[curiouscat]

While selecting the best hydraulic fluid for a diaphragm pump (low dissolved air + high bulk modulus) I came across this Table in a Handbook that surprised me.

Do hydraulic fluids really absorb that much air? 40% (v/v) sounds like a lot of air. And is the solubility of air so much lower in water than oils?

[Enthalpy]

1.5% volume air in water is reasonable.

I have not measured the dissolved air in hydraulic fluids I used, but bubbles were already nasty enough. They're the very first worry if seeking stiffness.

Dissolved air changes the bulk modulus a lot, even if it doesn't bubble out. Degassing first is good practice.

Not exactly Freon TF, but one perfluorinated compound is known to dissolve oxygen. Experiments showed rats "breathing" the liquid.

Anyway, fluorinated compound are not stiff at all, and silicone neither (<1GPa), so these two are out of the list.

Bulk modulus isn't widely documented but you can deduce it at 1atm from the sound velocity, whose square is the modulus divided by the density. K gains easily 30% between 1atm and 350bar. One table in the Hdbk of Chemistry and Physics, for chemicals.

Hydrocarbons are not very stiff (~1.5GPa); have a look at brake fluids instead, called "Dot-1" to at least "Dot-4". They can be phosphates, pentaerythritol esters... and their modulus is documented because it matters there, their dissolved gas as well.

The stiffest liquids tend to be viscous (honey, dry glycerine, glycol, polyols...) and hygroscopic. Their modulus changes a lot with the distance to the melting point (often near RT!) and water contents (2.2GPa already). Usable stiff liquids use to be ol-water mixtures.

Because a hydraulic fluid needs many qualities (wet metals, defoam, protect against corrosion, lubricate well... If not, adios the pump, quickly) I recommend to concentrate on well proven commercial formulas despite being not quite as stiff. Hydrolub is one commercial brand among others for polyethylene glycol +water, ready to use, where the proportion of water adjusts the viscosity to reasonable values; please check the vapour pressure if relevant to your use.