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What is an "impeller transducer" with regard to a fuel flow gauge?
Is such a fuel flow meter the same as described on page 3.14?: "Some flow gauges actually measure fuel pressure with a transducer and transmit it to the instrument which is calibrated in fuel flow."
If so, and if the flow reading is higher than normal, is this only a result of a blockage in the fuel line or filter, or could it also be caused by an overly-rich mixture?
A fuel flow meter with an impeller transducer uses fuel flowing over the impeller(s) to generate the signal sent to the instrument displaying the fuel flow. The fuel flow indication increases as the impeller rotation increases.
If there is a fuel line blockage, fuel will cease to flow and the impellers will stop turning. In which case the fuel pressure indication will drop. A rich mixture will mean more fuel is flowing so the fuel flow indication will increase.
The fuel pressure guage described on 3.14 uses a transducer to convert pressure measurements into a signal for the instrument which represents this information as gallons/hr (or lbs/hr). Fuel line blockages will, in this case, cause incorrect readings since the pressure in the line increases (due to actions of upstream pumps for example) and yet no fuel is actually flowing due to the blockage.
1. There are at least two kinds of fuel flow transducer:
a) one which uses an impeller and actually increases it's reading in proportion to actual fuel flow, and
b) another which measures fuel pressure and increases it's reading in proportion to fuel pressure, which may, or may not, also be in proportion with actual fuel flow.
2. The use an overly rich mixture will register as a higher than normal fuel flow if the impeller transducer is the one in use.
3. This impeller transducer has nothing to do with the impeller of a supercharging compressor.
And a final question:
4. Will the fuel-pressure-type transducer also register an overly rich mixture as a higher than normal fuel flow?
If the mixture is rich, yes there will be more fuel flowing and this will show up as a higher fuel flow in an impeller type. I would imagine a gauge with a fuel pressure transducer would do the same as long as the fuel pumps were doing their job properly.
An overly rich mixture would give you a "higher than expected" fuel flow (not necessarilty a "higher than normal"). Expected fuel flows for various flight conditions are often in the POH so you can check and see if the fuel flows you end up with are actually linng up with those predicted by the manufacturer.
No, the impeller transducer is a completely different item. "Impellers" and "Transducers" are generic terms.
Wikipedia defines an impeller as a rotor inside a tube or conduit used to increase (or decrease in case of turbines) the pressure and flow of a fluid. A transducer is some device which converts some form of energy (e.g. pressure, rotations of an impeller) into another (e.g. an electrical signal).
From these definitions you can see where the name "impeller transducer" comes from in terms of a fuel flow gauge (even though the impeller transducer is obviosuly not trying to change the pressure or flow of the fuel).
4.) Yes, more fuel flow = higher reading on this type of gauge. Again, it would give you a higher reading but not necessarily "higher than normal". The reading would most likely be totally normal for that mixture setting though not necessarily optimal for the flight conditions.
