Technical Question

Ichabod · Post by **Ichabod** » Wed Nov 22, 2006 10:38 am

I am in Hydraulics for a living and deal with pressure (PSI) and flow every day. I can not get my brain around cm h2o. I assume that is a measurement of pressure and not flow. Could someone explain this to me?

Thanks again,
Mike

DreamStalker · Post by **DreamStalker** » Wed Nov 22, 2006 10:43 am

That is correct. cm H2O is a measurement of pressure (force per unit area) ... it is the pressure required to elevate a column of water by X number of cenitmeters.

Flow is measured as a volume per unit time. In other words, flow will always have some component measure of time in the unit(s) used ... ie. cubic feet per second, liters per minute, gallons per day, bushels per week, barrels per month, acre-feet per year, etc.

- r

Doodeedoo · Post by **Doodeedoo** » Wed Nov 22, 2006 10:51 am

The cm of water unit measures pressure. This is the metric measure. If you want to have an idea of how much this is in imperial units, one cm of H2O is equivalent to 0.0142 pounds per square inch. Thus, 10 cm of water is 0.142 pound / sq inch. (compare with tire pressure of 30 psi)

Best regards,

Doodeedoo

Ichabod · Post by **Ichabod** » Wed Nov 22, 2006 10:58 am

That now makes sense to me. Does anyone know the maximum flow of my machine? Not that it really matters, I am just curious. My machine is basic and I don't have the ability for the feedback that most of you on the forum have so I can just be curious about stuff.

Thanks

Goofproof · Post by **Goofproof** » Wed Nov 22, 2006 1:09 pm

10 cm of water pressure in a car tire, would be called a flat. Most machinea have a flow spec of 30 liters per minute at maxium pressure. Which doesn't make since, as mask flow rates are higher than 30 lpm. Jim

curtcurt46 · Post by **curtcurt46** » Wed Nov 22, 2006 6:37 pm

Ichabod,
I too don't understand the Maximum Flow, but for what its worth the book says:

Max Flow: <35LPM
measured in accordance with EN ISO 17510-1@8, 16.5, and 25 cm H20@ 500 ml with BPM set to 10, 15, & 20 BPM @ 23 degrees C (+ or - 2 degrees C), 50% RH (+ or - 5%), and an atmospheric pressure of 101.54 kPascals.
I follow all but don't know what BPM is.
Let us know if you can lend some light on understanding.

DreamStalker · Post by **DreamStalker** » Wed Nov 22, 2006 6:58 pm

curtcurt46 wrote:Ichabod,
I too don't understand the Maximum Flow, but for what its worth the book says:

Max Flow: <35LPM
measured in accordance with EN ISO 17510-1@8, 16.5, and 25 cm H20@ 500 ml with BPM set to 10, 15, & 20 BPM @ 23 degrees C (+ or - 2 degrees C), 50% RH (+ or - 5%), and an atmospheric pressure of 101.54 kPascals.
I follow all but don't know what BPM is.
Let us know if you can lend some light on understanding.

BPM = Bars Per Minute = rate of pressure increase [woops ... ommit this part per Christine's post below]

Pressure is affected by gravity so the elevation or distance from the Earth's center of gravity will affect pressure measurements relative to atmospheric pressure (101.54 kPascals). Also, the compostion of the air (or gases composing the air) affect the measured pressure which is why Relative Humidity (RH) is standardized. In addition, because of the Universal Gas Law -- Pressure times Volume are proportional to a universal gas constant times Temperature (PV=nrT). Therefore, Temperature (23 degrees C) and Volume (500 ml) also affect the measure of pressure.

When measuring pressure it must be measured realtive to specific standards that affect it's value. All of the ENISO numbers refer to the standards used.

christinequilts · Post by **christinequilts** » Wed Nov 22, 2006 7:15 pm

BPM on all my BiPAP ST and now ASV stuff is Breaths per Minute.

Bregory · Post by **Bregory** » Wed Nov 22, 2006 7:46 pm

If it is as goofproof says a rated flow of 30 lpm at the maximum pressure, it makes sense that there could be more flow at lower pressures. I don't know much about this type of blower but generally they have this type of characteristic. Pressure increases as flow decreases and flow increases as pressure decreases. So at lower pressures there is for flow.

I'm not sure I'm helping but this could shed some light on why your leak rates could be higher than 30lpm if the blower is only rated for 30 lpm.

curtcurt46 · Post by **curtcurt46** » Wed Nov 22, 2006 7:51 pm

DreamStalker,
Thanks for the additional information.
Can you explain the difference between machine flow of 35 lpm max and a mask leak rate of 32 lpm at 10 cm. I know they are different, but don't quite have a grasp. Leak and flow must be different.

poopedknot · Post by **poopedknot** » Wed Nov 22, 2006 9:38 pm

The Puritan Bennett GoodKnight 420 specs say their max flow is 120 l/m including leak rate, could it be that Respironics's max flow is not including mask leak rates?

rested gal · Post by **rested gal** » Thu Nov 23, 2006 4:49 am

I kinda' liked this explanation about how breathing out while standing in a swimming pool feels:

Feb 28 2005 subject: Science behind CPAP Pressure
rwguinn wrote:
I have posted what the pressures mean elsewhere.
for example, 25cm H2O is about 10 inches of water. That is a low pressure. To demonstrate:
The center of my lungs is about 10 inches below my chin. If I stand in a swimming pool, with the water up to my chin, the water exerts a pressure exactly equal to what a person on maximum CPAP therapy of 25cm H20 would feel. The difference is that it helps me exhale, and I have to inhale against it.
CPAP helps me inhale, and I have to exhale against the pressure.
Neither one helps me breathe. It is static pressure.

Some other interesting old topics on that board:

Aug 08 2005 subject: Who thinks what? And what the experts say? Does the pressure

May 02 2005 subject: Formula for calculating volume of air for given pressure

Mask