moisture & "bubbles" over O2 sensors

Sutty

I'm not sure how this could work as moisture which condenses on the cell faces must arrive in vapour form, I can't see that a scrim would stop it. Does the scrim affect the WOB? (Just cos I can't see how it would work doesn't mean I won't try it!)

Neil

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AD_ward9

In the O.R. designs being put forward by Deep Life, the problem is addressed by:

1. Screening the sensor design to ensure it does not retain films of water on its face. See the water immersion test in the report. Some sensors are much more sensitive to water on their face than others.

2. Correct orientation of the sensors (i.e. downwards or sideways).

If the sensors are oriented at 120 degrees, then one sensor is almost guaranteed to be facing upwards in normal diving. This allows condensation to collect on it. This problem exists with the unit you refer to, but is not present on the Boris for example.

Divers are in the head down position for very short parts of the dive, at the start when they are descending, so both condensation and PPO2 risks are lowest in this phase. Having the sensors faces either vertical or face down is the solution to this problem. Puffing gas at their face is not the solution: sensors do not like any differential pressure, and it does not take much to force contamination through the front membrane.

Alex

Sutty

Alex, any thoughts on using a piezo unit to vibrate the cell to dislodge water droplets? Would existing sensors cope with vibration like that?

Neil

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Janos

Personally, I didn't notice any increase in WOB. I did notice less moisture, although there is still some. However I put them scrim in relatively early in my Rebreather career (around 25 hours) so I don't have that good a baseline.

As to the mechanism, I think the scrims will stop water vapour, (ie small droplets) but not gaseous water. The vapour phase will definitely be present if water is condensing.

Janos

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AD_ward9

It is not necessary if the sensors are simply put in the right orientation. K.I.S.S.

Interesting little fact: piezos do not work in helium. I do not know the physics, but every piezo device we have ever tested, fails when subject to helium under pressure.

Lungs are really bad at measuring WOB. Worth doing something to measure the breathing resistance on a machine. We have been surprised on quite a few occasions, just how simple changes cause big effects.

Alex

Sutty

I was thinking more of an add-on to existing units.

Interesting they don't work in helium - most piezos work at resonant frequency - this would probably change in a helium atmosphere, maybe thats the reason.

Neil

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AD_ward9

Piezos actually fail completely. That is, they do not work normally in air afterwards. Very odd phenomenon. Easy to demonstrate with things like piezo disks used for speakers, or piezo microphones. Problem also affects piezo resistive pressure sensors.

As I said, I do not know the physical cause.

I will post an example helium compatibility report onto our web site sometime next week, explaining how we screen components to helium sensitivity, showing what we tested and the results. We have repeated tests on piezo parts many times, always with the same result. The only things that get through are sealed from the helium, such as crystal oscillators. This problem makes it important to use multiple oscillators in a diving system for processor clocks etc.

Alex

Edit: Report on helium susceptibility of components posted onto the Design Verification section of OR portion of DL site