FX Typical air difference after a fill

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Plinkalot

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Nov 8, 2024
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Greetings,

Because of physics and the gas laws, there will always be pressure differences just after a fill... hot air cooling down and all that.

I'm wondering what the typical air pressure difference is after a fill? I have an Impact M4 Sniper.

How long does your gauge take to normalize?

BTW, my compressor takes 10 minutes to fill 130 bar.

Thanks
 
Last edited:
Plinkalot said:
Greetings,

Because of physics and the gas laws, there will always be pressure differences just after a fill... hot air cooling down and all that.

I'm wondering what the typical air pressure difference is after a fill? I have an Impact M4 Sniper.

How long does your gauge take to normalize?

BTW, my compressor takes 10 minutes to fill 130 bar.

Thanks
Click to expand...
As you may know -

It's NOT the "gauge", that normalizes, it's the "air pressure". The gauge just lets you know, that the air is back to ambient temperature.
And...as you may guess, these gauges with the tiny little gauge faces, are NOT very accurate when it comes to reading fine increments of pressure. Get a digital gauge, or a quality, 2" face gauge for that.

Mike
 
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MikeVV said:
As you may know -

It's NOT the "gauge", that normalizes, it's the "air pressure". The gauge just lets you know, that the air is back to ambient temperature.
And...as you may guess, these gauges with the tiny little gauge faces, are NOT very accurate when it comes to reading fine increments of pressure. Get a digital gauge, or a quality, 2" face gauge for that.

Mike
Click to expand...
I "believe" (with a pinch of salt) the large gauge on the tank when filling and use the small on-board gauges as a reminder (from the last shooting session) when I should refill.

Numbers wise, the gauges are of questionable accuracy but should be relatively consistent in indicating the pressure.

During a shooting session I refill after the appropriate shots or magazines being done.

When testing or tuning I'll top off the reservoir after ever 25 shots (I usually shoot 5 groups of 5 shots) to keep the fill pressure in a typical and at a reasonably consistent level. Find that topping off frequently and slowly minimizes heating/cooling in the reservoir. That may be over-kill, but that's what I do and I'm a creature of habit.

Just my 2 cents.

Cheers!
 
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Vana2 said:
I "believe" (with a pinch of salt) the large gauge on the tank when filling and use the small on-board gauges as a reminder (from the last shooting session) when I should refill.

Numbers wise, the gauges are of questionable accuracy but should be relatively consistent in indicating the pressure.

During a shooting session I refill after the appropriate shots or magazines being done.

When testing or tuning I'll top off the reservoir after ever 25 shots (I usually shoot 5 groups of 5 shots) to keep the fill pressure in a typical and at a reasonably consistent level. Find that topping off frequently and slowly minimizes heating/cooling in the reservoir. That may be over-kill, but that's what I do and I'm a creature of habit.

Just my 2 cents.

Cheers!
Click to expand...
Vana2,

What guages are you speaking of? My M4 has two gauges, the tank pressure and the regulator pressure.

My compressor's gauge and the gun's tank gauge agree with each other when filling. This indicates to me that both gauges are accurate.

LOL, I'm the exact opposite when it comes to filling my tank. In fact, the last part of any tune for me is to maximize rounds per bar so I don't have to fill the tank as often.

Thanks for your reply.
 
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bchannell said:
Pretty simple math.
If you fill to 250 bar and the cyl temp is say 110F then the cyl cools to rt of 72F the pressure in the cyl would now be 233 bar.

The gage should normalize as fast as the cyl cools down.
Click to expand...
Thanks for that.

My compressor has a temp gauge. Is that the cyl temp?

Do you leave your compressor connected to the gun during cool down? Is that how you determine "rt of 72F"?

Is there a formula to use to calculate how you got from 250 to 233 bar?

Thanks again
 
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No, compressor cylinder temp would be much higher, as there are temperature losses by the time the air reaches the air cylinder.
The formula is one of the gas laws, ie the relationship of pressure to temperature, assuming the confined space remains the same.

((Pi +14.7)/(Ti + 459.67) X (Tf + 459.67))-14.7

I believe I wrote that right:0
Pi = initial pressure
Ti = initial temperature
Pf = final pressure
Tf = final temperature

Pressure and temperature must be converted to absolute values, ie
temp F to Rankin
pressure psi to absolute pressure, which accounts for atmospheric pressure

then at the end you must subtract atmospheric pressure to get to psig (gage pressure)
 
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Plinkalot said:
Vana2,

What guages are you speaking of? My M4 has two gauges, the tank pressure and the regulator pressure.

My compressor's gauge and the gun's tank gauge agree with each other when filling. This indicates to me that both gauges are accurate.

LOL, I'm the exact opposite when it comes to filling my tank. In fact, the last part of any tune for me is to maximize rounds per bar so I don't have to fill the tank as often.

Thanks for your reply.
Click to expand...
Plinkalot,

I'm stepping out on a limb and making a statement that all gauges are less that 100% accurate 😁

High (lab) quality gauges are accurately calibrated and the large dial faces make it easier to correctly read the real pressure.

I have several FX PCPs so I'm looking at the same gauges as you. Gauges on other brands are no better or worse.

The small gauges on our PCPs, tanks and compressors are not "top of the line" and the coarse graduations and thick indicators make a precise pressure reading impossible. Do you read of the left edge, the right of use the middle of the indicator? IMHO, it doesn't matter if they are "accurate" or not - as long as they are consistent.

What I'm saying is that the gauges on our PCPs are for reference, they are accurate enough for their intended purpose.

Because nothinng is perfect, there is a manufacturing tolerance (plus or minus some value... say +/- 5 bar on a gauge), a "window" of acceptance between too little and too much. The gauge on my Impact may be on the low end of tolerance and yours on the high end and we're both indicating the same pressure where we are actually 10 bar different... and that's perfectly fine for our applications.

That real world difference is the reason that you using my tune numbers (or visa-versa) won't yield the same results... but they your chronograph is likely different than mine anyway 😉

The fact that your gauges agree is bonus! Enjoy!

Cheers!
 
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Vana2 said:
Plinkalot,

I'm stepping out on a limb and making a statement that all gauges are less that 100% accurate 😁
Click to expand...
Accurate was a poor choice of words. They agree so there are inaccurate to the same degree.

What I was looking for was in the nature of what bcchannell posted about temperature/pressure loss over time.

My pressure difference from immediately after a fill to about 3 hours later is about 20 bar.
 
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Plinkalot said:
Accurate was a poor choice of words. They agree so there are inaccurate to the same degree.

What I was looking for was in the nature of what bcchannell posted about temperature/pressure loss over time.

My pressure difference from immediately after a fill to about 3 hours later is about 20 bar.
Click to expand...
Yeah, there's always a loss of pressure. The amount depends on temperature and pressure differentials.

It is what it is, I just try to top of the tank regularly rather that shooting it to the bottom of the fill.

A number of my PCPs have low volume, low pressure capability so I'm used to filling every 30 shots or so. The shot-count on my higher pressure bottle guns spoilers me.

Cheers!
 
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