# Compressor question and suggestion for spray booth

I'll start with the spray booth. I took an old disfunctional computer =
monitor and ripped it of its content. Then I wrapped it in tape to make =
it airtight, drilled a hole in the back and attached a 230 V small fan =
to the outside of the back. Then I stuffed the back of the inside of =
the old monitor case with a "Hepa-filter" cut to size. Not fancy at all =
but works quite well with acrylics/alcohol. The Hepa-filter seems to =
absorb most of the fumes/dust. This was made in an afternoon and works =
really great. I guess you could improve on the design...
So for the question: I have a compressor of a brand called "Werther =
International"(?). It's a small compressor with a diminutive air-tank. =
The question I have is how to read the pressure gauge. The more I think =
of it the more confused I get. The compressor is a fairly simple design =
with a "waste-gate" attached as a shunt (in parallell) with the =
air-outlet. By opening the shunt I can lower the output pressure. The =
pressure gauge is, as I understand it, measuring the pressure at the =
outlet. Now say I adjust the shunt so the gauge reads 20 psi when I'm =
not pressing the trigger on the airbrush. Then I press the trigger so =
the gauge reads 15 psi. Am I spraying with 15 psi or 20-15=3D5 psi? =
The more I press the trigger, the more air comes out but at the same =
time the measured pressure get lower. So how do I read this gauge? If =
the gauge measures the spraying pressure the only way to spray at 30 psi =
is not to press the trigger at all!
Thanks.
I made an "article". See it at =
So for the question: I have a compressor of a brand called "Werther International"(?). It's a small compressor with a diminutive air-tank. The question I have is how to read the pressure gauge. The more I think of it the more confused I get. The compressor is a fairly simple design with a "waste-gate" attached as a shunt (in parallell) with the air-outlet. By opening the shunt I can lower the output pressure. The pressure gauge is, as I understand it, measuring the pressure at the outlet. Now say I adjust the shunt so the gauge reads 20 psi when I'm not pressing the trigger on the airbrush. Then I press the trigger so the gauge reads 15 psi. Am I spraying with 15 psi or 20-15=5 psi? The more I press the trigger, the more air comes out but at the same time the measured pressure get lower. So how do I read this gauge? If the gauge measures the spraying pressure the only way to spray at 30 psi is not to press the trigger at all!
Since there are no other takers on the compressor question who are familiar with your setup, I shall give it a try. Here are my two cents; The required spraying pressure is the static pressure and should ideally not drop when operating the airbrush. For a correct readout of the static pressure the gauge should be mounted at a spot in the system where the air is in (almost) stationary state, like an air-tank for example. When the gauge is mounted in an airflow like at the air-outlet, the static pressure will be lower depending on the flow speed. Which is normal. This will not explain the significant pressure-drop you are experiencing though. As I understand it you can adjust the pressure by opening a simple choke-valve which vents surplus air into the environment right? In that case I think you are missing a device that (partially) shuts this valve when operating the airbrush to compensate for the pressure drop caused by the airbrush. It is my suspicion that your compressor setup was originally designed for use in static pressure applications and not for airbrushing. Nevertheless I see no reason why it can't be used for airbrushing. Either you leave the setup as it is and take in consideration possible occurrence of paint-blotches while triggering the airbrush (so not triggering on the model). Or you could make a closing valve yourself. Judging from your spray booth you are most likely capable of doing the latter. Here is a picture of the pressure valve on my setup which is an inverse mounted bicycle tyre valve.
small rubber hose covers a small vent hole and can be slided for pressure adjustment. The cloth peg holds the hose in place. HTH.
This sort of pressure drop is common, when the trigger is open the reading on the gauge will indicate the pressure in the line and therefore at the airbrush, there is always an increase on the gauge when the trigger is closed. almost all professional painters will set pressure with the trigger open. Though to be honest most have been doing the job long enough that they would rarely use the gauge at all relying instead on experience
Actually Olle, fluid systems and electronics have quite a lot in common. You could model your setup as a battery (compressor) short-circuited by two parallel connected resistors (airbrush and "waste gate") The two parallel resistors will increase the current (flow) but will also decrease the voltage (pressure) at the battery. So you need a switch to switch between resistors. The faster the switch, the better. The tank can act as a condensator to even out irregularities. Good luck with it.
"Olle Bjurstam" schreef in bericht news:A1KZa.69573\$ snipped-for-privacy@news2.bredband.com... Magnificient explanation. The "waste gate" I have on my misengineered contraption should CLOSE to compensate for the pressure-fall! That way the pressure gauge should show me the correct pressure. Indeeed I think my compressor is for a static setup. I Think I understand what's going on here. (I'm an electronics engineer but working with software. Pressures, temperatures, fluids in motion etc. has always been greek to me.)
The "waste-gate" on my compressor is simply a plug with two holes in the thread which is screwed into a threaded socket in the compressor. If I remove the plug and replace it with something that closes when the pressure becomes to low it would work I think.
Alternatively I screw it shut and ADD a device similar to yours.
Thanks for the idea and explanation.
Let's see, where did I put that bicycle valve...
The problem is perhaps not the pressure drop per se, but how to = understand what pressure is actually measured. I think I understand it a little = better now. The key seems to be to switch the non-regulating waste-gate that I = have for some kind of a regulating one, one that closes when the pressure = gets below a certain limit. As Bassie Adriaensen (?) suggested in another = answer my compressor seems to be made for a static usage (a bit odd since the = instructions tells me not to run it for longer periods than approx 20 min). But then = it seems to be of Italian make ;-)
I looks as if it was made for an airbrush with fixed air flow and moving = needle, if such an airbrush exists at all (shold be my first very old Humbrol with = extrernal mixing and virtually no means to regulate air-flow).
Well, anyway, thanks for the input.
One more suggestion before I'm off; Shouldn't there be a membrane, or something like a steel ball underneath the spring like you say? Perhaps there are parts missing. You're welcome and I hope you will be modelling soon. And don't feel stupid because you obviously don't seem to be to me. Or I'm just as stupid, who cares. We can't know everything... fortunately.
"Olle Bjurstam" schreef in bericht news:pWLZa.69654\$ snipped-for-privacy@news2.bredband.com... You're right. The equations are the same. I think the key point (for me) was that I thought the compressor was made for air-brushing (so I was told) and all I had to do was to understand how to "tune" it. I never thought about the need to have a regulating shunt instead of the one fitted. Your answer actually was a "revelation". Feel a bit stupid...
After looking at the shunt alreday present I can see that they could have made it "regulating" by very small means. There is a spring in it which only makes the "valve" stay in place (it increases the force on the treads so the screw won't unscrew). If they had redesigned it a little and made the spring press on a membrane instead it would have worked better I guess.
I'll see what I can do with this.
No membrane - nothing missing.
Hard to explain without drawings=20
Used fixed font: +-----+ ()+---| | ()| O | ()+---| | +-----+
This is the "plug". It's hollow but the left side is capped.
Just before it gets thicker there are 2 holes. In the far left there is a rubber ring. The thick part is threaded almost all the way out to = the right. You screw this one into the seating so that the rubber ring comes in = contact with the seating. The more you screw it the more air-tight it will be.
When midly "screwed" air leaks past the rubber ring, into the two holes and out of the plug.
Around the part with lesser diameter (to the left) in the picture there is a spring increasing the friction in the threads.
It's a cheap design. =20
snipped-for-privacy@telia.com (Olle Bjurstam) wrote in :
I see what you mean now. You're right.
"Olle Bjurstam" schreef in bericht news:yGMZa.69686\$ snipped-for-privacy@news2.bredband.com... No membrane - nothing missing.
Hard to explain without drawings
Used fixed font: +-----+ ()+---| | ()| O | ()+---| | +-----+
This is the "plug". It's hollow but the left side is capped.
Just before it gets thicker there are 2 holes. In the far left there is a rubber ring. The thick part is threaded almost all the way out to the right. You screw this one into the seating so that the rubber ring comes in contact with the seating. The more you screw it the more air-tight it will be.
When midly "screwed" air leaks past the rubber ring, into the two holes and out of the plug.
Around the part with lesser diameter (to the left) in the picture there is a spring increasing the friction in the threads.
It's a cheap design.
Well the "plug" seems to be meant for venting air allright. In the meantime I googled "Werther International" and found the following pages;
They show a lot of professional looking equipment. Perhaps your compressor is among them. With "static applications" I was thinking of things like pneumatic plungers or other devices that require some fixed pressure without continuous fluid flow. But I can't find specific info on these sites about that. The setups shown on the pages seem to have the "classic" layout without the "switch" device. But then you will need the whole lot (electric pressure switch, moisture trap, reducing valve including gauge) which is a lot more expensive. Frankly I have very little practical knowledge of this kind of stuff (compressors and such), so I was hoping someone else could help you out. Perhaps you should try something like alt.talk.politics.misc for it ;-)
LOL.
Well I'm contemplating buying a bigger compressor anyway. Tax-return is = coming so perhaps it's time to upgrade. See If I can find a reasonably cheap and quiet = compressor with a bigger tank.
The one I got sort of works, but it is a little too weak. I'll try to = find a "real" pressure regulator (they are not that expensive) and fit it onto it and = then we'll see what happens.
Thanx for an educating discussion.
"Olle Bjurstam" schreef in bericht news:jV3_a.57513\$ snipped-for-privacy@news1.bredband.com... LOL.
Well I'm contemplating buying a bigger compressor anyway. Tax-return is coming so perhaps it's time to upgrade. See If I can find a reasonably cheap and quiet compressor with a bigger tank.
The one I got sort of works, but it is a little too weak. I'll try to find a "real" pressure regulator (they are not that expensive) and fit it onto it and then we'll see what happens.
Thanx for an educating discussion.
If you are going to use it for airbrushing I think I'd choose one such as the blue ones from the second page. They are suspiciously resembling a (Italian) refrigerator compressors which are very durable and silent. Again good luck and have fun.
Regards, Ron a.k.a. Bassie Adriaensen (my artist's name)

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