Pelton Turbine Wheel Design
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.
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®.