Pressure Drops from hose

[Finny]

Just wondering if anyone knows the amount of pressure loss from flexible hose.
I don't have a gauge at the gun.
I do remember seeing a formula but it seemed very complex.

If I have 100psi exiting the tank
and a 15m length of flexible hose, (10mm it says)
what would be the pressure at the gun ?

[MeFB]

Assuming that you have a constant 100psi at the tank, which you won't, then it comes down to flow rate. No flow = no loss in pressure. As flow increases, so will the losses. It's probably more important to know if your compressor has the puff to keep up.

[Finny]

Thanks, I hadn't considered that it would vary, but that makes sense.
I have a 3hp 17cfm, which I bought new, but really hasn't had too much work yet.

I was probably looking for a rough guide.
Under 90% use (On - 90%, Off - 10%). Would I have 90, 70 or 50 psi at the gun. Or a range (from 80 to 90)
Yes I could buy another gauge, but my spending is limited.
And it already has 2, one where it comes out of the tank, and another just before the flexible hose joint.

[ekstevo]

If you only have 15 meters of hose don't worry about pressure loses alone the way. Id have 20 meters and a 12 cubic foot compressor it keeps up with my star gravity gun. reg set on around 60 psi.

[Finny]

Found a note on pressure drop which I thought was interesting.



But I decided to buy a gauge to go on the actual gun.
Didn't check this but it appeared that 75psi leaving the compressor, ended up being a consistent 50psi at the gun when spraying.

[rosco]

Hi Finny,
sorry I didn't get to this one earlier.
I should probably ask why you are concerned with any pressure drop in the hose line..?
I read you are using a 10mm inside diameter (pic) air line - and should be commended for keeping away from the little 6mm.

At 15m you are probably losing around 10% of your static pressure through a choked outlet at the end of the line, and this is heavily dependent on what the orifice of the choke is, how wide the spray pattern of the gun is and indeed - the viscosity of the paint - but I believe we must appreciate that with industry standard spray application connectors and an appropriate air line - the critical threshold is what is actually coming out of your gun.... there are so many variables that it would probably be foolish to approximate what pressure is passing through the gun with a known (depending on how accurate the gauge is) delivery pressure from your supply connector.. a gauge near the gun would be cumbersome - and would indicate only what pressure is likely to be entering the gun.....

My suggestion would be to ensure that your regulator/filter do not impede the free passage of delivery air.... there are some very el-cheapo regulator/filters on the market - I initially had one of these and when using a little 3 cfm compressor with a touch up gun, struggled to achieve any form of fan using 6mm hose..... until I ditched the regulator and purchased a "real" one.... if you can track one down and are prepared to pay the price - Norgren - I don't believe there is a better one on the market for back-yarders - the quality and performance are worthy of the outlay ... it's black, has an orange backed water indicator (will take a pic if you need one).

With a compressor of 17 cfm - you outdo my little industrial Peerless twin by 3 cfm.... I could up the motor and reduce the size of the driven pulley (the same air pump is used on the Peerless 17) - but I manage quite well with it and have not had to wait to regain pressure/volume - even with the largest of my guns.

I would like to know what gun you intend to use... most of my full body/roof work was done with an suction fed Iwatta W77 with a 2.0 mm needle - stated to use minimum of 50 psi - but I found it more advantageous to increase this to 60 at the final regulator/filter before it reached the air hose... I have absolutely no idea what was coming out of the gun or through the hose - but my belief is that you might be looking for a suggested starting point...?
I have dabbled a bit with gravity fed guns using acrylic - with excellent results ... but I prefer the Iwatta for laying down big jobs...

In reality, I would suggest you experiment a little with your gun settings for the paint - you'll quickly learn what the gun needs by the pattern you get.....
With a suction gun, if you have the gun pressure (and I assume it has a pressure knob) at full, you paint is at the correct viscosity for the atmospheric conditions and you find that the pattern is split - you don't have enough pressure/volume....it will more likely "spit" and flutter - rather than spray a full fan....
In contrast, if you have to "throttle" down the gun pressure valve by a lot - you are probably over-working your regulator/lines.

If the gun does not have a pressure valve - you are very much dependent on the supply regulator, the length of the air line will of course cause the pressure variation to balloon high/low a bit as you operate the gun trigger.... but at a constant delivery and flow once the air-line/gun balance - there should be little difference in result....

With a gravity gun, my understanding is that the paint will continue to flow by the weight of it in the cup - but it won't atomize correctly, nor will the "wing" jets fully flatten out the fan - resulting in a heavy, coarse and somewhat mottled coverage... I don't have any documentation or much experience with gravity guns... these comments are entirely from experience... and mainly using the smaller 0.5 /1.0 / 1.5 mm touch up guns...

Many people spray vehicles with a 12 cfm compressor - with a gun which has a needle orifice of 2.0 mm.... this is really asking a lot - especially if using 6mm hose... the compressor will begin to struggle as soon as it reaches the lower setting of the pressure governor....

Most don't appreciate the key to all this is not only pressure - but volume...and recovery (the ability of your supply line to quickly return to its static setting).

Use large connectors, 10 mm or even 12 mm hose, a quality/high output filter regulator and a compressor capable of maintaining pressure/volume for the gun you intend to use.... from what I read, you have most of this covered..

Sorry about the detail... fingers running on the red line again.....

frats,
Rosco

p.s. - didn't read the two lines at the bottom of your above post...... drop of 25 psi - your gun uses a lot of volume..... now that you know the drop - set your regulator to 75 and remove the gauge for this job.....

[Finny]

Awesome Rosco.
I do miss your feedback.
Lots of great info, as always.
I'll check my guns. I don't think I have the details on them anymore.

[rosco]

I get a "bit" carried away when I post, Finny.... won't ever change, I believe the forum understand this.....

At the end of the day, ditch all your concern regarding pressure drops etc...... if you can get your gun to shoot paint with a result which brings a smile and a slight note of pride from your work - you're there .... and we should be asking you for advice...

frats,
Rosco

p.s. - how'd I do?... shortest one from me for a while, folk.....

[jack_fc]

I get a "bit" carried away when I post, Finny.... won't ever change, I believe the forum understand this.....
frats,
Rosco

p.s. - how'd I do?... shortest one from me for a while, folk.....

I'm disappointed, Rosco.....

(like many others, I enjoy your lengthy but very informative posts)

[Finny]

I have a few guns now.
Some in better condition than others.
One my son used a few years ago, and when opened was still half full of paint.
Which ended up on the floor of my garage, because I didn't expect it to be full of paint.

Anyhow current guns.




And this was the pressure gauge/regulator that I use on the guns.
Fairly quick to swap it over on each gun, but as you said Rosco, once I know the pressure at the gun is in range, I probably don't need it.
Just need to check that the pressure doesn't change as I adjust the screw. Otherwise it could be quite different when removed.
Its not difficult to swap, just screws on and screws off, so I might leave it in place.


Unfortunately the Tradetools is the newest and probably the best condition, with a 2mm.
It's what I will probably use for the body in the end.
The gravity feed gun doesn't even have a name, I think it was a SCA (supercheap brand) and I found it difficult to use. It was also a HVLP gun and I'm not sure it if needs to have different pressure to the others.
I only got it for doing the roof because I thought if less likely to hit to car with the gun.

Just praying for some good weather so I don't get any more clouding in the paint. Humidity isn't getting under 60% too often.

[rosco]

Thanks Finny,
do not for one moment look at the outside of any gun to note its condition... yes, poorly cleaned outside - but it's the internals which must be strictly cleaned after use... and the nozzle/fan cap..
I don't believe you can appreciate how disappointing and frustrating it is to almost complete a flawless pattern and coating over a panel - then have some 'fleck" of paint dislodge and bury itself deep into your layers.... if one breaks away - there's every chance anything else will let go as well.....

I like the Iwatta - they are an exceptional brand and were used in large proportions in the trade before pressure drums came on the scene.

Sadly, and I mean this with great ambition for you - it is the wrong time of the year to spray - unless you have access to a humidity and temperature controlled booth... high humidity and low temperatures will fight you every step of the way. You might manage to lay one coat down with some degree of success - but the next one is likely to bloom up and dishearten you....

It gets worse..... if you do manage to lay your coats down - and trap any humidity within (which I firmly believe will occur) - you won't know until the weather dries up and the humidity tries to get out.... causing blisters in the now semi-cured acrylic...

I like to spray with no more than 50% relative humidity... and won't consider even mixing up paint unless I know the temperature is going to be at least 24 deg. C.... they are my base limits....trying to find that down here at this time of the year is only likely in a booth.... you might have higher opportunity up where you are - at least the humidity should be a lot lower than during summer for you, but you really need the temperature to and low humidity to afford yourself success.....

Back to guns - I'm a victim of my own insanity/obsession here, Finny - I completely strip every gun after every job.... not tomorrow, but now!
As soon as the last of the paint is laid down, I leave the spray area and start my routine.... empty paint cup, blast 1/3 cup of thinners through onto something until it runs clear.... the disconnect everything and start to strip the gun.... piece by piece, cleaning it in solvent then wiping it dry - cleaning again before air-drying it with an air jet... lightly smear some thin oil over the needle, the valves and packing washers.
I leave everything just nipped up by fingers then wrap the entire gun in linen and pack it away..... this is the degree of my insanity - but my locked steel cabinet holds every gun in this condition ready for a quick strip, clean and final assembly....

I note that you are using the same connectors as I - and I believe these to be the easiest and most user friendly - they also have large porting which affords a high volume of delivery when needed.

As a suggestion for spraying your roof - before I commenced mine - I fitted some overhead hooks to the ceiling of my garage and suspended the air hose from them. I spent a day making up a platform on which to walk along each side of the car and did quite a few "dry runs" with gun, hoses and clothing to ensure I wasn't going to drag anything into the paint.... amazing what you'll find can easily brush over the roof as you lay your patterns....

Speaking of which - what worked for me was to commence at the edge of one side and work to the middle - quickly run around to the other side and pick up the wet edge and continuing outwards to the other edge.... I learned to achieve this when applying the primer coats... it was a cinch to follow the same procedure when laying down the top coats.
But I do heartily agree - suction cup paint guns and roofs (rooves?) are another chapter and require a fair bit of thinking through before actually commencing.
I found it advantageous to have lighting on both sides of the vehicle - just above the height of the roof... this afforded opportunity to watch the reflection of the pattern as it went on - and is my indicator as to how slow/fast/close/far I work as I pass along the run.... I like to achieve a high gloss (wet) reflection but just enough to achieve one... to slow/near and it will puddle up and run.... hope that makes sense...

Ok - enough - for those who were disappointed with my previous brevity - I do very much hope I have quenched you needs.....

Finally, Finny - if you find the urge to attempt cleaning of your chosen (if not all) gun.... I would suggest soaking the head in thinners for a day or so... but be very careful not to submerge the air valve assembly - the seals will perish if subjected to a dunking for a sustained period... If acrylic was used in the gun, a volatile (Motospray Superglow) thinners will more than likely soften it and allow you to clean out the passages and tip assembly....

frats,
Rosco

[Finny]

Thanks again for the ideas Rosco.

I'm sure I will get some decent days when it comes time to paint. But I'm still not quite there yet.
I will clean out the Iwata gun, as it was pretty good. It was the one my son used that still had the paint.
The Sampson I don't know much about , it was one my father had.
The other three. The Gravity feed, the Tradetools and the Scorpion, are all guns I have brought New and I have followed a very through cleaning process after every use so far. Probably just the outside that I haven't worried about too much.

When I did the roof on the wagon I went from front corner to rear corner, back and forth, and swapped sides just the once when I got to the middle line of the roof.
Would have been a lot easier it I could have just turn the whole car on its side.
Don't think I could handle the constant swapping of sides, and the wagon roof does make it a bot more challenging.

[rosco]

Finny, I must have confused you with my dribble...... your method is exactly what I do for the roof... start at the edge of one side then work towards the centre - on reaching the centre, I change sides (walk around to the other side) and pick up the wet edge then continue towards the edge of the remaining side..... i.e. - work from one side backwards and forwards working away from myself across to the centre, stop at the centre - change sides to stand from then work outwards towards myself backwards and forwards from the wet edge at the centre until I reach the final run immediately in front of me...

Hope that clears it up..... like muddy water....

frats,
Rosco

[Finny]

I can get confused really easy, so don't take all the credit Rosco.
But I still think you have a different approach.

They say a picture paints a thousand words, so I probably say just as much as you do.
The following is what I did, and was planned to do again next time.
Hope you like my wagon picture.

[rosco]

Your picture - my thousand words, Finny - yes, that's my procedure...

frats,
Rosco