume)

P2V2 = k (final pressure * final volume)

∴ P1V1 = P2V2

This equation can be used to predict the increase in the pressure exerted by a gas on the walls of its container when the volume of its container is decreased (and its quantity and absolute temperature remain unchanged).

Boyle’s Law - Statement, Detailed Explanation, and Examples

No, the relevant law of physics here is Gay-Lussac's law (Temperature vs. Pressure with volume being constant:

Note that Pressure and Temperature must be in absolute values for the formula to work.


Absolute temperature scale can refer to:

• Kelvin scale, an absolute-temperature scale related to the Celsius scale
• Planck temperature scale, an absolute-temperature scale based on absolute zero and the Planck temperature
• Rankine scale, an absolute-temperature scale related to the Fahrenheit scale

Gay-Lussac's Law - Statement, Formula, Detailed Explanation

 

[tbone1004]

@JackOfDiamonds not sure what you're getting at, but the temperature in this case is what is relevant since V is fixed. That said, the T doesn't work unless you actually measure the tank because calculating based on the pressure change is going to give you irrelevant information since there is no "time" factored in there to account for the heat sink abilities of the tank itself.

what's the CFM on it? Quick google looks like ~3cfm but what are you measuring? Even though you don't think you're "hot filling" because the pump is slow, what Jack said above isn't entirely incorrect as the gas coming into the tank is VERY hot. It's not as hot as the equation would have you believe because it's going through cooling stages, but it is still quite hot and if you shoot an IR gun at the tank before and after the fill, you will find that the temp has gone up a bit. That delta T is what you want to use to estimate the pressure change, just remember to use Kelvin not F or C.
What @RayfromTX said though is most useful though. It's way too much of a hassle to try to calculate it since it will change based on how empty the tank way, the delta in ambient temperature and water temperature, etc. They usually settle down after 2-3 hours back to ambient, so you can always just check the pressure right after filling, then wait overnight and check again. That said I usually overfill everything about 10% higher than what I want my final pressure to be and it tends to be close enough. It's not enough if I'm filling quickly out of a bank and the tanks are really empty, and if I'm just "topping off" a tank then it's too much, but for what I would call a "normal" fill out of a compressor like that, 10% is about right in my experience.

 

[BoltSnap]

@JackOfDiamonds not sure what you're getting at, but the temperature in this case is what is relevant since V is fixed. That said, the T doesn't work unless you actually measure the tank because calculating based on the pressure change is going to give you irrelevant information since there is no "time" factored in there to account for the heat sink abilities of the tank itself.

what's the CFM on it? Quick google looks like ~3cfm but what are you measuring? Even though you don't think you're "hot filling" because the pump is slow, what Jack said above isn't entirely incorrect as the gas coming into the tank is VERY hot. It's not as hot as the equation would have you believe because it's going through cooling stages, but it is still quite hot and if you shoot an IR gun at the tank before and after the fill, you will find that the temp has gone up a bit. That delta T is what you want to use to estimate the pressure change, just remember to use Kelvin not F or C.
What @RayfromTX said though is most useful though. It's way too much of a hassle to try to calculate it since it will change based on how empty the tank way, the delta in ambient temperature and water temperature, etc. They usually settle down after 2-3 hours back to ambient, so you can always just check the pressure right after filling, then wait overnight and check again. That said I usually overfill everything about 10% higher than what I want my final pressure to be and it tends to be close enough. It's not enough if I'm filling quickly out of a bank and the tanks are really empty, and if I'm just "topping off" a tank then it's too much, but for what I would call a "normal" fill out of a compressor like that, 10% is about right in my experience.

What he is saying is that it is experimentation and trial and error until you get a feel for it.

 

[icechip]

Great answers! Thank you, guys!

 

[Billy Northrup]

My buddy puts a shop fan that blows over the compressor and across the tank. Topping of an AL80 with 60cf takes about 20 minutes @3cfm. Like Ray says overshoot a bit and it should settle in the ballpark. On a few dive ops we crash filled 3400-3500 and the would cool off to 3100 but those tanks were filled in a few minutes and were smoken hot.

Not sure what the duty cycle on that compressor is, but adding a whip and fill two tanks at a time would slow the fill but might too much run time.

 

[tbone1004]

what I've seen in a quick google says I wouldn't run a splitter, I suspect the duty cycle is 50% or better and I wouldn't really want to run it more than 20-30mins at a time.....