incorrect fuel flow readings

[microflying]

I am having trouble getting a sensible reading from my fuel flow meter. It's ok to start with then in flight it will suddenly come up with high fuel flow warnings and the meter has jumped from around 20l/hr to 36-37l/hr at the same engine power setting. The problem is worse at higher engine power settings but the figures it gives can not be right. Reduce the engine power by 100 rpm and it makes sense again. But only for a while. The meter is mounted forward of the firewall and is more or less level.

 

[pbennett]

If it's a device similar to a Floscan, I'd be starting by suspecting vapour or air bubbles in the turbine. All the sensor manufacturers give specific instructions to avoid this. The key seems to be to have the fuel outlet higher than the inlet to ensure any bubbles are swept downstream. You may need to reduce the source of bubbles such as heat affected fuel lines (use firesleeve) or something cavitating the fuel upstream such as a faulty boost pump (unlikely). A leaky pickup could also allow air to be entrained with the fuel.

Peter

 

[schristo]

i am experiencing the same problem with my 'red cube' sensor... i have tried using a blast tube on it and it appeared to help for a while but the unit is reporting flow numbers that are clearly wrong. the numbers are stable, not erratic, but are crazy. for around 10 hours the numbers make sense and the totalizer closely matched actual usage based on refill at the pumps.

 

[schristo]

my fuel flow readings are now rock solid after i wrapped the 'red cube' in fire sleeve.  where mine is located, between the fuel meter and the distributer block, there is not a lot of cooling air and it apparently gets hot enough to effect the impeller flow within the transducer.  

for troubleshooting - the symptom was creeping high fuel flow readings.


update
after several hours the readings again began to creep around... i secured the connector with additional ties to ensure no vibration and the readings are again stable... i think that the root issue is really vibration at the wire connection rather than heat buildup at the transducer.

 

[microflying]

Sorted the problem by locating the impeller cockpit side of the firewall. Must have been getting too hot engine side. Readings are sensible now and rock steady. Very accurate.