Living at altitude & a manometer

[mraroid]

I live at about 2,225 m (about 7,300 feet).  I do not know if my old unit (Philips Respironics PR System One AUTO 750P) will compensate for my altitude.  I just read that the unit I have ordered (Dreamstation Auto BiPAP DSX700T11) has some internal adjustment for altitude.

Well, no way to know for sure.  I read this very excellent tutorial here on building a water manometer:

http://www.apneaboard.com/forums/Thread-...ew-dollars

It had a link to suppler # 2 for a "store bought" manometer.  I thought it would be best to test my broken, one foot in the grave Philips Respironics and see what it is putting out as well as checking my new unit when it arrives.

I ended up buying  (from suppler # 2) the unit they have for sale ($35.00 + free shipping in the US).  But now that I bought it, I wonder how accurate it it?  I think a water manometer build is in my future.

Anyone living at altitude report any issues?  I occasionally spend the night in Quito which is about 2900 m (9,400 feet). I want my unit to work and not sure if I need to adjust anything....

mraroid

[quiescence at last]

I am guessing 9400 ft is high even for the new adjusting type. but, if you are on auto settings, it will still adjust to your needs. you just may not be able to say average was really 13.5 cm, cuz it might actually be more like 13.3 cm. what what what? .2 cm inaccuracy? no big deal.

Hey, the moderate concern is that altitude affects oxygen percentage, and thus may result in brain and body confusion. generally it leads to higher CA for a while, but if you return to lower (relative) altitudes in a few days, it will just be a blip.

QAL

[Sleepster]

I think a water manometer build is in my future.

With the machine on, insert the end of the hose into a tub of water. Bubbles will of course form. Lower the end of the hose until the bubbles stop. The depth of the hose end at which the bubbles just stop forming is the pressure. For example, if it has to be submerged to a depth of 15 cm to get the bubbles to stop, the pressure is 15 cm H2O.

[Sleeprider]

Of course a manometer measures differential pressure, not absolute pressure. So a tube of pressurized air at an absolute pressure, inserted into a tub of water can be submersed deeper at high altitude, just as water boils at a lower temperature at high altitude (note baking instruction for half-baked ideas takes longer at high altitude). There is a great article on this called "Manometer Basics" on the Sensormag site. I'd do the link, but I have not had good luck with posting links in spite of being a montor, so do a search.

The Dreamstation adjusts up to 5000 feet MSL and I'm not quite sure what it does. The pressure applied in CPAP and BiPAP is also a differential pressure, so while it takes less absolute pressure to apply 5 cm of pressure at 9000 feet than it does at sea level, I have no idea why that matters. If you find out, let us know and I'll put it my vast reservoir of not-so-important facts that I constantly reference.

I would think that differential pressure is not going to change with altitude, as long as there is not a near vacuum, because it only measures pressure against atmospheric pressure, which is not a constant with altitude. I'd far rather discuss the adiabatic lapse rate for temperature with altitude which is a constant. This raises another interesting point as absolute pressure is a function of both altitude and temperature. So even though the atmosphere gets colder as we go higher, air pressure drops faster due to less overlying air mass. If you think about it enough, you might just condense into a cloud of rain.

I really should have saved this for my 6000th post.

[OMyMyOHellYes]

I suspect that the "pressure adjusting" feature is more about marketing than therapy. CPAP operates on the principal of differential pressures, adding only a little tiny small bump in overall pressure to your airway.

And it is not using an absolute pressure of say 8 cm of water. It is operating at a pressure that is 8 cm higher than the surrounding air.

One thought to consider for anyone that has used or contemplated using a CPAP or like device on a commercial flight - the pressure in the cabin of pretty much any airliner is going to the equivalent of being at 8,000 above sea level. Some newer lines like the 787 may have a better pressure differential and cabin pressurization may only be 5,000 feet.

OMMOHY

[ajack]

I think a water manometer build is in my future.

With the machine on, insert the end of the hose into a tub of water. Bubbles will of course form. Lower the end of the hose until the bubbles stop. The depth of the hose end at which the bubbles just stop forming is the pressure. For example, if it has to be submerged to a depth of 15 cm to get the bubbles to stop, the pressure is 15 cm H2O.

That my friend is a gem of lateral thinking. So simple and effective.

[justMongo]

I don't see a problem with linking to the article referenced by Sleeprider:
http://www.sensorsmag.com/components/manometer-basics

I prefer an analog gauge like the Dwyer Magnehelic Series 2000 Differential Pressure Gauge, Range 0-20 cm WC. It's easier to see the slew rate on an analog gauge; with a digital, it's blur of numbers. The model 2000-20CM is available from Bezo's online emporium for $63.50. It is a differential gauge with two ports.

[mraroid]

JustMongo....

I wished I lived in the US where I could just order something and have it delivered. The post office in Ecuador is not working, so anything that Ecuador will allow into the country needs to arrive via FedeX, DHL or some private carrier. And then you pay the import duty fee. I did this once. A hand full of capacitors a few resistors and some diodes. FedeX shipping - $165. Import duty on the $12.00 worth of parts? $52.50.

I really like that analog Dwyer Magnehelic Series 2000. But I think I will need to build my own water manometer. I need it for my broken PR System One AUTO 750. The new unit I ordered - DreamStation Auto BiPAP DSX700T11 probably works fine. But it will be fun to test it.

Thanks everyone for the good feedback

mraroid

[justMongo]

...So a tube of pressurized air at an absolute pressure, inserted into a tub of water can be submersed deeper at high altitude...

I am going to disagree with this statement. Although what is said about differential and absolute pressure is true.
It takes the same pressure to lift a 13 cm column of water at sea level as it does at 10,000 feet above mean sea level.
Why, because the pressure head of a 13 cm column of water is the same. P = rho x g x h. rho is the density of water, g is the gravitational factor, and h is the height of the column of water. g does not change significantly from sea level to 10,000 feet MSL.

Now, here's how I think a CPAP machine works: The CPAP pressure sensor is an absolute pressure sensor. The machine will spin up to try to produce the set pressure (in this case 13 cm-water) at whatever altitude it is at. There is a rated ceiling altitude for CPAP machines because the air is less dense at higher altitude. The blower turbine will spin faster to compress air and comply with the set pressure, until it cannot comply because of maxing out the blower RPM. If the machine can comply -- that is to say, put out 13 cm-water at 10,000 feet, it can lift a 13 cm column of water.

The same goes for bubbles in a tub of water. it's not the air pressure on the surface of the water that's a factor, it's the gravity head of the water.
If the machine can put out 13 cm-water pressure, it can blow bubbles at depths at or less that 13 cm deep.

Yes, water will boil at a lower temperature at 10,000 feet, that's vapor pressure of water versus the air pressure.

I hope that makes sense.

[Sleepster]

The Dreamstation adjusts up to 5000 feet MSL and I'm not quite sure what it does.  The pressure applied in CPAP and BiPAP is also a differential pressure, so while it takes less absolute pressure to apply 5 cm of pressure at 9000 feet than it does at sea level, I have no idea why that matters.  

In other words, CPAP machines are concerned only with the difference between the pressure inside the hose and the pressure outside the hose. So when the CPAP pressure is set at, for example, 10 cm, the pressure inside the hose is 10 cm greater than the pressure outside the hose,* making the pressure in your airway 10 cm greater than the pressure in your bedroom. This is why pressurizing your bedroom (making it a hyperbaric chamber) will not take the place of a CPAP machine, hose, and mask!  

Therefore one would naturally wonder why it's necessary for the machine to make adjustments for altitude. If I have 10 cm of pressure in Houston (at sea level) it means the same thing as having 10 cm of pressure in Denver (the mile high city).

I think the reason is because of the way CPAP machines measure flow rate. They pass air first through one pressure sensor and then a second pressure sensor. The drop in pressure between the two sensors can tell you how fast the air is moving, but that determination is a function of the air density. The air in Denver is less dense than the air in Houston, so this has to be compensated for if the machine is to be able to accurately determine flow rates. Knowing the flow rate is necessary because the machine needs to know how fast to spin the impeller to maintain the pressure. This the real engineering feat of the CPAP machine: it maintains the same pressure regardless of how fast the patient is inhaling (or exhaling).

*Strictly speaking, the air in the hose is moving, so the pressure is not the same all along the hose. To deliver the correct pressure at the mask end of the hose the pressure at the machine end must be a bit higher. This is why you have to tell the machine what kind of mask you have (or in the case of Respironics machines, set the resistance).