Quote: (Originally Posted by
paulraymaekers)
Alex, that sounds weard to me..
I have been interpreting that WOB and respiratory pressure has to be 'plotted' against displaced volume, and not measured: exactly the graph from wich we can mesure the WOB
as far as I have seen a breathing machine like the ANSTI does in no means mach a human body in dead volume.. already the connection hoses from the piston to the mouthpiece, length minimum 3/4 meter, diameter 50/60mm
I have always interpreted the calibration orifice beiing used just to check volume displacement against pressurediff..
am I completely wrong, or do we express ourself wrongly??
paul
The displaced volume is the piston movement. You can measure it. It means the Lissajou is the displaced volume against pressure.
The pressure is relative pressure. The issue is relative to what? This is where the calibration Lissajou comes in. If you put the calibration orifice on a true sine flow machine, relative to the outside of the mouthpiece, you do not get a WOB of 3.3J/L at 50msw. It you put the reference in the volume being moved with the tubes similar to a human, then you get the reference Lissajou exactly.
We are aware the ANSTI machines do not match a human. That is why we spent a lot more than an ANSTI machine on one that does match a human, including the bronchial resistance.
This may, or quite likely may not, be what the EU Committee intended to achieve, and the woolly wording of where the reference is other than its hydrostatic position, does not help one bit. They may have also come up with the calibration orifice by mistake: transparency in the process would be most welcome. It a tester were to take the reference point directly outside the mouthpiece, one can chop almost 0.8J/L from the graph I posted: that is how large these differences in measurement method can be. DL reads EN14143 as a safety document for human safety, and given the indicator on the calibration Lissajou they use, then it appears to be correct to do the proper safety test and measure relative to the moving volume with the right breathing machine. NORSOK is more explicit with this, in that the breathing simulator should simulate a human.
For example, take the DL Sports rebreather we have had sitting about for quite a while, as the safety case progresses slowly forward. It has a true WOB of 2.19 J/L, and 2.68 J/L on a human (40msw, 75lpm RMV, 3 litre stroke, air). We can remove the "+b" part of the equation just by measuring the WOB relative to a suitable point in the water (at the suprasternal notch position), and chop 0.8J/L off this figure, to claim 1.4 J/L to 1.88 J/L, but frankly, that is not the true result when used on a human diver.
Alex