  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.
