The simplest and most effective means of monitoring impurity levels is a plugging meter, which measures the temperature at which solids are precipitated. Figure 4.7 shows the principle of a plugging meter. Sodium flows through an orifice, the pressure difference across which is monitored. As the sodium temperature is reduced the pressure dif­ference rises when impurities are precipitated. A plugging meter is a valuable practical guide but gives little indication of what impurity is causing the blockage. It can tell the operator that an impurity is present, but not what it is.

Oxygen and hydrogen concentrations can be measured separately (Hans and Dumm, 1977). The oxygen concentration can be meas­ured by an electrolytic meter in which the electrolyte is a ceramic (ThO2 doped with YO2) that separates the sodium, which forms one electrode, from an air reference electrode. The potential generated depends on the oxygen concentration in the sodium. The main prob­lem is that the ceramic electrolyte is very brittle and susceptible to thermal shock, so the temperature of the sodium has to be controlled very carefully.

Hydrogen concentration can be measured by utilising its ability to diffuse through a nickel membrane into a carrier gas such as argon.

The concentration of hydrogen in the argon can then be measured by a katharometer, which depends for its operation on the marked effect of the hydrogen on the thermal conductivity of the mixture. Alternatively the membrane can be evacuated by a vacuum pump of known pumping speed. The hydrogen pressure upstream of the pump, which can be measured by an ionisation gauge, depends on the rate of diffusion through the membrane and therefore on the hydrogen concentration in the sodium.