Appreciate the responses and realise its hard to diagnose over the internet.
If it wasn't clear, the gun sprays great and has no fluctuations during spraying. It was just the initial trigger pull for the first 1/2 second where the pressure wouldn't drop super fast
to the final working pressure and whether that was normal. I'll try dropping the tank regulator pressure further and see its impact (previously I ran it at 60psi)
I received some assistance about my hose setup from this forum a year or so ago after my systems inability to hold 2 bar at the gun due to too much air hose and quick connect fittings.
I made changes based on the advice I received and currently have:
120 litre (30 gallon) tank with 350+l/min FAD compressor so I can run this HVLP no problems.
No quick connect fittings except on gun, otherwise all high flow fittings.
8 metres (25 feet) of 1/2 inch hose
1/2 inch port commercial 3 stage filter setup
7 metres of 3/8 inch hose to gun
The air regulator at the gun is a Devilbiss unit so not cheap (but probably made in China). One air regulator at the tank.
General Discussion. Make yourself at home...read, ask and answer!
The Devilbiss air regulator you have at the Base of your gun is not an air regulator at all its and adjustable restriction device a simple valve open it all the way or remove it from the system entirely.
Open the Devilbiss regulator at gun all way open and keep it open & then adjust air pressure at your regulator at compressor with trigger pulled.
If you still see pressure fluctuations replace Regulator on the compressor.
This is NOT an ideal set up, Air regulator at compressor is probably the problem these are generally cheap junk on smaller compressors from Box store retailers. Your compressor is also way to small for the job at hand. 350 Liters per min FREE AIR equates to 12 cfm FREE AIR this means CFM @ 0 psi.
When your compressor comes up to working pressure CFM production drops dramatically! (ANY and all COMPRESSOR pumps do this)
I would expect your CFM out put at 90 psi to be less than half of your free air rating its probably 4-5 cfm at most.
Most good compressors MFG have a Free air rating and then one at 90 psi.
Its the rating at 90 psi is the one you have to pay attention too. 90 psi is an compressors working pressure as most all air tools use 90 psi as their working pressure and will be the CFM your spray gun uses while painting, if your Gun requires 12 cfm but compressor only makes 5 cfm at working pressure your going to have radical pressure & FLOW fluctuations while spraying. Just because you have pressure does not mean you have the same FLOW.
Depending on actual cfm out put of your pump itself your 30 gallon tank will be depleted of required air flow in just a short while long before the pump actually kicks on.
The tank is turned on by PSI in the tank NOT available air volume.
Unless your using a very special very low cfm gun which they do make they are called LVLP (Low volume low pressure) your going to have problems.
https://www.eastwood.com/concours-lt-hv ... 1-3mm.html
This LVLP gun Only requires 4.5 cfm! Its not an HVLP, its a LVLP most all hvlp guns are air hogs!
Their are a few other mfg of LVLP guns out their, I have not tried any of them but If I were to try one it would be the one from Eastwood at least they do have people who test the stuff they sell.
A LVLP gun will spray slower with a smaller spray pattern as compared to what most all of us use. Which are name brand HVLP's or RP compliant guns which all use a great deal more air that put out more paint and have much bigger spray patterns.
When I say more air I am not saying more air pressure BUT More air Volume at low pressure CFM requirements are every thing. do not confuse Pressure with volume, they are two very different things entirely! I cant stress this enough!
Link to acceptable Air regulators.
https://www.grainger.com/category/pneum ... WP13816485
Speed air or Wilkonson Parker all good names just go to your pipe size 1/2 inch these wont break the Bank. Grainger is available world wide.
You'll be fine with what you have for spraying Primers & epoxy as these are generally sanded after application anyways so doesn't really matter.
Where your gonna have problems is spraying Base coats and Clears. get a LVLP gun and a much bigger compressor.
A&P Mechanic, FCC General radio Telephone Operator
Line Maintenance A&P Mechanic and MOC Tech specialist.
The compressor I have is a high quality brand and the tank regulator is really nice. The only limitation is I have single phase power thus I'm limited to 3hp.
In my thinking - the scenario in my original post happens with the compressor off so the compressor output is not a variable. Also the compressor can outpace what the HVLP gun is drawing while spraying. The issue I have to mindful of is with HVLP is my compressor will run more often so I need to have good moisture filtration.
I'll have to rig up a short length of hose and run the gun and see if it happens, then I will know its just what a spray gun does as the needle opens. I'll also drop the tank pressure down as low as I can get it to hold the working pressure at the gun.
At the end of the day, not really a big issue, if I was being really pedantic I would just pull the trigger for 1/2 second away from the panel and then start spraying.
A few comments:
Firstly we measure things a little differently. For us a 3.2hp or 2400W motor is our maximum on single phase 240V. Depending on efficiency of the motor(s) and pump(s) this can provide anywhere from 350 to 475 litres/minute (FAD) which is more than enough to run most spray guns. From what I understand from Adam's posts, his compressor is towards the bottom end of this range.
My AZ3 (RP, not HVLP) has a 1.3mm tip and theoretically uses 200 l/min. The HVLP specs for 1.8mm seem to indicate 340 l/min at 2 bar. So your compressor, at 350+ l/min is sufficient, but has little excess capacity, certainly for this gun. If, as you say, the compressor shuts down at times during spraying then you're probably okay. In your situation what I'd do is consider changing the HVLP gun for RP. You'd lose about 40mm of fan width (340mm to 300mm) but pick up enormously with reduced air usage of only 215 l/min. The RP also has more transfer efficiency, so you'd get some savings in paint usage as well.
Tank size, at 120l is more than sufficient. I run the same size tank and have no problems at all. After all, tank size only provides a buffer between a fully charged tank and the time when the compressor has to begin recharging. Of course, it helps to keep the air inside a little cooler, allowing time for water to condense out.
Talking about condensation, water traps or cooling pipe runs between the compressor and your gun are the least efficient and introduce restrictions that can cause variations at the gun. A cooling array between the compressor and the tank is much better. Saves your tank too, since water doesn't gather there.
Now, back to this drop in pressure when you move the trigger from air only to air plus fluid. As I said before, the extent to which this happens depends on the internal design of the gun. Although I already knew, I ran a few tests again this morning to confirm. The AZ3, from 90psi static, drops almost immediately to about 32psi when the trigger is first pulled and then takes another half second or so to settle at 30psi. Going to the second position the pressure (at inlet) drops another 3psi, just like yours. From 60psi static the effect is slightly more dramatic with a drop of about 3psi moving to the second position with the cup empty. Adding some thinners into the cup makes the drop almost unnoticeable from 90psi and only 1psi from 60psi static.
However, repeating using a Pro Lite shows none of this behaviour at all and I checked also with a 1.8mm Star and it's the same as the Pro Lite.
So, it's a gun design issue. I'd suggest more so on an HVLP gun than an RP. Nothing I think to worry about if you pull the trigger before you get on to the panel, but I'd probably increase the static pressure to maybe 75psi to reduce it.
What else can you do? Comments above about removing regulators from guns and, instead, using a wall regulator are valid. Fine if you're using just one or two guns but if you have multiple guns or share the booth with someone else then it means time setting up for each gun change. I used to do it, and had a chart taped to the wall detailing ideal settings for each gun. Worked, but was a time user when I didn't have time. Quicker to have a regulator on each gun.
That said, one of the problems with most regulators is that the passage through the middle is usually too narrow. On some regulators I have drilled it out so it's the size of the internals of a high flow fitting. That has made a big difference, improving air flow (volume) considerably and reducing the difference that always happens when you use a regulator at the gun to set the regulator on the wall, then remove the gun regulator only to find that the improved flow has changed all your hard work.
Appreciate you taking the time to test your actual gun.
I'm priming my truck one panel at a time as I metal finish each one so it's not a problem at the moment but I could see how I could run into trouble if I was using the gun for an extended period with the FAD so close to the gun air consumption. The HTE kit nozzle / air cap kit to convert the AZ3 from HVLP to HTE is about the same price as a new gun unfortunately.
One question - with the AZE the fan adjuster has 1 / 2 / 3 / 4 on the know....is it 4 full turns out to maximise fan width? I just turn mine out till I see the thread but my fan width on this gun never seems as big as the specs - I could be spraying too close on my test sprays though maybe.
No problem. Really it took about 30 seconds to pick up a gun, squirt about 20ml thinners into the cup, plug it into the air hose and pull the trigger.
The numbers on the knob are really just there to help with minor adjustments.
With a little thinners in the pot, screw the adjuster all the way in (but don't force it). You should have a round spray pattern. Now unscrew the knob until the fan pattern stops getting wider. At this point the ends might be a little untidy so screw back in a quarter or half turn until the pattern is even. Check on paper if you like. You may have to fine tune this if using a much thicker paint like a HS primer. Test spray at 200mm distance from paper.
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