Home-made manometer. Questions before posting DIY...

For those techie engineer types out there, I have a question for you: I built a manometer using foam-core, a piece of see-through acrylic tubing and various pipe-fittings. I created a pdf with centimeter markings every half centimeter, but I'm not exactly sure that's right. That way the scale can be printed out and lined up with the 0 water level on the manometer and it should measure correctly. The scale should be linear and at 50% - correct? If someone could please confirm, I would be grateful.

It might be easier to use a plastic ruler with a metric scale rather than printing a pdf. pdf's can be inaccurate.

Absolutely agree with using a ruler, rather than introducing another source of error.

Remember also: assuming you're building a U-tube type, you can either 1) zero the ruler at the no-pressure water level of both arms, and then DOUBLE the reading of the pressure-elevated side, or 2) zero the ruler for each measurement at the level of the low side, and read the pressure directly at the elevated side, being careful to maintain a good horizontal perspective for reading.

Also, the tubing should be wide enough that there isn't significant capillary effect.

Good luck!

Using a cm ruler these are my readings:

This doesn't look right.
What's odd is that this same setup measures my M-Series Pro set at 11.5 as being 11.0 - which was confirmed by the manometer in the sleep lab. My M-series pro is low by about half a cm h20 when set to 11.5. I asked them to check.

What am I not understanding here?

Maybe there is an adjustment for your mask type. Try zeroing out that.

Make it easy on yourself......

viewtopic.php?f=1&t=49152&p=450105&hili ... er#p450105


Den

Three thoughts

1: Was EPR off? Maybe it thought you were breathing out!
2: Altitude? But it should automatically compensate for that.
3: Don't the cpap manometers use a ball in tube arrangement? That allows an airflow and the machine won't think the tube is blocked.

What if you add an airbleed/leak to your system before the meter. The machine should compensate for the leak and you should still get a reading.

Hi DD

Can't imagine that its the mask... it compensates Flow for leak... but pressure is pressure and both you and the machine are at the same altitude. Got to be RAMP, or EPR... methinks 3/8 inch tubing with water is just fine. Take a look at this installation...

viewtopic.php?p=34083#p34083

Hope it helps.
Nord

I'm at a loss for explanations; I've also described anomalous manometer results:

Velbor wrote: Uncertain of the accuracy of either machine’s pressure setting, I used a single-calibrated-tube water manometer to measure the dynamic output pressure of each machine set at 10 cmH2O. (By dynamic, rather than static pressure, I mean that a Y connector was used to allow "leak" through the venting port of an otherwise blocked-off Activa mask, while the pressure measurements were being taken.) Somewhat surprisingly, the ResMed indicated 11.8, and the Respironics indicated 11.2 cmH2O.

I then attached each machine to opposite ends of the same U-tube water manometer, to balance their static pressure outputs against each other. Somewhat surprisingly, in view of the 0.6 cmH2O difference in individual pressure noted above, the same 10.0 pressure setting on each machine did result in equal water levels in each arm. There were no observable oscillations or fluctuations in the water levels over time.

viewtopic/t48375/viewtopic.php?f=1&t=48375

I've not seen a meaningful difference between "static" and "dynamic" pressure measurements on the same machine. The "single tube" manometer I used had too thin a tube, and capillary effect error was likely a factor.

A rule of thumb occasionally mentioned on the board is that any actual pressure within 1cmH2O of setting is "within spec." Seems reasonable enough, though I'm not sure where that figure originated. I've read that some machines have a "pressure adjustment" function built into the display screen and firmware, but I've never seen one. Yet, given all the discrepancies, it's difficult to know what to trust for setting purposes.

ResMed S7 and S8 machines (what about S9 ?) ask whether a 2m or a 3m hose is being used. The only use of this information I can imagine is that the machines assume a greater pressure drop at the far end of the longer hose, and so may slightly increase the actual output pressure at the blower. Yet your readings are too low. Raises the question: are you measuring pressure near the blower, or at the far end of the tubing? In a static (no flow) condition, there should be no difference, anyway.

Presumably your bluish coloration is just a bit of food coloring in the water, which shouldn't change density to any significant degree.

Will be curious to hear of your efforts to resolve what's going on. I had pretty much given up. Good luck.