Can using a longer hose necessitate a pressure change?

dsm · Post by **dsm** » Wed Jun 13, 2007 7:38 pm

[quote="tangents"]Hey dsm, I'm going to have to take that bet...

I think the way the machine operates is to produce pressurized air. The pressurized air produces a flow. The combination of the pressure and flow in your throat keeps it open. The basic gas formula is pv=nRt.
p is pressure
v is volume
n is number of moles of gas
R is a constant number
t is the temperature.

Any length of hose will still have a constant volume throughout the night. So for a constant pressure produced by the CPAP, everything in the formula is constant, therefore the length of hose whouldn't matter. Just as many air molecules should be coming out the mask end as are going in the machine end, and the pressure should be the same, too. The NUMBER of air molecules in the hose is greater (although constant), so that makes the LOAD on the machine greater. That's the only reason I can think of for the pressure to decrease with hose length, if the machine can't pump out sufficient air volume to maintain pressure.

I'll be curious to know what your measurements tell!

Cathy

dsm · Post by **dsm** » Wed Jun 13, 2007 7:51 pm

oceanpearl wrote:Someone please explain to me how there could possibly be "friction loss". pressure in a vessel cannot be less at one point than another.

Considerations -

Air is compressible
Air as John M says, is subjected to losses down a long tube.

But there is a very good reason why some brands of xPAP have a separate air pressure sensing line inside or outside the main air hose. It is because it is the most accurate way to measure the pressure and events taking place at the mask. It allows for varying length air lines. The separate air line doesn't face the same losses as the air is not really flowing through it as it does through the main Air Circuit hose. That ancillary line is sensing static pressure. It is a cost cutting exercise that most manufacturers adopted to not use the ancillary air line.

When Autos 1st came out, they couldn't work with full face masks due to the effect of air buffering in the mask and how that skewed what the Autos detectors were seeing. Nasal prongs kept this buffering small enough that they did work ok. The manufacturers got around that limitation 2 ways 1) was to add the ancillary air line 2) was to enhance the predictability in the software in the machines.

#2 there is a delay in what happens at the mask & when the machine detects it (air compressibility issue again) with fluid that is not an issue hence why air is good in suspension of cars (Citroen & Mercedes) & why fluids are good for hydraulic applications (rams, bulldozers etc) where compressibility is not wanted.

DSM

_________________

CPAPopedia Keywords Contained In This Post (Click For Definition): hose

Guest · Post by **Guest** » Wed Jun 13, 2007 9:07 pm

"Someone please explain to me how there could possibly be "friction loss". pressure in a vessel cannot be less at one point than another."

It is more of the same on - Someone that doesn;t know what they are talking about so you don;t need to concern yourself with it. If one were to increase to hose length to, say, 40 feet from six feet, it would make a difference, but increasing from 6 feet to ten feet would not introduce enough resistance to matter.

ddpelp · Post by **ddpelp** » Wed Jun 13, 2007 10:22 pm

OSAGuy

Bingo!!! Yep exactly what Respironics has already said 10 footer is fine.

jennmary · Post by **jennmary** » Wed Jun 13, 2007 11:57 pm

Why be snippy? How is it helpful to make remarks to the extent that someone does not know what they are talking about?
He is simply testing a theory. I myself have wondered the very same thing and would be interested to hear what the results of his little test are when he is done.

dsm · Post by **dsm** » Thu Jun 14, 2007 12:03 am

[quote="ddpelp"]OSAGuy

Bingo!!! Yep exactly what Respironics has already said 10 footer is fine.

dsm · Post by **dsm** » Thu Jun 14, 2007 2:36 am

[quote="Anonymous"]"Someone please explain to me how there could possibly be "friction loss". pressure in a vessel cannot be less at one point than another."

It is more of the same on - Someone that doesn;t know what they are talking about so you don;t need to concern yourself with it. If one were to increase to hose length to, say, 40 feet from six feet, it would make a difference, but increasing from 6 feet to ten feet would not introduce enough resistance to matter.

dsm · Post by **dsm** » Thu Jun 14, 2007 5:54 am

The results of 1st tests.

In the end it took 3.0 hrs just to get this data & write this post. I went to get the F&P221LE but it was at the bottom of a pile & in its box so I grabbed a Healthdyne Quest that allows the output pressure to be adjusted with a screwdriver & is set using a manometer so at least I was able to set it precisely. Also the Tranquility Quest (respironics) doesn't use a pressure transducer - just pumps out pressure dialled up with a screw driver & set with a monometer.

I then got 3 hoses

1 x 10 foot
1 x 6 foot
1 x 1 foot

I chose a Mirage Std UMFF mask (but also did validity comparisons against a Lge UMFF - no difference).

I set the manometer to 0 & tested it. Then dialled up 16 CMS on the healthdyne. The 8 tests I did were

1 - block air exit at at machine end & check CMS
2 - unblock the air exit at the machine end & measure CMS
3 - Put the UMFF mask on & place against face & hold breath
4 - breath at slow steady rate & measure in & out CMS

5 - block air exit at mask end & check CMS
6 - unblock air exit at mask end & check CMS
7 - put on mask & not breath - measure CMS (at mask end)
8 - breath steadily & measure in/out CMS (at mask end)

I did the above tests
- 1 time for each hose with machine set at 8 CMS
- 1 time for each hose with machine set at 16 CMS

I will load the data into a spreadsheet & add it to my web site tomorrow (tis 9pm here)

But a summary of the results shows these points & some results that I don't understand at all ...

Point 1
Anytime I blocked the air hose at any pressure & no matter what length, the CMS was identical to what the machine was set for (this confirms the point I made that air in an ancillary air line doesn't move and will accurately measure static pressure so length doesn't matter.

Point 2
Air in the main Air Circuit that doesn't move will also exhibit the same pressure any where in the hose no matter what length (same as for an ancillary pressure line)

point 3
When I removed my hand from blocking the air hose and let the air blow free, the manometer showed drastically different data depending on which end it was at.
e.g. With a 10' hose & machine at 16 CMS & hose blocked & manometer at machine end the CMS shows as 16 but unblock the hose & let the air flow freely & the pressure dropped to 10 CMS (manometer at machine end)

Shift the manometer to the mask end of the air hose & let the air flow free & the manometer dropped to -1 (this looked like venturi effect had kicked in as the air was moving so much faster than when the manometer was at the machine end).

point 4
In contrast to point 3 - the difference in CMS reading between a 6' hose and a 10' hose was nill while the hose was blocked & no matter which end the manometer was at.

point 5
With machine set for 16 CMS, the measured CMS with the hose unblocked (air blows free) & manometer at the machine end for 6' hose was +6.75 & a 10' hose was +10.0 (this shows that the length of a hose will affect the amount of back pressure detected at the machine end allowing that the hose was unblocked.

point 6
With the above set up but manometer at the mask end (air blows free) the readings were for 6' hose -1.8 CMS & for 10' hose -1 (this appears to reflect the higher venturi effect of air exiting faster from the 6' hose (less resistance)

point 7
This bit has me a bit foxed any one with a theory please explain it ...
(as stated at the top, the quest is a straight cpap & AFAIK doesn't have a pressure transducer to monitor exit air pressure - checked my photos of its insides) ...

If I then fitted the UMFF mask and put it on my face (no breathing) & allowing it has a high fixed leak rate, I was expecting the CMS reading to drop below 16 due to the fixed leak BUT in all cases & all length hoses & no matter where the manometer was, the CMS rose !!!. Why ? (the machine was not increasing its output ?)

e.g. machine at 16 CMS 10' hose mask on face (no breathing) the CMS went to 17 CMS when manometer at machine end & 16.25 when manometer at mask end. Both were above what the machine was set to & in both cases although I was not breathing & the air was venting thru the fixed leak holes, the CMS rose. I could understand this if the machine had a pressure transducer & was adjusting the motor up to compensate for the fixed leak. So this result has me foxed. The only diff to when the air wasn't moving (hose blocked) is that it was leaking out the mask at the fixed leak rate. This effect was consistent across all hoses.....

so with the mask on my face & no breathing & for a 6' host the readings were 17 at machine end & 16.5 at mask end ???

for a 1' hose the readings were 17 CMS at machine end & 17 CMS mask end.

SUMMARY:
I believe that the tests show conclusively that when the air is not moving the pressure will be constant at any point in the tube which agrees with the point made that pressure in a vessel won't vary around the vessel - but that comment fails to take into account air moving through a long tube as it does with a cpap machine. It is not static..

When air moves through a long tube it meets resistance & what none of us explained before was that the faster the air moves the greater the resistance thus the greater the pressure drop. But I do agree that it was only approxt 0.5 CMS difference between a 6' & 10' tube at 16 CMS when I was breathing with the mask fitted. That is hardly anything to be concerned about (on that point OSA Guy is correct).

But OSA Guy may want to kindly revise his comment in regard to length makes no difference. It sure does

Anyway, it was an interesting experiment & the results were not all I expected but some aspects of this now make sense (faster the air travels, greater the resistance) even if the increased pressure when the mask is fitted doesn't.

Cheers

DSM

roster · Post by **roster** » Thu Jun 14, 2007 6:06 am

jennmary wrote:Why be snippy? How is it helpful to make remarks to the extent that someone does not know what they are talking about?
He is simply testing a theory. I myself have wondered the very same thing and would be interested to hear what the results of his little test are when he is done.

I see the "snippy police" were active last night.

DreamStalker · Post by **DreamStalker** » Thu Jun 14, 2007 7:40 am

Anonymous wrote:"Someone please explain to me how there could possibly be "friction loss". pressure in a vessel cannot be less at one point than another."

It is more of the same on - Someone that doesn;t know what they are talking about so you don;t need to concern yourself with it. If one were to increase to hose length to, say, 40 feet from six feet, it would make a difference, but increasing from 6 feet to ten feet would not introduce enough resistance to matter.

Guest -

By your own logic, you should not concern yourself with posting on this forum ... so go try and keep the masses ignorant elsewhere.

DSM -

Air is also subject to the priciples of fluid dynamics (as it too is a fluid although not a liquid). Your results show that under static conditions, there are no differences in pressure. However under transient or dynamic conditions ... as you are observing, and others have noted, friction in addition to the fluid compressability of air does affect pressure.

Nevertheless, these pressure effects induced by different lengths of tubing are likely insignificant to PAP users but may become slightly more significant for users with the following:

1) high pressure Rx

and/or

2) masks having high CO2 vent flow rates.

and of course ...

3) increased tube length and/or air circuit volume.

Additionally, as previously mentioned, it would seem logical that the detection/recording mechanisms of auto machines may also be affected by tube length ... to what degree, I have no idea as I do not fully understand the working mechanisms of auto machines.

So ... although I too agree with Guest's concluding statement, I disagree that CPAP knowledge should only be kept in the hands of privileged RT's or that patients should maintain their ignorance about their treatment and only concern themselves with giving up their minds and money to them.

John_M · Post by **John_M** » Thu Jun 14, 2007 8:02 am

dsm wrote: If I then fitted the UMFF mask and put it on my face (no breathing) & allowing it has a high fixed leak rate, I was expecting the CMS reading to drop below 16 due to the fixed leak BUT in all cases & all length hoses & no matter where the manometer was, the CMS rose !!!. Why ? (the machine was not increasing its output ?)

e.g. machine at 16 CMS 10' hose mask on face (no breathing) the CMS went to 17 CMS when manometer at machine end & 16.25 when manometer at mask end. Both were above what the machine was set to & in both cases although I was not breathing & the air was venting thru the fixed leak holes, the CMS rose. I could understand this if the machine had a pressure transducer & was adjusting the motor up to compensate for the fixed leak. So this result has me foxed. The only diff to when the air wasn't moving (hose blocked) is that it was leaking out the mask at the fixed leak rate. This effect was consistent across all hoses.....

I think this shows three things.

1/ The pressure drop down a 10' hose under normal operating conditions is 0.75 cms H20

2/ Somehow the properties of the machine (most probably the characteristics of its pump) is that its output pressure rises when its delivering about the normal flow rate. This is a property of the machine and most probably varies between machines.

3/ The manufactures of the machine got it about right. The machine was set to deliver 16 cms H20 and actually delivered 16.25 cms H2O at the mask.

Thanks dsm for all those measurements. I am looking forward to seeing your results as this will tell us how the pressure drop down an operating hose changes with hose length. If a 10 ft hose has a pressure drop of only 0.75 cms H2O it doesn't sound a big deal.

John M

_________________

CPAPopedia Keywords Contained In This Post (Click For Definition): hose, manometer

tangents · Post by **tangents** » Thu Jun 14, 2007 8:33 am

John M Wrote:

2/ Somehow the properties of the machine (most probably the characteristics of its pump) is that its output pressure rises when its delivering about the normal flow rate. This is a property of the machine and most probably varies between machines.

This, I believe, is the pump operating under load. The CPAP is designed to have resistance (flow limitation) at the mask end. I believe, dsm, that that is why your open-ended tests (free flow) gave pressure readings so much less than the machine setting, and the pressure rose very close to the setting when you put the mask on.

Very interesting thread. Thanks for donating your time last night to test this theory, dsm. Great reading!

Cathy

Goofproof · Post by **Goofproof** » Thu Jun 14, 2007 10:13 am

tangents wrote:John M Wrote:
2/ Somehow the properties of the machine (most probably the characteristics of its pump) is that its output pressure rises when its delivering about the normal flow rate. This is a property of the machine and most probably varies between machines.
This, I believe, is the pump operating under load. The CPAP is designed to have resistance (flow limitation) at the mask end. I believe, dsm, that that is why your open-ended tests (free flow) gave pressure readings so much less than the machine setting, and the pressure rose very close to the setting when you put the mask on.

Very interesting thread. Thanks for donating your time last night to test this theory, dsm. Great reading!

Cathy

Test results may also be flawed due to location of testing. Down Under, fluids flow the opposite direction, from fluids north of the equator. The spiral wrap of the hose, may effect the flow due to the spin factor. One location the flow would spin, with the spiral, the other would have to jump over the wire, and would waste more energy. Jim

DreamStalker · Post by **DreamStalker** » Thu Jun 14, 2007 10:25 am

Goofproof wrote:
tangents wrote:John M Wrote:
2/ Somehow the properties of the machine (most probably the characteristics of its pump) is that its output pressure rises when its delivering about the normal flow rate. This is a property of the machine and most probably varies between machines.
This, I believe, is the pump operating under load. The CPAP is designed to have resistance (flow limitation) at the mask end. I believe, dsm, that that is why your open-ended tests (free flow) gave pressure readings so much less than the machine setting, and the pressure rose very close to the setting when you put the mask on.

Very interesting thread. Thanks for donating your time last night to test this theory, dsm. Great reading!

Cathy
Test results may also be flawed due to location of testing. Down Under, fluids flow the opposite direction, from fluids north of the equator. The spiral wrap of the hose, may effect the flow due to the spin factor. One location the flow would spin, with the spiral, the other would have to jump over the wire, and would waste more energy. Jim

Makes perfect sense to me

dsm · Post by **dsm** » Thu Jun 14, 2007 3:09 pm

Goofproof wrote:
tangents wrote:John M Wrote:
2/ Somehow the properties of the machine (most probably the characteristics of its pump) is that its output pressure rises when its delivering about the normal flow rate. This is a property of the machine and most probably varies between machines.
This, I believe, is the pump operating under load. The CPAP is designed to have resistance (flow limitation) at the mask end. I believe, dsm, that that is why your open-ended tests (free flow) gave pressure readings so much less than the machine setting, and the pressure rose very close to the setting when you put the mask on.

Very interesting thread. Thanks for donating your time last night to test this theory, dsm. Great reading!

Cathy
Test results may also be flawed due to location of testing. Down Under, fluids flow the opposite direction, from fluids north of the equator. The spiral wrap of the hose, may effect the flow due to the spin factor. One location the flow would spin, with the spiral, the other would have to jump over the wire, and would waste more energy. Jim

Mask

Additional Comments: Encore Pro 1.8

Can using a longer hose necessitate a pressure change?

Pressure drop doesn't sound like a big deal