Pelton Turbine Wheel Design

The McMillan Company micro-turbine flow sensor works for both gas and liquid applications. A miniature sapphire shaft floating in a sapphire bearing produces very little friction. In most cases, units will function reliably for years with little or no maintenance.

Because the turbine wheel is so small and lightweight, the customer can expect the unit to function across a large dynamic range, usually 10% or lower of the maximum flow rate (20% of maximum flow rate for gas applications). Repeatability of turbine flow sensors is excellent; since the design inherently has no zero drift, no adjustments need to be made to maintain repeatability.

Figure 1: Breakdown of McMillan turbine sensors

Standard McMillan turbine design (Figure 1)
Most McMillan products utilize a small, Ryton® turbine wheel. The rotation of the Pelton-type turbine wheel is linear over a large dynamic range. To sense flow rate, these flow sensors use an electro-optical system. This system consists of an infrared diode beaming light through a glass window onto the turbine wheel. “Spokes” deposited on the turbine wheel alternately reflect and absorb the light. The reflected light is sensed by a photo-diode, and advanced electronics convert those pulses into a signal proportional to flow rate.

Figure 2: PTFE McMillan turbine sensors

Figure 3: Turbine wheel assembly

McMillan turbine design made with Teflon® PTFE (Figures 2 & 3)
Certain liquid flow products (the Model 106, I-106, 106F and 401) made with Teflon® PTFE use a slightly different optical system. This system allows them to see through even seemingly translucent or opaque liquids, such as ink or polishing slurries. This type of design allows usage of a sub-minature microturbine wheel about the size of a quarter in both diameter and thickness. The turbine wheel is then supported on a very small sapphire shaft held in position by two sapphire bearings. 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. This design allows these products to be used for ultraclean applications - no particles are generated.

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 holes, spaced evenly around the center of the wheel. As the wheel spins, an infrared beam is projected through a Teflon 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.

* Patent Numbers 4,467,660; 5,542,302; 5,728,949; DE 19680105 T1;
GB 0163785; GB 2302175B; GB 2332064B; Japan 1770103.
Other patents pending.

Ryton - Reg TM Phillips 66
Teflon® is a registered trademark of DuPont. Only DuPont makes Teflon®.