Holding pressure and running pressure

[Guest]

Hi,

I have just purchased an airbrush compresser with tank .

Although not used in anger yet I have noticed when I adust the pressure gauge it fills as required and holds fine.

My question is, on activating the airbrush the gauge reads a lower 5+ psi less.

So which pressure reading on the gauge should I use as the spraying pressure for my paint.The holding pressure or depressed airbrush pressure.

Thanks Darren

[Guest]

Mine does that too Darren, so I take the running pressure as being right.

[Guest]

As the airbrush is not spraying at the holding pressure, the dynamic pressure would be the relative one. Temperature / diameter and length of hose/ viscosity of the paint mix / pressure drop through valving and particle filters all have a bearing on the delivery pressure at the nozzle. Having said that, I have a small needle valve on the supply hose at the airbrush end, and I crank my compressor to max., and use this valve to regulate the airbrush pressure. I spray onto some scrap, and when I am happy with the results, I start on the model. Remember start spraying off the model and finish off the model. Cheers Derek

[Guest]

Great advice, thanks for taking the time to reply

Daz

[Guest]

Darren if it is 5psi that is a lot. Mine does about 1PSI. With past compressors I would say on average max 2psi and if that. Most is just a fractional dip in the needle.

For 5PSI I would think that there is a leak some where in the airbrush. Is this loss of PSI in all the settings ie 10 psi or 30 psi as a starter. Does your pressure stay the same after the tank has filled and the compressor turned off for say 30 minutes. Also have you a quick disconnect brush to hose these I found are notorious for loosing pressure. OK when they are stagnating. Pick the brush up and they can leak.

Worth just painting the brush with a washing up /water liquid to see if you are getting bubles any where, ie a leak, from some point in the brush.

Laurie

[Guest]

Mine does that too Darren, so I take the running pressure as being right.

I agree, although as Laurie says, the drop (5 psi) is bigger than my setup (about 2 or 3 psi).

[stona]

I agree, although as Laurie says, the drop (5 psi) is bigger than my setup (about 2 or 3 psi).

Mine drops about 5 psi too.

I think it is more to do with the brush and the pressure at which you spray. I spray at a dynamic pressure of 35 psi so a 5 psi drop is more explicable than if I was spraying at 15 psi.

Cheers

Steve

[Guest]

Interesting. I have a pressure sensor/adjuster on the Compressor as per normal. Also I have installed at work top height, so that I can adjust pressures easily, an in line pressure sensor adjuster.

The work top one loses only about 1 psi when using the airbrush. But just had a look and the one on the compressor loses 3 or 4 psi. Now why would that all be ? When releasing the air on the airbrush all goes back to normal.

Then it must be release of the air from the tank into the air hose and airbrush which lowers the pressure. I wonder if this happens on a compressor without a tank ?

All academic Darren as your system seems to be average.

Laurie

[Guest]

Then it must be release of the air from the tank into the air hose and airbrush which lowers the pressure. I wonder if this happens on a compressor without a tank ?

Laurie

My old tankless one did the same thing Laurie - I think it's the pressure gauge as I don't believe they're particularly accurate.

[stona]

I think it's the pressure gauge as I don't believe they're particularly accurate.

Amen to that. They are hardly precision instruments, they serve to give a rough idea of the pressure at the reservoir. When I say that I spray at 35 psi I'm sure that the actual pressure is within a few psi either side of that, but no more than that

The main thing is that it's the "displayed pressure" that works for my set up and paints.

Cheers

Steve

[Guest]

Agree with all of that except what begs the question is why do they loose pressure ? Where does 5psi disappear too.

In a less academic mode the answer is to set the pressure with the trigger pressed. This at least gives a starting point. Providing it remains at that pressure when air brushing then that is all that is wanted. I think the pressure things are reasonably accurate but just not in portraying the actual true pressure in numerical terms.

Laurie

[Guest]

I don't think it goes anywhere Laurie, I think it wasn't there to begin with!

My thought is that before the AB trigger is depressed the pressure is built up against the gauge & because of less-than-perfect build quality / materials, the gauge gives a higher reading. Imagine it as the gauge parts flexing.

Once air passes through the AB, the reading is accurate.

[rickoshea52]

This is my two cents, with some reference to Pascal's Law and my hydraulics systems training.

If a pressure is applied to fluid in a vessel, that pressure will be felt throughout the fluid without reduction and react at right angles to the confining surfaces of the vessel.

So what you have is a vessel gradually being filled with compressed gas until the pressure within the tank reaches a given pressure at which point the pump will cut out and pressure maintained. When the tank pressure drops to a certain level the pump cuts in again to replenish the reservoir.

The pressurised air in the tank passes through an orifice in the regulator valve body to the airbrush. When the airbrush is operated air passes through this orifice and a pressure drop occurs; the gauge on the compressor measures the regulated air flow pressure and not the pressure of the air in the reservoir.

The accuracy of the gauge is not that great and I would guess that none of us ever considers having them calibrated.

[Guest]

.

[Guest]

Wow!!! Can't believe how much this post had moved on, I didn't seem to get any update message.

Thanks for everyone's help I shall keep an eye on it and see if it causes any issues.

I haven't even sprayed paint with it yet, so quite a learning curve anyway.

Paint ordered and on the way from SMS ;-)