The resistive probes are local sensors with an electrically conducting tip (Cr/Ni wire) being in direct contact with the liquid. The probe is supplied with an alternating current flowing from the probe tip to their hull pipe acting as the other electrode. Due to an electric isolation made by glass or ceramic material between the stainless steel wire and the hull pipe, the closure of the electrical currents is achieved via the liquid. Thus, the gas contact at the probe wire is detected by an interruption of the current.
Measuring principle of the resistive probes
If the probe is supplied with a single wire tip the local void fraction a can be measured. A double wire probe, where the two electrodes are installed with a distinct displacement in flow direction, allows in addition the determination of the bubble velocity from the time delay of the signal between both electrodes. Further, the bubble chords can be yielded from the product of the velocity and the measured gas contact time at the probe. Methods to transform the chord-length distributions into bubble-size distributions have been suggested by several authors.
The measuring principle is ideal for liquid metal applications, because the huge difference in the electrical conductivity between the gas and the liquid causes very sharp signals with high amplitudes easily to evaluate by a threshold method.
Typical signal delivered by a single-wire resistive probe in a sodium/argon bubbly flow