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Max. return flow rate = 5 gallons / 0.25 hours = 20 gallons per hour
Now, add in the engine’s consumption fuel rate at this RPM, which should be readily available
from the manufacturer’s documentation for the engine, and calculate the maximum supply flow
rate as follows:
Max. supply flow rate = Max. return flow rate + Max. consumption flow rate
For this example, let’s assume that the engine manufacturer’s documentation shows that the
fuel consumption of the engine at the maximum RPM is 5 gallons per hour. We can calculate
the maximum supply flow rate as follows:
Max. supply flow rate = 20 gallons per hour + 5 gallons per hour = 25 gallons per hour.
The 25 gallons per hour maximum supply flow rate is within the specifications of the M1RSP-
2R-E8 fuel flow sensor, so the M1RSP-2R-E8 is appropriate for this fuel flow application. If the
maximum supply flow rate had exceeded the M1RSP-2R-E8 specifications, you would have
needed to select the next larger fuel flow sensor, the M2RSP-2R-E8.
1.6.6 Accuracy of Diesel Fuel Flow Measurement
Even though the fluid flow sensors used with the FFM100 are rated at ±0.25% accuracy, since
the fuel consumption measurement is calculated as the difference of the supply and return fuel
flow rates, the accuracy of the fuel consumption measurement when operating in differential
mode for a diesel engine depends on the ratio of the flow rate of fuel consumed by the engine
to the flow rate of fuel supplied to the engine.
Following is a worst-case analysis of diesel engine fuel consumption measurement accuracy
with a 4:1 supply rate to consumption rate ratio (this ratio is typical of many diesel engines).
This assumes that both the supply and return fuel flow sensors are at the limit of their
specifications in the way that will cause the most inaccuracy in the fuel consumption
calculation.
Actual Supply Flow Rate: 80.0 LPH
Actual Engine Fuel Consumption: 20.0 LPH
Actual Return Flow Rate: 60.0 LPH
Measured Supply Flow Rate: 80.2 LPH (This assumes that the supply sensor
reads 0.25% high)
Measured Return Flow Rate: 59.85 LPH (This assumes that the return sensor
reads 0.25% low)
Calculated Fuel Consumption: 20.35 LPH
Error: 1.75 %
The resulting 1.75% error value is what appears in the FFM100 specifications for differential
flow rate accuracy.