Can using a longer hose necessitate a pressure change?

Feels like I need more air with the longer hose. Is this possible? Seems to make sense. My AHI is very good, but I feel like I am really trying to "pull in" the air at the lower setting (which is 10, by the way).

Also, would I feel like I am getting less humidity? AM I getting less humidity?

Thanks.

Blessings...Mary

Mary,

How long is the hose ?

The std length is 6' if you go to 10' it may verywell affect your CMS setting.

In fact I have both 6' & 10' hoses plus can set up a cpap & use a manometer at the mask & will test what happens. (may take a day before I get back).

Am also interested in what might show. Reason I say this is I have done tests with a Bipap Auto & can show that the ipap epap gap shows as 2 CMS at the machine end of the hose using a dial manometer, but when I place the same manometer at the mask end it shows an (approx) 1.25 CMS gap rather than the 2 at the machine end.

So am leaning toward a 10' hose showing an guestimated 0.5 pressure loss over a 6' hose.

DSM

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

I am in total agreement with Tangents. It's just like water pressure.

tangents & oceanpearl are correct the longer hose is fine. When I first got into this in March the 6 foot hose was too short for my bed moving during the night I bought a 10 foot hose and THEN thought about if it made a difference. I emailed Respironics direct and their folks said you could use two 6 footers together and they have tested their machine, per the Respironics rep stated in the email their machine will read pressure just fine I was worried also about reaction to apneas etc but they claim no problem .. Sooo I guess it is if we believe the mfg. I do .. and from the last few months seems they are right

If I can find the old email from them I will post it here...

i never was very good at physics, which explains why i didnt make it far as an engineering student, but one thought: would the length of the hose affect air pressure over distance in any way due to friction? especially given the 'wavy' texture of some hoses. though, even if it didnt it might be marginal at best.

[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

as short as it is here is the cut and paste of the reply direcly from Respironics

====

Mr. XXXXX, We have been told that you may use 2 6ft sections of tubing without any problems occurring with the autoalgorythm. The 10 foot tubing will work fine.

Regards, Service at Respironics


--------------------------------------------------------------------------------

From: Comments
Sent: Monday, April 09, 2007 10:27 AM
To: Service
Subject: FW: Contact Us - Information Request

tangents wrote: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.
----
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.

Here are some other factors to consider.

As air is moving down the hose, there is a frictional loss as the air moves down the hose. so there is deltaP due to frictional loss.

deltaP here is the difference in air pressure between the beginning of the hose and the end of the hose.

The longer the hose, the more friction , the greater the deltaP. This is most probably the deltaP that dsm is measuring. So with a longer hose on CPAP/BIPAP its easy, just increase the pressure settings to compensate for the extra deltaP.

But detection of stoppages, apnea's, is another story. When they happen, there is an immediate pressure change at the body as the increased resistance of the apnea causes a drop in flow rate and increased pressure at the apnea site. This pressure change propagates back up the hose to the machine. The increased volume of the longer hose will diminish and most probably slow down the rate of rise of pressure at the machine thus giving the apnea "signal" the machine receives a different shape and reduced amplitude. Will the machine recognize this ? Could it miss it? Is it possible that machines will give falsely lower reports of AHI due to a longer hose?

Also a longer hose will most probably reduce the effectiveness of an APAP.
When an APAP machine senses an event, it will respond by increasing its pump speed and thus raising the source pressure. This will cause increased volume to flow down the hose. But a longer hose has a greater volume to fill so it will take longer for the effect of the increased pump pressure to be felt at the apnea site. Is this important? I don't know.

I wish I could calculate or measure these effects, but right now I can't. The manufacturers of machines have presumably investigated all this stuff and say a 10 ft hose is OK.

I once wanted to have a 10ft hose. Snoredog cautioned me against it. I want the best treatment I can get. I re-arranged things so I am now happy with my 6ft hose. The machine is located next to my bed on a specially constructed shelf so it sits 1" below the top of the mattress, is next to the mattress on my side and has an elbow joint so the hose comes directly out of the machine pointing towards the hook on the wall.

John M

Sidebar on leakage rates.

The old-timers on the board keep on stressing the importance of keeping leak rates down.

Are they concerned about the increased deltaP due to the increased flow rate causing a difference between source pressure and delivered pressure?

Are they concerned about the inability of the machines to deliver the increased volumes of air required with an increased flow rate?

Are they concerned about the fact that the pressure signals due to apneas will be diminshed with high leak rates?

Or, sigh, have they just observed that high leak rates reduce the effectiveness of treatment?

Hey, thanks, John and Seamonkey! Those are indeed interesting considerations. I knew in my head there was an analogy to stringing too many Christmas lights on a circuit causing the last string to dim, but couldn't make the connection to air flow and frictional losses. Eureka! It'll still be interesting to see dsm's measured pressures. I will cheerfully eat my words if I'm proven wrong, and a 10 foot hose isn't the same as a 6 foot.

Cheers,
Cathy

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

Here are my thoughts on this. (which are not based on any scientific evidence)
I've DO have two 10' hoses.....neither of which I've ever used, so I can't comment on any personal experiences with one.

With regard to testing with a manometer at a constant pressure, you probably would not see any difference.
The only scenario(s) where it might make a difference is on an Auto, with regard to the detection, response and scoring of events......and, with regard to humidity coming from a heated humidifier, it would have a larger/longer surface to cool the air.

Other than that......

On the subject of "Science", I was saddened to see that Don Herbert passed away. I'm sure many of us grew up watching his TV show. It doubt that it helped me in my school science studies, but it was always fun and interesting to watch.
I've pasted one of the stories from the WEB, below.

Den (still believing it's not the length of the hose that counts, but how you use it)


TV's 'Mr. Wizard' Don Herbert Dies at 89
Jun 12, 6:41 PM EST

The Associated Press

LOS ANGELES -- Don Herbert, who as television's "Mr. Wizard" introduced generations of young viewers to the joys of science, died Tuesday. He was 89. Herbert, who had bone cancer, died at his suburban Bell Canyon home, said his son-in-law, Tom Nikosey.

"He really taught kids how to use the thinking skills of a scientist," said former colleague Steve Jacobs. He worked with Herbert on a 1980s show that echoed the original 1950s "Watch Mr. Wizard" series, which became a fond baby boomer memory.

In "Watch Mr. Wizard," which was produced from 1951 to 1964 and received a Peabody Award in 1954, Herbert turned TV into an entertaining classroom. On a simple, workshop-like set, he demonstrated experiments using household items.

"He modeled how to predict and measure and analyze. ... The show today might seem slow but it was in-depth and forced you to think along," Jacobs said. "You were learning about the forces of nature."

Herbert encouraged children to duplicate experiments at home, said Jacobs, who recounted serving as a behind-the-scenes "science sidekick" to Herbert on the '80s "Mr. Wizard's World" that aired on the Nickelodeon channel.

When Jacobs would reach for beakers and flasks, Herbert would remind him that science didn't require special tools.

"'You could use a mayonnaise jar for that,'" Jacobs recalled being chided by Herbert. "He tried to bust the image of scientists and that science wasn't just for special people and places."

Herbert's place in TV history was acknowledged by later stars. When "Late Night with David Letterman" debuted in 1982, Herbert was among the first-night guests.

Born in Waconia, Minn., Herbert was a 1940 graduate of LaCrosse State Teachers College and served as a U.S. Army Air Corps pilot during World War II. He worked as an actor, model and radio writer before starting "Watch Mr. Wizard" in Chicago on NBC.

The show moved to New York after several years.

He is survived by six children and stepchildren and by his second wife, Norma, his son-in-law said. A private funeral service was planned.

I have used a 10 foot hose for over a year, this month I went with a 6 foot because it was here. Changing the hose made no difference in treatment, except that the hose is a little short. And that's using APAP.

The machine works correctly using a 10 foot hose, over 10 foot isn't advised especally with APAPs due to the chance of response being altered.

The ribs in the hose do offer resistance to flow, but as long as the machine can maintain it's output level the pressure drop doesn't affect treatment.

John_M

Are they concerned about the inability of the machines to deliver the increased volumes of air required with an increased flow rate?

Are they concerned about the fact that the pressure signals due to apneas will be diminshed with high leak rates?

Or, sigh, have they just observed that high leak rates reduce the effectiveness of treatment?

These are what concerns me, and my 10 foot hose doesn't affect any of them. Jim

Can't say for certain what a longer hose does to the pressure. I do know that my machine has a setting option for 'tube length', 2 m and 3 m. Don't think they put that in there for nothing. This may be something unique to ResMed machines, though.

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.

Hi Oceanpearl

What you say is true but isn't this really a dicussion of the pressure being less in one vessel than in another

Sleep deprived

JimW

think you are right but your machine is manual settings the one I have auto senses hose see above what Respironics advise no problem