Originally posted by Laurence Stein DDS
I beg to differ. What you call noise is compensated at the second stage adjustment and we do not live in this horizontal world we have imagined. The cracking pressure on your reg is measured in inches of pressure and in fact it is just a fraction of an inch between cracking and flow. The 0.5 psi/ft is nearly an order of 10 greater than when compared to a fraction of an inch cracking pressure and assuming a tank is about 2 and 1/2 feet long, there is nearly 3 pounds of difference in the horizontal position. This is no longer "noise".
Swimming in a horizontal position this might not be noticeable but descending and ascending (and possibly tired) on a line, lobstering, checking holes--for those who spearfish--I can assure you that it does matter and it does wear you out.
At least my Monday is starting off on an entertaining note...
How many mistakes, let me count the ways...
1) Your math is wrong, 2 1/2 half feet would be 1.25PSI, Ill even round up to 1.5PSI, which still isn't close to 3PSI.
2) Directly comparing cracking pressure and IP pressure is like comparing apples to oranges. If the valve orifice that the gas moved through in the 2nd stage was 1 square inch in area, you might have a point, but it's a tiny fraction of an inch by design, lessening the mechanical force required to keep it closed. In addition the valve opening is assisted [typically] by a fulcrum on the lever that's against the diaphragm which allows those inches of water to overcome the either-side-of-the-valve delta.
3) As I stated in my first reply, if the 1st stage is lower than your second by (we'll use your number) 6 feet/3PSI it'll deliver 3PSI MORE IP to the 2nd stage, in theory making breathing EASIER assuming the typical downstream 2nd stage valve.
4) This also goes for your on your back example. Great theory but you got it backwards. On your back the IP delivered to your 2nd is GREATER not less so it should make your breathing easier, not harder.
5) Youve confusing how a snorkel works versus how a second stage works. Ignoring for the moment that your reasoning is backwards, (points 3 & 4) you would be correct for a snorkel. But again, the 1st and 2nd stage combination of regulators isnt a snorkel.
6) Using your 3PSI (6 foot number) again, if you were trying to overcome a 6 foot deficit of pressure you bet itd wear you out -- in one breath specifically since NO ONE can pull air through a 6 foot snorkel from the surface!
7) Lastly a qualitative argument -- if you were right, the hookah systems we use at the Aquarium wouldnt work. For the small exhibits we leave our cylinder topside on the path, hook a 1st stage up to it that has a very long hose to our 2nd stage and we go to the bottom of the exhibit which is about 6-8 feet. And -- sorry, youll have to take my word for it -- we can breathe just fine.
Important career tip: Stick to dentistry.
However, I have this nagging suspicion from your authoritative statement Remember your first scuba lessons. In your first post that youre a SCUBA instructor. Please, please tell me youre not.
Originally posted by Laurence Stein DDS
I suppose you have never experienced the increased inhalation effort while lying on your back? It IS caused by the relative difference in the first and second stages horizontal position and it IS caused by a relatively minor difference of about 6 inches.
Yes, I've experienced the harder breathing. No, it's NOT caused by the pressure differential between the 1st and 2nd stage (again, you've got it backwards). Time for you to stop listening to the dive shop monkeys and reason this through for yourself. I assume you had to take some physics along the way to becoming a dentist. Take a deep breath, forget what the local shop and/or mainstream instruction has told/taught you and figure this one out for yourself. Its really quite simple once you get past the diveshop/training voodoo stories.
Roak
) pole! 
