IP pressure drop upon inhalation

[DivesWithTurtles]

How much should the ip pressure drop on a good hard breath?

The reg is an Apeks TX-100. The tank had 3000psi. I hooked up an ip gauge to the inflator hose and got a solid 138psi with no creep over a couple of minutes (spec is 135 +/-5). Took a couple of good hard breaths, and with each inhale the ip dropped to about 123, then jumped back to 138 immediately after each inhale.

Is that much drop right as expected?

 

[DA Aquamaster]

Less IP drop usually reflects faster response time and greater flow rate, so less IP drop is better, and less IP drop is associated with higher performance first stages.

So what is "normal" depends on the performance of the first stage design.

I get around 8 psi with a Scubapro Mk 25 and 10 psi with a Mk 17, and about 20 psi with a MK 2 or Mk 3. So 15 psi is still a comparatively good number and is about what I would expect for an ATX-100.

 

[redacted]

Less IP drop usually reflects faster response time and greater flow rate, so less IP drop is better, and less IP drop is associated with higher performance first stages.

So what is "normal" depends on the performance of the first stage design.

I get around 8 psi with a Scubapro Mk 25 and 10 psi with a Mk 17, and about 20 psi with a MK 2 or Mk 3. So 15 psi is still a comparatively good number and is about what I would expect for an ATX-100.

And in this situation, it may be just as much a question of the response time of the guage and the method of connection than of the regulator. I use a Craftsman compression guage. With my SP Mk10 and the guage connected to the inflator hose, I can get a 10 to 25 psi drop depending on how much load I put on the 2nd (normal breathing to strong purge).

 

[Rick Murchison]

Don't forget IP hose ID and length. The longer the hose and the smaller the ID, the more the drop at the entrance to the second stage, even if there is no drop at the first stage. The drop you cite at the end of the LP inflator hose should be a pretty good representation of the drop at the first stage, and I would classify it as "excellent performance."
And of course the real bottom line is WOB at the mouthpiece, where IP is just one factor in several.
Rick

 

[DA Aquamaster]

Pressure is pretty uniform despite the size of the restriction and is a separate issue from flow rate.

This morning I took a high performance second stage (a D400) that will sustain a freeflow at about 65 SCFM. I ran the test with the same tank and Mk 17 first stage but swapped my 7 foot hose after the first trial for a 20 inch hose. The IP drop was the same - 22 psi - in an established full blown freeflow regardless of hose lenght. And with a hard inhalation (a much lower flow rate) the IP was still the same at 10 psi regardless of hose lenght.

If you want another example, consider your SPG, the restiction is very tiny and the flow rate is very small so it takes a few seconds for the SPG to come up to full presure even with 3000 psi behind the incoming gas.

So in a sense you are correct, a small restriction can have a serious impact on flow rate and a lower or higher flow rate will potentially impact IP drop with a lower flow rate aiding a first stage struggling to maintain IP. But a longer hose of a given restriction is not going to have any effect on IP, at least until we get to VERY long hose lenghts, where pressure to the second stage itself may drop due to dynamic flow/drag effect. But then, you have another factor coming into play - the internal volume of that very long hose provides a very large internal volume of IP air upon which to draw which will again provide a buffer to a regulator with a slow response time.

You are also correct that the second stage makes a difference. If you have an high performance second stage in full freeflow mode at 65 SCFM you can expect significanlty more drop in IP than if you had a low performance second stage with a flow rate of only 30 SCFM.

But in general, the higher the flow rate the first stage is capable of producing, the less IP drop you should see.

 

[donacheson]

Pressure is pretty uniform despite the size of the restriction and is a separate issue from flow rate.

This morning I took a high performance second stage (a D400) that will sustain a freeflow at about 65 SCFM. I ran the test with the same tank and Mk 17 first stage but swapped my 7 foot hose after the first trial for a 20 inch hose. The IP drop was the same - 22 psi - in an established full blown freeflow regardless of hose lenght. And with a hard inhalation (a much lower flow rate) the IP was still the same at 10 psi regardless of hose lenght.

.

All you're really saying is that WITH THE SETUP YOU USED, pressure drop in the hose is probably not significant but that in the first stage dominates. Your data contradicts the second half of your first statement; pressure [drop] does depend upon flow rate.

 

[DA Aquamaster]

What I am saying is that:

1. That for a given inside hose diameter, the drop in pressure will be the same regardless of hose lenght (within practical limits).

2. I agree that the IP drop is meaningless if you don't specify the test conditions. A hard inhale on a regulator results in relatively little flow rate and will inflict a commensurately small hit on IP (ie: 10 psi with a Mk 17 D400). On the other hand, a sustained full blown freeflow has a much higher flow rate and inflicts a much larger hit on IP (ie: 22 psi with a Mk 17 D400).

3. I am also saying that a long hose will not cause a greater drop in IP and if you are blaming an excessive drop in IP on a long hose, you are mistaken and need to look elsewhere for the problem.

Just for kicks I just did the same tests with the same D400 second stage on a Mk 3 first stage and got a 35 psi drop in IP. This was exactly what was expected from the comparatively low performance first stage. But again there was no difference in IP drop between the long or short hose during a sustained maximum rate freeflow.

So it's pretty safe to say that regardless of first stage performance, adding a long hose should not reduce IP as in a worst case full blown freeflow from a high performance second stage situation, the extra lenght made zero difference in IP on high, medium and low performance first stages.

4. Finally, I am saying that all other things being equal and under the same conditions, a higher performing first stage will have less IP drop than a lower performing first stage.

 

[redacted]

What I am saying is that:

3. I am also saying that a long hose will not cause a greater drop in IP and if you are blaming an excessive drop in IP on a long hose, you are mistaken and need to look elsewhere for the problem.

Just for kicks I just did the same tests with the same D400 second stage on a Mk 3 first stage and got a 35 psi drop in IP. This was exactly what was expected from the comparatively low performance first stage. But again there was no difference in IP drop between the long or short hose during a sustained maximum rate freeflow.

So it's pretty safe to say that regardless of first stage performance, adding a long hose should not reduce IP as in a worst case full blown freeflow from a high performance second stage situation, the extra lenght made zero difference in IP on high, medium and low performance first stages.

When you say long/short hose I'm fairly sure you are refering to the hose attached to the operating 2nd and that make good sense. But I am under the imperssion that the lenght of hose (or lack of hose for a direct connection) to th IP gauge will also cause a variance in observed IP drop. I expect the most accurate reading (and greatest drop) would be obtained with the gauge connected directly to an LP port.

And I'm pretty sure that when you refer to flow rate it is in the 1st stage between the HP & IP chambers and not that of hoses.

 

[DA Aquamaster]

As long as the smallest restriction in the inflator hose is adequate to support the very very small flow required to equalize the amount of air in the IP gauge, it will have no effect on the IP drop as measured by the gauge whether it is on an LP port, on the end of an inflator hose, or attached between the LP hose and second stage. Worst, case, the needle will lag very slightly and that actually may not be a bad thing. If the gauge were on an LP port and there was substantial turbulence there, the needle could possibly dance a little. Placing it at the end of a hose dampens any inaccuracy that would otherwise result.

Personally, I prefer an in-line gauge between the LP hose and second stage as it is the IP at the second stage that really counts. Practically speaking, I have also found that very few gauges actually agree with each other and will vary a few psi. So its entirely possible that you could have a gauge on an LP port, an in-line gauge at the second stage and a gauge on the end of an inflator hose and get three different readings, but the difference would be due to gauge error, not pressure drop in the hoses. I suspect this is where some of the confusion develops.

With regard to regulator flow rate, it's the flow rate at the second stage that matters as it is always the limiting facotr that will dictate the maximum flow rate that will occur in the first stage (unless of course you purge two regs at the same time.) After all if you did use some type of hose with a very tiny ID that restricted flow rate, you would get less IP drop due to the reduced flow rate, but that would not mean your first stage was performing better, it would just mean less performance was being demanded of it.

 

[donacheson]

What I am saying is that:

1. That for a given inside hose diameter, the drop in pressure will be the same regardless of hose lenght (within practical limits).

2. I agree that the IP drop is meaningless if you don't specify the test conditions. A hard inhale on a regulator results in relatively little flow rate and will inflict a commensurately small hit on IP (ie: 10 psi with a Mk 17 D400). On the other hand, a sustained full blown freeflow has a much higher flow rate and inflicts a much larger hit on IP (ie: 22 psi with a Mk 17 D400).

3. I am also saying that a long hose will not cause a greater drop in IP and if you are blaming an excessive drop in IP on a long hose, you are mistaken and need to look elsewhere for the problem.

Just for kicks I just did the same tests with the same D400 second stage on a Mk 3 first stage and got a 35 psi drop in IP. This was exactly what was expected from the comparatively low performance first stage. But again there was no difference in IP drop between the long or short hose during a sustained maximum rate freeflow.

So it's pretty safe to say that regardless of first stage performance, adding a long hose should not reduce IP as in a worst case full blown freeflow from a high performance second stage situation, the extra lenght made zero difference in IP on high, medium and low performance first stages.

4. Finally, I am saying that all other things being equal and under the same conditions, a higher performing first stage will have less IP drop than a lower performing first stage.

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.

3. Not necessarily so. Gotta measure the IP at both ends of the hose.

4. True, but only for the change in IP at the 1st stage output.