Compressor filling rate; does it change as tank pressure increases?

[The Chairman]

The physics: Nonlinear gas volume/pressure relationship.

 

[The Chairman]

We usually use STP (Standard Temperature Pressure) to figure out how gasses will compress and mix. For the most part, especially if you're pumping air which is a complex gas (more than 2 elements), that works fine. My first answer is correct: your filling will slow as you get to your target pressure. That's completely normal and to be expected on a personal compressor. Just be patient and keep the temps down. It's also to be expected that the pressure will decrease after the tank cools. Over-pumping it by 10% will probably accommodate the cooling loss.

 

[tbone1004]

Understood and well stated. Temp is always the gorilla in the "room" [tank...]. Another flow rate changer for me was how often and how much I opened my condensate drain....as this was my first use, I was a tad bit over zealous...and that slowed the bottle charging...

"Non-linear" love it...kinda like "stasis"...must exist but hard to nail down...appreciated your input...

the best part is the condensate drain timing changes based on time of year. Basically it's trial and error. Not sure which pump you have or how it is configured, but basically you want to keep the purge time relatively fixed, I usually go for 5secs for the intermediate water traps and then 3secs for the final oil/water separator. Ideally the end of that time is when the last of the condensate is really gone so you will adjust your time intervals to get enough water in there to make the blowdown worthwhile. All depends on your compressor but the 3-5secs every 15mins is a good starting point. At this time of year with low humidity you may be able to go 30-40mins between purging, and in the middle of summer it may be every 10mins depending on the intake air

 

[tbone1004]

We usually use STP (Standard Temperature Pressure) to figure out how gasses will compress and mix. For the most part, especially if you're pumping air which is a complex gas (more than 2 elements), that works fine. My first answer is correct: your filling will slow as you get to your target pressure. That's completely normal and to be expected on a personal compressor. Just be patient and keep the temps down. It's also to be expected that the pressure will decrease after the tank cools. Over-pumping it by 10% will probably accommodate the cooling loss.

and the overfilling is totally legal btw for @Wallowa the working pressure stamped on your tank is at normal atmospheric conditions so you can overfill quite considerably so long as once the tank equalizes temp at 70F it is not higher than the working pressure. This is often misconstrued by most recreational dive shops who think the stamp on the wall is the absolute max pressure come hell or high water

 

[The Chairman]

the best part is the condensate drain timing changes based on time of year.

IE based on the relative humidity and temps. Filling in an air-conditioned space is best if you can stand the noise. Low humidity and low temps are best.

and the overfilling is totally legal

He's not restricted by any laws since this is for his personal use. For others, sure.

 

[rx7diver]

Does a Hp Scuba compressor fill rate decrease as the tank pressure increases? Will going from 500psig to 1,500psig take less time than going from 2,000psig to 3,000psig?

Thanks...

Yes. This non-constant (decreasing) fill rate is one of the reasons why scuba compressor adverts will often report a SCFM fill rate (Standard Cubic Feet per Minute fill rate), the time it takes to fill an Al 80 from 500 psig to 3,000 psig. At least, this is my understanding of the reasoning. (I'm NOT an engineer.)

rx7diver

 

[Wallowa]

IE based on the relative humidity and temps. Filling in an air-conditioned space is best if you can stand the noise. Low humidity and low temps are best.

He's not restricted by any laws since this is for his personal use. For others, sure.

Thanks yet again guys...I am well aware of rated vs working pressure 'recommendations' for the bottles including the relationship between temp/pressure in a fixed container [tank] and here [3,895 ft absl] where humidity is normally very low, relative humidity is to be watched but not a big issue...neat thing is that I am not in need of a few more CF and more psi since a lot of my diving will be at altitude and in 40 F HOH [which will decrease pressure] but last fill was at air temp of 40F....ocean will be around 55F...adding that I will be at depths which will limit BT and hence gas requirements [even factoring in higher gas consumption rate]...with shortened BTs and safety/decom stops I will rarely need more than 80cf while diving with rule of thirds...anyway, do enjoy the facts and theory behind them and I learned from your posts....cool you are willing to share the collective knowledge...thanks.

Ps...if needed I have two sets of triples [120cf/90cf] and a double [100cf]...but they can only 'get me into trouble'.. by thinking I can go deeper and stay longer...no end to that cycle!