Pressure regulator positioning for HVLP guns

[Patrick Chase]

Inspired by a discussion with John a while back, I decided to try moving some of my pressure regulators (particularly for smaller/touch-up guns) "off-gun". My guns were already set up with 6 to 8 foot, 3/8" diameter "lead-in" hoses that are directly attached to the gun via swivel fittings (no quick-connect fittings). Moving the regulator is therefore a simple matter of moving it from between the gun and swivel fitting to the opposite end of the hose. Doing so makes the gun much more maneuverable in tight spaces.

The main concern with this sort of configuration is pressure loss. HVLP conversion guns take relatively high airflows at low pressure, and that's basically the worst case for loss in a hose (just as low-voltage, high-current is the worst-case for transmission loss in a wire). The issue is compounded by the fact that it's flow-dependent - adjusting, say, the fan valve on the gun can change the flow rate and therefore the pressure drop. This turns out not to have been as serious an issue as I thought. though. Gates has a handy pressure loss calculator here, and for a typical conversion gun taking 10 scfm at 20 psi, the loss amounts to 0.16 psi/foot, or ~1 psi at 6 feet for a 3/8" ID hose.

Note that smaller hoses are very, very bad news. The corresponding loss rate for a 1/4" hose is 1.37 psi/ft (i.e. about 8x worse than the 3/8 hose). Conversely 3/4" hoses only lose 0.004 psi/foot under the same conditions, which is why they're sufficient for non-conversion (i.e. turbine) guns.

All in all it's a positive change IMO. I have to reach a bit further to adjust pressure, but the maneuverability benefit is worth it.

[John TenEyck]

I'm glad the change has worked out for you Patrick. I had a mini regulator right at the inlet of my old HVLP gun, but my new gun has a regulator at the inlet that controls the pressure in the 3M PPS cup. Adding the mini gun pressure regulator to that was very unwieldy so I moved it 20' upstream, in front of the 3/8" retractable hose that goes to the gun. I haven't noticed any appreciable change in how I have to adjust the pressure regulator or gun settings. I guess a 3.5 psi change isn't significant with the products I'm using. One of these days I'm going to follow your lead and make up a 6 or 10' whip and move the cup regulator to the upstream side. That will make the gun "smaller" and more maneuverable in tight quarters.

John

[Warren Lake]

I dont like length in the gun. in the past i removed the regulator and put it in the hose at my left hand, I always have the hose in my left hand. When manufacturers got smart I got Sata stuff and a nice compact gun with a digital regulator in the side of the gun, the copiers followed likely several manufacturers have some config of that now. Its excellent when you are pushing the limits to be able to easily read a digital readout and see the pressure difference even one pound an make.

[Patrick Chase]

One thing I should have mentioned: Positioning the regulator remotely only works with diaphragm-style pressure regulators, that provides a relatively constant output pressure independent of flow rate.

A lot of common "air adjusters" (for example the cheaper 3m one) are basically cheater valves with pressure gauges, and the resulting pressure reduction is flow-dependent. They don't reduce output pressure at all when the gun isn't firing. The problem with using one of those on the end of a hose is that you end up with a bunch of high-pressure air in the hose when you don't fire the gun, and you have to bleed that out before the flow stabilizes (for example by pausing for a little bit with the trigger partially back such that the air is engaged but the needle is closed). Such adjusters work much better when directly connected to the gun, since there isn't as much "dead volume" for that to accumulate.