Air Venturi Scuba tank fullness?

This a very simple Answer to you question...
Look at the Gauge on your tank.....also to what pressure do you fill your gun to???
What is obvious... You can't fill a gun above the pressure of Your Tank!!!!
So when the tank's pressure is getting close to you gun fill pressure...... Time to Fill Your Tank!!!

Also in the Resources tab, there is simple Fill Calculator!!

Pressure will always equalize. You will eventually reach the point where the air Venturi tank won’t fill your gun to its max level and that will be obvious on the gauge then. You won’t have to close the valve when filling it will just stop. What is the max fill pressure for the hw100.

On a side note. People get way too focused on the tank needing to fill their gun up to the max. This approach wastes so much useable air. Whether you have a regulated gun or not they generally operate best somewhere below the max fill of your gun. When your tank gets to the same level as the max fill of your gun run the tank tethered to your gun and get every last bit of useable air out before you get it refilled. They are going to charge you the same if you take it in at 3000 psi or 2000 psi.

A "dead head" on the end of the fill hose is what I use. It blocks the hose therefore reads tank pressure.

Airguns of Arizona has a calculator to tell you how many fills your tank can make of your gun. But the more challenging thing is partial fills. I have two guns that can be filled to 300 bar but that is also the limit of what I fill my tank to. So I cannot fill the gun fully even once. I prefer to fill to 250bar. You can put your desired fill pressure into the AOA calculator so it can handle this. But if you ask to fill your gun to 300 bar it will tell you zero or a negative number.

You can still get a useful fill of your gun as long as the tank pressure is higher than the regulator pressure. But it becomes a hassle to keep running back to the tank at some point. I usually refill my tank when it gets down around 3200 psi. That's nearly 500 psi lower than my desired 250 bar fill pressure but I still get a useful number of shots when the tank is over 3000. I would put something like 3000 or 3200 into the AOA calculator.

It's not a propane tank. What the pressure gauge shows is what is in the tank.

Not useful. Basic math then.

JimBentley said:

A "dead head" on the end of the fill hose is what I use. It blocks the hose therefore reads tank pressure.

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This DeadHead is a plug you can snap into you Foster QD Socket.....
It made to check pressures... Also check for leaks in you fill setup!!

The OP stated he had an air Venturi tank so I assume it’s a 4500 scba

EZ fill valve [or China clone] makes it nice,,, when you open the A/V valve, the EZ holds the line pressure so you see what is in the tank.
as other have stated , the A/V valve shows line pressure, not tank pressure, when the valve is closed

I have a deadhead wired to my compressor and tank to keep dirt out

My used Scott tank has a gauge on the tank but it's hard to read to a precise value. I could dead head the fill hose but the pressure doesn't go down that fast so I just refill the tank when I am getting too small a fill. I can see the pressure on the fill set gauge which is a nice big gauge so that's what I use but it is also on the gun, of course. The pressure falls some as the air cools, however.

As mentioned above, it's not necessary to fill to gun's max limit for MOST airguns. There should be a "sweet spot" where you get the most efficiency from your gun. Mine CAN fill to 250bar but shoots most efficiently from 240 down to 120 so I never fill past 240bar. It's just a waste of air.

Unregulated maybe?

JohnWicck said:

I’m curious. What are you noticing that you feel that 240bar is more efficient than 250?

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My gun shoots pinpoint accurately in that window. It's never an inaccurate gun but at max fill there's a difference of 1/8" - 1/2" in POI (50 Yards). Takes the gun 3 or 4 shots to "settle". If I fill to around 240ish there's no needing it to settle.

Gerry52 said:

My gun shoots pinpoint accurately in that window. It's never an inaccurate gun but at max fill there's a difference of 1/8" - 1/2" in POI (50 Yards). Takes the gun 3 or 4 shots to "settle". If I fill to around 240ish there's no needing it to settle.

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What gun are you talking about here? Regulated or unregulated. Don't confuse the OP with information on a platform that is not relevant to his usage.

To the OP. It appears that the HW100 has a max fill of 200 bar. So, once your 4500psi / 315 bar fill tank drops below 2900psi/200bar you will start to notice fewer shots per each fill from the tank. You can still keep using the fill tank to fill the HW100, you'll just get fewer shots per fill each time. But - this will let you get a little bit more bang for your buck on each fill of the big tank.

Also - check your tank after they fill it at the store. Sometimes stores will do the full 4500 psi fill and sometimes their reserve tanks will only fill to ~3800 psi. Probably matters less as a diver, but for our purposes push for a full 4500 psi fill so you can get the most number of fills for your money.

I have two guages on both of my tanks. One guage reads tank pressure and the other reads fill pressure into the gun.