Hydrodynamic properties of dilute mixtures of He3 and He4 were investigated below 100 mK by measuring the thermodynamic properties of the solution in various flow impedances as functions of the He3 velocity. The He4 component in the impedances was practically always macroscopically at rest. If the flow impedances consisted of a bundle of metal or glass capillaries, a distinct critical velocity was observed above which the He4 chemical-potential difference across the impedance was nonzero. The critical velocity of capillaries with diameter d roughly follows a ln(d)/d dependence. In the critical-velocity region, the amplitude of the pressure fluctuations was observed to be relatively large and strongly velocity dependent. The characteristic time for transient effects in the flow system changed abruptly at the critical velocity. There are indications for the existence of a second critical velocity. In flow impedances with an annular cross section, no critical velocities were observed.