IP pressure drop upon inhalation

[DivesWithTurtles]

1. That doesn't make sense (to a physicist). BTW, did you determine the pressure drop with two measurements, one at each end of the hose between the 1st and 2nd stages? If you only measured at the 1st stage end, all you've measured is the change in IP at the 1st stage (as a function of flow rate). The IP will be different at the other end of the hose - at the 2nd stage input - and it's the difference between these two measurements that defines the IP drop in the hose.

(Note: I don't know squat.)

Huh? Do you mean gases do not exert pressure equally in all directions in a container?

Wouldn't the only time that a difference in pressures would be noted at opposite ends of the hose be during the extremely short lag before equilibrium is reached (whether the equilibrium is in a sealed container or a steady state flow)?

(Note again: I still don't know squat.)

 

[DA Aquamaster]

DivesWithTurtles gets it.

To those who still don't...for the last time - I understand what you are trying to say...air passing though a small hose will encounter drag/resistance on the sides of the hose, flow will be reduced and the pressure will be lower at the other end.

With a much more viscous fluid like water traveling through a much longer hose, it is even true, and it's one of the reasons why you can pump more water through a 5" fire hose than you can two 3" fire hoses.

But the fact is any difference in IP drop in the hose is not going to be significant or observeable with the viscosities, flow mechanics and distances that we are dealing with here. You can say it's so all you want, but you better be saying it with your dive buddies Santa Claus and the Easter Bunny, because believing in it is just not enough to make it true.

 

[donacheson]

DivesWithTurtles gets it.

To those who still don't...for the last time - I understand what you are trying to say...air passing though a small hose will encounter drag/resistance on the sides of the hose, flow will be reduced and the pressure will be lower at the other end.

With a much more viscous fluid like water traveling through a much longer hose, it is even true, and it's one of the reasons why you can pump more water through a 5" fire hose than you can two 3" fire hoses.

But the fact is any difference in IP drop in the hose is not going to be significant or observeable with the viscosities, flow mechanics and distances that we are dealing with here. You can say it's so all you want, but you better be saying it with your dive buddies Santa Claus and the Easter Bunny, because believing in it is just not enough to make it true.

Of course it's not significant so long as get enough air to breath. On the otherhand, 15 or 20 years ago (some? most? all?) reg manufacturers went to larger bore hoses between the 1st and 2nd stages. Whether or not this was just for hype or was to help increase air delivery to the diver I don't know.

 

[Zaphod]

OK, this takes me back about 15 years, but I think the issue is that with the regulator locked up, the dynamic pressure is zero. When we inhale, the flow rate determines the dynamic pressure as 1/2 * p * u^2 with p as the density and u the velocity of the medium (this is the second term in Bernoulli's law). Because the pressure gauge is typically perpendicular to the flow, the total pressure (as measured by the pressure gauge) is p(total) = p(static) - p(dynamic). So, as the flow rate goes up, the total pressure on the gauge should go down at least according to Bernoulli. Of course, this depends on how your pressure gauge is setup. I think this is opposite of what is being touted here as law.

 

[DA Aquamaster]

I think we are saying the same thing.

It takes me back to last weekend, but the Bernoulli tube bolted on the side of my plane that powers the needle on the Turn and Bank Iindicator agrees with you. It's pretty low tech but is none the less designed to accellerate incoming air over a curved surface in order to cause a drop in pressure to produce sufficent vaccuum to power the gyro in the TBI. And it does this very well as long as the velocity of the airflow is above about 20 mph and below the speed of sound.

My regulators all agree with you as well - to the extent that the higher the flow rate, the greater the IP drop overall in a given first stage - that's been stated before.

But the major issue in question is whether the hose lenght itself causes a drop in IP, and I have not been able to demonstrate that it does that to any measurable degree in a scuba regulator.

An issue you do bring up is the potential for variations in pressure in specific areas of the regulator when gas is flowing through the first stage that could produce different IP readings depending on the local flow conditions. And again, it is something you would want to control for in your test conditions.