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Surface waves on both superfluid 3 He and 4 He were examined with the premise, that these inviscid media would represent ideal realizations for this fluid dynamics problem. The work on 3 He is one of the first of its kind, but on 4 He it was possible to produce much more complete set of data for meaningful comparison with theoretical models. Most measurements were performed at the zero temperature limit, meaning T < 100 mK for 4 He and T ∼ 100 µK for 3 He. Dozens of surface wave resonances, including up to 11 overtones, were observed and monitored as the liquid depth in the cell was varied. Despite of the wealth of data, perfect agreement with the constructed theoretical models could not be achieved.

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A concentration-saturated helium mixture at the melting pressure consists of two liquid phases and one or two solid phases. The equilibrium system is univariant, whose properties depend uniquely on temperature. Four coexisting phases can exist on singular points, which are called quadruple points. As a univariant system, the melting pressure could be used as a thermometric standard. It would provide some advantages compared to the current reference, namely pure 3 He, especially at the lowest temperatures below 1 mK. We have extended the melting pressure measurements of the concentration-saturated helium mixture from 10 mK to 460 mK. The density of the dilute liquid phase was also recorded. The effect of the equilibrium crystal structure changing from hcp to bcc was clearly seen at T = 294 mK at the melting pressure P = 2.638 MPa. We observed the existence of metastable solid phases around this point. No evidence was found for the presence of another, disputed, quadruple point at around 400 mK. The experimental results agree well with our previous calculations at low temperatures, but deviate above 200 mK.