Longer hose cause pressure loss?

[georgepds]

The pipe pressure loss for a 2 m (~6 foot) pipe is given by the following
P = [1/2 row v^2] cd (wetted area)
= [0.5*1.225*1]*1*[2*0.08]
= 0.1 Pa
Or converting to our favorite unit ( cm of h20) , the pressure drop for a 2 m (~6') pipe is 0.001 cm h20 . For a 6 m pipe, it would be double.

This number is so low compared to the ~10 cm h20 for a typical cpap setting it is negligible. It does not matter if the pressure loss is 0.001 or 0.002 cm H20, it is 4 orders of magnitude (ten thousand times) smaller than the delivered pressure.

If you want to use a long hose, don't let pressure considerations alter your decision

--G

~~~~ details~~~~~~~
row ~1.225 kg/m^3
v ~ 1 m/s = ( volume flow/ pipe cross section area)
cd ~1
pipe diameter = 1" (0.0254 m)
pipe circumference = pi*diameter = 0.08 m
pipe length = 2 m
wetted area = pipe circumference * pipe length

[dsm]

The pipe pressure loss for a 2 m (~6 foot) pipe is given by the following
P = [1/2 row v^2] cd (wetted area)
= [0.5*1.225*1]*1*[2*0.08]
= 0.1 Pa
Or converting to our favorite unit ( cm of h20) , the pressure drop for a 2 m (~6') pipe is 0.001 cm h20 . For a 6 m pipe, it would be double.

This number is so low compared to the ~10 cm h20 for a typical cpap setting it is negligible. It does not matter if the pressure loss is 0.001 or 0.002 cm H20, it is 4 orders of magnitude (ten thousand times) smaller than the delivered pressure.

If you want to use a long hose, don't let pressure considerations alter your decision

--G

~~~~ details~~~~~~~
row ~1.225 kg/m^3
v ~ 1 m/s = ( volume flow/ pipe cross section area)
cd ~1
pipe diameter = 1" (0.0254 m)
pipe circumference = pi*diameter = 0.08 m
pipe length = 2 m
wetted area = pipe circumference * pipe length

George,

A good formula but you haven't taken into account the speed of the air down the tube & the frictional losses which are substantial.
Using a dial manometer and a pressure of 12 CMs and breathing normally, the loss is approx 1 CMs per 6'.

Cheers DSM

[dsm]

Shooting from the hip, pressure drops in tubing of that diameter don't hold a candle to pressure drops from the holes in the CPAP mask.

The bigger problem is likely control. The more air volume you have in the system you could see one of two effects
1) The volume attenuates the pressure fluctuations in the mask back at the transducer and from the machine to the mask causing the machine to be less responsive in attempting to compensate for breathing
2) The volume becomes tuned to the frequency of breathing. Like an organ pipes of different lengths or like a pop bottle filled to different levels. This could actually amplify pressure fluctuations from breathing for all devices and confuse an APAP like crazy.

John,

Your logic is on track, but the comment 'confuse an APAP like crazy' is perhaps a little harsh

The machines seem able to deliver acceptable therapy despite the measurable losses observed over a 6' tube. What I have noticed from having used a variety of machines that have/don't have, an added proximal (or internal) pressure sensing line is that I do need to set the CMs about 1 CMs higher to get the same therapy results on the machines that don't run the added sensing line to the mask.

i.e. machines that all have an added mask pressure sensing line ...
PB42x family
PB330
Vpap Adapt SV

Those that don't run a mask pressure sensing line ...
Respironics Bipap family
Respironics M series Cpaps & pre M series cpaps
Resmed Vpap III family & earlier
Resmed Autos (excluding Autoset-T and the Vpap Adapt SV )
F&P
Etc:

DSM

[dsm]

Just to add to this often confusing subject ...

When air is not moving in a tube the pressure will equalize end-to-end in the tube (i.e. as an example, the air does not flow at all in the secondary mask pressure sensing tubes - they are completely static & thus can accurately measure pressure at a distance).

Where there is no airflow, If there is say 12 CMs at the mask end of the tube, no matter how long, there will be 12 CMs at the machine end and a sensor there will read 12 CMs. This is in essence what George has pointed out (very accurately). But, as soon as the air starts to flow two things begin to happen ...

1) there are frictional losses that increase as as the speed of the air increases
2) the air flow begins to become turbulent and can cause standing waves which will vary depending on the diameter of the tube & its length & speed of the air and how the air enters the tube (smoothly or turbulently).

#2 is not too big a deal at the speeds we breathe at - pipe organs rely on standing waves to generate particular frequency sounds & the diameter of the pipe also affects the volume of a particular frequency (combined with the length of the pipe).

#2 standing waves do cause pressure variance from one peak to the next in the standing wave - i.e. if you could sample the pressure at each inch along a cpap tube and air was rushing through at say one litre per minute (minute volume), and a standing wave of say 3 ft developed, the pressure will vary a small amount up and down at each sampling point plus will additionally vary downward with length due to frictional losses. But to restate, I believe this aspect is minimal at the speeds we breathe through the air hose.

Just to repeat a fair rule-of-thumb - at 10 CMs and with a 6' cpap tube, expect a loss around 1 CMs when breathing at approx 0.5 lpm of Minute Volume (the reality may be more like 1 CMs of pressure loss at 0.550 lpm of Minute Volume and 12 CMs of pressure).

DSM

[georgepds]

The pipe pressure loss for a 2 m (~6 foot) pipe is given by the following
P = [1/2 row v^2] cd (wetted area)
= [0.5*1.225*1]*1*[2*0.08]
= 0.1 Pa
....
George,

A good formula but you haven't taken into account the speed of the air down the tube & the frictional losses which are substantial.
Using a dial manometer and a pressure of 12 CMs and breathing normally, the loss is approx 1 CMs per 6'.

Cheers DSM

Hi,

Could be wrong, but not for the reason stated.

The speed of the air and the frictional losses are in there. The drag coefficient cd represents the skin friction losses, and v is the mean velocity in the tube. The "extent" of the tube is caught in the wetted area.

But, if you say you used a manometer, I defer to measurement

-G

[DSM(guested)]

The pipe pressure loss for a 2 m (~6 foot) pipe is given by the following
P = [1/2 row v^2] cd (wetted area)
= [0.5*1.225*1]*1*[2*0.08]
= 0.1 Pa
....
George,

A good formula but you haven't taken into account the speed of the air down the tube & the frictional losses which are substantial.
Using a dial manometer and a pressure of 12 CMs and breathing normally, the loss is approx 1 CMs per 6'.

Cheers DSM

Hi,

Could be wrong, but not for the reason stated.

The speed of the air and the frictional losses are in there. The drag coefficient cd represents the skin friction losses, and v is the mean velocity in the tube. The "extent" of the tube is caught in the wetted area.

But, if you say you used a manometer, I defer to measurement

-G

George

Somewhere in this forum is a long thread going back 2+ years where I ran a series of tests at various pressure, with various machines & using both 1 x 6' hose & 2 x 6' (12') hoses & I published the results showing manometer readings with static flow & with dynamic flow at a range of pressures.

Trouble is I can't find that thread as I sure don't want to do all those tests again

Also I am quite certain there is a statement in one of the respironics manuals that mentions the dynamic losses in a 6' hose & it matched my research. But again, I have searched but can't find that posting.

If I do I will extract the data & put it on my own site so I know where it is

Cheers

DSM

[DSM (login is failing)]

George

I just did a more thorough search & found some of the data I posted back in 2007 ...

viewtopic.php?f=1&t=21043&p=182502&hili ... f6#p182502

Cheers DSM

[dsm]

Here was an earlier set of tests & post (see the post that shows as being at 12:03 am )

viewtopic.php?f=1&t=21043&st=0&sk=t&sd= ... rt=15#wrap

DSM

It starts off as follows (am having lots of trouble trying to get the correct link to the right post ...

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)

etc: etc: etc:

[john_dozer]

Shooting from the hip, pressure drops in tubing of that diameter don't hold a candle to pressure drops from the holes in the CPAP mask.

The bigger problem is likely control. The more air volume you have in the system you could see one of two effects
1) The volume attenuates the pressure fluctuations in the mask back at the transducer and from the machine to the mask causing the machine to be less responsive in attempting to compensate for breathing
2) The volume becomes tuned to the frequency of breathing. Like an organ pipes of different lengths or like a pop bottle filled to different levels. This could actually amplify pressure fluctuations from breathing for all devices and confuse an APAP like crazy.

John,

Your logic is on track, but the comment 'confuse an APAP like crazy' is perhaps a little harsh

The machines seem able to deliver acceptable therapy despite the measurable losses observed over a 6' tube. What I have noticed from having used a variety of machines that have/don't have, an added proximal (or internal) pressure sensing line is that I do need to set the CMs about 1 CMs higher to get the same therapy results on the machines that don't run the added sensing line to the mask.

i.e. machines that all have an added mask pressure sensing line ...
PB42x family
PB330
Vpap Adapt SV

Those that don't run a mask pressure sensing line ...
Respironics Bipap family
Respironics M series Cpaps & pre M series cpaps
Resmed Vpap III family & earlier
Resmed Autos (excluding Autoset-T and the Vpap Adapt SV )
F&P
Etc:

DSM

You're probably right. The natural frequency inside the system is probably too high. A dimension of 6' has a natural frequency of ~165 Hz.

But there's a few things that may make suggestion not seem quite as really far fetched.

You only need to light off a quarter wavelength which cuts the frequency requirement by a fourth.

The system isn't necessarily just the tube as a rigid vessel. The tube is flexible to a degree and some masks in my experience are very flexible possibly drawing down the effective stiffness and thereby the natural frequency.

Then the volume of air in the entire system may come into play as well. that includes air in the water tank, in the CPAP base unit and the users respiratory system. The addition volume could also lower the natural frequency.

Finally, the fan used to create the pressure may generate frequency content that approaches the natural frequency of said system depending on its vane count and spacing. Meanwhile the motor itself, running off of AC current, could have 60 Hz (and multiple of 60 Hz) artifacts. As well as having an artifact related to its own rotation speed. This is not to mention the system's motor (not just pressure) control.

[dsm (guested again)]

Good points John,

One thing I do know is that the motors are nearly all DC brushless and the newest ones have variable speed controllers. Some of the vanes on the blowers are shark fin profile to aid smooth airflow.

Cheers

DSM

[CollegeGirl]

Hey Victoria,

In regards to your much less complicated topic - - hose hanging -

I'm sure you probably haven't had a chance to go through every single one of those wonderful posts on the links RG posted, so I wanted to call your attention to the cheap, easy item I use:

The 3M Cord bundler.



It's being shown "in use" - the gray cords going through it are just to show you how things hang in it.

You can usually find this at a local Lowe's or Walmart, or, barring that, you can generally order them quite inexpensively online.

As you can see, you would just put your hose through where those cords are. It's just big enough for that, and still allows the hose to move freely. And the best part is it attaches to the wall (or your headboard...or whatever else you want) with 3M Command strips - so you can take it down and move it whenever you like without damaging your walls or furniture. They come in packs of two, so I keep one on the wall above my bed, and one in my cpap case for travel. I've used it on countless hotel walls, and never damaged a single one. I couldn't live without mine!

Good luck!

[daveuk]

Resmed machines I have used have a hose length setting for 2 and 3 metres. If used with a Resmed integrated humidifier, the 3 metre setting is automatically disabled. Resmed's user manual says only 2 metre hoses should be used with a humidifier. The machine cannot compensate for the combined pressure loss caused by humidifier and 3 metre hose. I don't know if the same applies to other makes, but the principles are the same. If the machine can't compensate for the longer hose, don't use one.

[scrollinggrandpa]

Hi all. I am using a Resmed S9 with humidifier. I was using a 6 foot hose but changed to a 10 foot hose. I also use the Airfit P-10 mask. My pressure is supposed to be set at 11.

After changing to the longer hose I noticed I wasn't feeling as well in the mornings. I have one of those pressure testing attachiments so I put it on the end of the 10 foot hose and turned my machine on. The meter said I was only getting a pressure of 8 instead of 11. I know the 6 foot hose gave me a pressure of 11 at the end as I had checked it. I accessed my machine and uped the pressure out of the machine to 14 and the meter read 11 at the end of the hose. Guess this proves that a longer hose will require adjustments to your machine. Hope this helps someone.

[Goofproof]

No not if your machine is working correctly, all real machines have a 10 ft max, it's more a problem with data collection being dampened by too long a hose. Apap's might suffer signal loss too. Jim

[Wulfman...]

Hi all. I am using a Resmed S9 with humidifier. I was using a 6 foot hose but changed to a 10 foot hose. I also use the Airfit P-10 mask. My pressure is supposed to be set at 11.

After changing to the longer hose I noticed I wasn't feeling as well in the mornings. I have one of those pressure testing attachiments so I put it on the end of the 10 foot hose and turned my machine on. The meter said I was only getting a pressure of 8 instead of 11. I know the 6 foot hose gave me a pressure of 11 at the end as I had checked it. I accessed my machine and uped the pressure out of the machine to 14 and the meter read 11 at the end of the hose. Guess this proves that a longer hose will require adjustments to your machine. Hope this helps someone.

Did you change the internal settings?
The ResMed machines used to have settings for hose length, but I couldn't find them for the S9 in the clinical manual. Especially for the 10' tube option. But, they do have settings for the following. Some are automatically detected. There are still apparently different options for tube diameter.


From the manual:

The S9 device is compatible with the following tubing:

Tube Specifications Settings

SlimLine
Length: 6’ (1.8 m)
Inner diameter: 0.6” (15 mm)
Default setting

Standard
Length: 6’6” (2 m)
Inner diameter: 0.75” (19 mm)
If using the standard tubing, adjust the tube
setting in the Patient Setup or Clinical Setup
menus.

ClimateLine - Heated
Length: 6’6” (2 m)
Inner diameter: 0.6” (15 mm)
Automatically detected


Den

.