Calculation question

I'm a bit of a mathematical type. I can't find a formula anywhere for the question I'm trying to answer. So, we measure air pressure in terms of the pressure exerted by a column of water 1cm high at 4 degrees Celcius (where water is most dense - thus why frozen water bottles may burst).

Anyway, if we were to convert 1cmH20 to a certain number, say in terms of liters per minute (see where I'm going with this?) what would that number be? Note: I realize that these will be different numbers based on the diameter of the CPAP hose. Standard is 19mm while the Resmed slimline and climatelines are 15mm.

So, if I have a pressure of 12cmH20, that's the pressure of a column of water 12cm high or 12cm in diameter? What would the corresponding flow rate (after all CPAP is only an airflow generator) in terms of litres/minute be?

Roman Hokie wrote: So, if I have a pressure of 12cmH20, that's the pressure of a column of water 12cm high or 12cm in diameter?

12cm high

Roman Hokie wrote:What would the corresponding flow rate (after all CPAP is only an airflow generator) in terms of litres/minute be?

There's a human-diaphragm sourcing pressure at one end of this biomed circuit and a CPAP machine sourcing pressure at the other end. Those tandem pressure sources generate flow with respect stimulus/response driven impedance changes in biology as well. Hence it's really more of a tall biophysics problem than an inanimate physics equation. That's why the flow measured by your S9 ResScan graphs show wildly varying sinusoidal oscillations in flow measured at the pneumotach sensor...

Yeah. 12cm high. Right. You got me. You answered the question I asked - not what I meant to ask.

Should have been, "what is the airflow generated, in terms of litres per minute, at 12cm pressure?"

For grins, let's assume that it's not a biomedical circuit with the CPAP generating 12cm and the human diaphram physiologically offering counter pressure in sinusoidal rhythm. Let's assume that there is no mask at the opposite end of the 19mm or 15mm slimline hose and that the only pressure generated is from the CPAP airflow generator.

But, you are (of course) 100% correct about the diaphram, not to mention the possibility of leak, expected and not expected, from a mask if it was integrated into the system. (of course, the mask would leak 100% unless adequately sealed to something like, I don't know, a face).

pressure and flow (leak) aren't the same "dimensions" so can't be added or subtracted, just as you can't add velocity and acceleration as the "dimensions" are different

The analog to pressure in electricity is voltage. The analog to flow rate is current. The relation in terms of airflow is probably not as nicely linear as in electricity, but the basic formula is V=IR. This means I = V/R. In other words current is equal to voltage divided by resistance. For the airflow I would probably translate this as airflow is proportional to pressure divided by resistance.

This is why a mask leak does not automatically mean lower pressure. The blower increases the flow to maintain the same pressure as much as it can. Obviously there is a maximum flow rate that it can deliver. Once it reaches that flow rate, the pressure will start dropping as resistance decreases.

This is a bit to heavy for me on a Friday

Hmmm... I'm not sure, Jules. When doing fire protection work, I can calculate the flow generated from a hydrant from a psi (well, _I_ can't, but it can be measured) and vice versa, assuming a fixed x-section of pipe. So, if 1cm = .014223 psi then there stands to reason that, given the diameter of tubing (after all, the S9 DOES ask if the tubing is standard or slim), we can determine a flow rate.

I do get what you're saying about velocity being a dimension away from acceleration (and location being a dimension from velocity, all being vector quantities in 3D space).

But I seem to think there's some overlap in PSI and flowrate provided certain variables are answered.

I've also tossed my question to a fireman to see what he thinks.

I may be all wet (pun intended) and will gladly say so if it comes down to it.

Yeah, it's heavy for me, too, Uncle Bob. But I make it a point to learn something new every day. And I haven't yet today.

I would assume that the flow rate can be determined from pressure if you know the size of the "smallest" orifice the flow is through. In the case of a CPAP circuit this would be the vents on the mask not the hose diameter. However, it is a matter of not being able to add apples and oranges without this information.

No idea why I read through this entire thread, should've stopped at the first 3 words - now I have a headache from techno-overload

xena

Roman Hokie wrote: But I seem to think there's some overlap in PSI and flowrate provided certain variables are answered.

1 PSI is equal to 70.308 cm H2O @ at 4 °C. I'm still not sure of the purpose of your calculations since the human respiratory drive will always "drive" the resulting flow---that is unless you are using a (flow) volume ventilator on a patient without an intact respiratory drive.

I'd suggest an impractical resistor grid problem instead: http://xkcd.com/356/ j/k

Uncle_Bob wrote:This is a bit to heavy for me on a Friday

Yikes!! Me too!

rosie

jules wrote:I would assume that the flow rate can be determined from pressure if you know the size of the "smallest" orifice the flow is through. In the case of a CPAP circuit this would be the vents on the mask not the hose diameter. However, it is a matter of not being able to add apples and oranges without this information.

Vents, yes. Absolutely. Unless, for the purpose of the question, the hose is the ResMed Slimline (15mm diameter) and there is no mask or anything else attached to the other end of the hose.

I'm glad you guys and gals are still here, even those who wonder WHY they are. I'm fighting the school over financial aid stuff and needed this to take my mind off the stress.

so you are rigging the machine up not to a person but to flow out into the open air? such as when someone would flush a fire hydrant

Math:
My computer at work does least squares adjustment of a survey traverse in a fraction of a second.
Early surveyors, like George Washington did it on paper--spending hours (maybe more)
Mathematics is amazing; but I'm glad to have computers.