## 1 Introduction
As a result, the double-sensor probe can be applied to measure local flow void fraction, IAC and magnitude of velocity when the two-phase flow is very consistently stable bubbly flow, and the accuracy can be very high without too much care to the signal process and correction algorithm [34]. However, more assumptions and more complicated signal processing and correction algorithms are necessary if the flow becomes complicated or more parameters, e.g., bubble size and interface direction, have to be detected. Inevitably, more uncertainties would be produced in the application of single-sensor probe or double-sensor probe if employing these assumptions [43; 44; 45; 46]. As uncertain assumptions must be incorporated, it is hard for the double-sensor probe to obtain real local flow parameters in multi-dimensional two-phase flow. Meanwhile, the typical multiple-sensor probe, namely the four-sensor probe, as a promising alternative compared with single-sensor probe and double-sensor probe, is developed by a few researchers to omit the involvement of so many assumptions. According to measurement theorem of four-sensor probe, it is unnecessary to coordinate the connections of the sensors' tips to the direction of interface or direction of interface displacement velocity, while the void fraction, IAC and chord length can still be calculated. In addition, the interface direction can be obtained by the proper processing and trigonometric operation of the four series of signals from the four sensors [47; 48; 49; 50; 51]. However, the bubble velocity or interface displacement velocity is still unknown, except its component in the normal direction of interface [36]. It has been proved that this problem can be compensated by employing several four-sensor probes or multiple-sensor probe, five-sensor probe, six-sensor probe, etc. [52; 53]. With a much simpler assumption of the two-phase flow, obtaining the full velocity vector by usingmerely one four-sensor probe can also be realized. For instance, assuming a spherical or symmetric bubble shape and a bubble velocity perpendicular to the symmetric plane has been demonstrated to be a concise way to get the full components of bubble velocity [54; 55; 56]. The above simplifications when calculating the total interface velocity vector only apply to special cases. Considering this fact, the local interface velocity directions can be practically provided by prior measurement, flow simulation and even other legitimate assumption in many two-phase flows. It is necessary and significant for obtaining interface details to propose a new method to get the total interface velocity vector based on the known interface velocity directions. Besides the development of algorithm for four-sensor probe, the miniaturization of the probe and sensors are essential to guarantee the accuracy of measurement. According to the published literature [47; 50], both the diameter of sensor wire and the total front area formed by all four sensors have great influence on the bubbles' behavior. Thus, it is recommended to use as small as possible sensors and probe of cross-sectional area. With the assumption of a much smaller probe than the size of bubble, the error produced during the application of the probe stems mainly from the bouncing away of bubble from the probe and the slipping away of interface through the gap between sensors. Their errors or uncertainties are acceptable [36; 47; 49]. Therefore, with the solution of the many issues of four-sensor probe, including its fabrication and further miniaturization, the correction for its disturbance to original flow and the improvement of signal process algorithm, it is promising to be widely applied in multiphase flow measurement inside chemical reactor, oil piping, power generation facility and heat exchanger. In addition, the method will be easy to be adopted for different combinations of fluid components.