UHP Flow Sensor Principle of Operation
The McMillan UHP Flo-Sensors and Flo-Controller provide a proportional analog or digital outputs based on volumetric flow rate. Figure 1 shows the internal design.
The flow sensor design utilized in these units is McMillans patented microturbine design. McMillans microturbine wheel technology utilizes the Pelton turbine wheel concept. This type of design allows usage of a sub-minature microturbine wheel, only about 0.8 inches (20 mm) in diameter. The turbine wheel is then supported on a very small sapphire shaft held in position by two sapphire bearings - see Figure 2. Because of the very light weight of both the wheel and the shaft, the microturbine wheel virtually floats in the liquid. This flotation effect causes the turbine wheel to be suspended in the middle of the bearings and thus eliminates shaft and bearing wear. Therefore, no particles are generated (see Particle Generation Data).
As flow passes through the flow sensor, it is directed onto the teeth of the wheel using a precision-machined orifice, which is sized according to flow range. The flow is projected onto the wheel, spinning the wheel faster as flow increases. This speed increase is directly proportional to the increase in flow rate.
The microturbine wheel features 8 small windows, spaced evenly around the center of the wheel. As the wheel spins, a light beam is projected through a PTFE window and onto the wheel. A sensor on the other side of the wheel detects each hole and translates those signals into pulses. Thus, as the wheel spins faster, more pulses are generated. When the wheel stops, no pulses are generated.
The pulse output is then processed and converted to an output, either pulse, voltage or current.