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Over the last few days a lot of speculation and BS has been posted on the
difference between hose diameters, so I decided to do a little research an=
d
not spout off uninformed opinion. The following information is from BINKS =
Inc.
a major supplier of air hoses and painting equipment.

The 3 major factors that effect pressure drop in hoses are inside diameter=
,
length, and internal smoothness of the hose. I will try to describe the
effects that these 3 factors have in simple terms.

Internal diameter: Air flows through a hose faster in the middle then at t=
he
sides. This is due to friction of the air hose interface and shear between=
 the
air molecules close to the walls and those a little further away. In pract=
ical
terms a 1/4 inch hose will have up to 20 times more friction then a 1/2 in=
ch
hose.

Hose length: The pressure loss due to the friction in a hose is proportion=
al
to it=92s length, so a 10 foot hose will have two times the pressure drop =
as a 5
foot hose.

Hose smoothness: A rough internal surface can increase the friction by as =
much
as 50% compared to a smooth hose. Many paint shops where long air hoses ar=
e
used to supply the spray guns will use a nylon or Teflon lined hose to
minimize roughness.

The expected pressure loss due to the above factors are: (These are based =
on a
consumption of 10 CFM so any SCUBA applications will be much lower, the
comparison tables just do not go down the volume level we use)
=09=09=09=09=09=09         =09=09=09=09=09 Pressure drop (PSI)
Hose ID=09=09Pressure Drop per Foot (PSI)=093 foot hose=099 foot hose
5/16" =09=09=090.12=09=09=09=09=09=09=09=09.36=09=091.08
3/8"=09=09=09.06=09=09=09=09=09=09=09=09=09.18=09=09.54
1/2"=09=09=09.014=09=09=09=09=09=09=09=09.042=09.126

As you can see the pressure drops are low and I don=92t think you could te=
ll the
difference between them on a second stage whip. The decreased pressure dro=
p
will be quite noticeable in a 50-75 foot surface supplied deco rig so the
larger ID hose should be used and a lined hose might be justified. 

Now any painter will tell you that valves have a much greater effect on ai=
r
flow and pressure drop. As to the effect of internal first and second stag=
e
design, others on this list are much better informed then I am. I will sta=
te
that if you are using a surface supplied deco rig you should not use quick
disconnect fittings as these will cause a large drop in flow/pressure. If =
you
use a shut off valve where the second stage whips connect to the feed line=
,
use only ball or a large gate valve, needle valves just will not perform.
Anyone who has spent 20-50 minutes on a hard breathing deco knows how wipe=
d
out you will be when you get back on the boat.

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