I do not believe air mixes with water at these low forces. However, air does get into my cylinder while starting to form a seal and it forms a bubble at the top or bottom, depending on the direction of the cylinder. This reminds me that when I have a large air bubble in my cylinder it movies up and down the cylinder past my penis as I change directions. As the bubble moves up and down over my penis, I can not feel any change in the vacuum as it passes. Accordingly, this confirms, for me anyway, that the 5 hg pressure does not change as I switch from water to gas inside the same cylinder.
Next time I water pump, I am only going to fill the cylinder half way up with water and see if I can detect any change in pressure between the two mediums,
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This is why we recommend pumping with a gauge, dt. The penis is not reliable as a sensor of pressure/vacuum variations.
One thing I’ve noticed with water pumping is that my penis doesn’t feel dehydrated like it does when I air pump so maybe there’s more to it then just simple dehydration vs hydration.
If your penis is completely surrounded by water, and you’re using tap water, which is less salty than your blood and lymph, osmotic forces are going to cause a net fluid gain into your penis. In other words, your penis should wind up more hydrated after water pumping with tap water. That was the point I was making earlier about adding salt to the water you pump with. If you add enough - to where it is saltier than your body fluids - it would tend to draw fluids out of your penis.
And if your penis is surrounded by air, and especially if that air is at lower than atmospheric pressure, I would expect there to be fluid loss from your penis, so I would not be surprised by slight dehydration.
That makes me wonder a little bit about whether that may be a part of why there’s less overall donutting after water pumping. An overall gain of fluids to the penis might comparatively reduce what would otherwise be a more localized gain of fluids just below the glans.