I guess in that case it would have to be 16% out. No, never noted any differences in FF.
Peter wrote:
Could the fuel flow reading on one engine be 8% out without you noticing it?
If you would be talking 1 or 2% I would say yes. But 8% is quite a difference. I think chances of intermittend wiring or sensors issues are higher then software to go corrupt.
If you look at my Shadin writeup you will see I got a 7-8% error (totaliser over reading e.g. reading 10.8 USG/hr when it should be 10.0 USG/hr) purely because the totaliser was wired up using unshielded cable, and fairly obviously the indicator was picking up spurious pulses from the ignition system. The wiring was done by one of UK’s well known 145 companies 
I flew with that error for years, before getting the opportunity to re-wire it properly on the suspicion of the reason, and immediately the flow rate indication was exactly right with the K-factor configured with the value marked on the transducer.
So maybe a connection to a cable shield has come off?
But the above assumes that you are getting a flow rate indication error, which you say you aren’t getting. So I have no idea how this can possibly add up. If you set say 50 USG FOB and both totalisers are showing 5 USG/hr flow rate, then after 5 hours your FOB must be 25 USG.
Peter
How does using unscreened cable cause the Floscan transducer to give out more pulses per second which is the only way it could overread?
http://www.floscan.com/html/blue/itemimages/page/201%20gas%20200a.jpg
The transducer as supplied is wired un-screened. I do see that the installation manual does show a screened harness:
On the actual installation wiring diagram there is no screening shown:
I’m intrigued. Firstly they guts of the system are a couple of Shadin sensors working as Peter has previously described. I’m interested in you 65USG figure as total useable is 76.4USG but I’m guessing you’ve got that one covered. Secondly, and not wishing to be condescending, are you sure you’re filling the tanks each time? The shape of the DA42 tanks is such that you can get a significant difference even if you think you’ve filled to the rim. Also, you can get small air locks between the sub-tanks and we often find a wing-waggle whilst refuelling allows you to get more in. The aux tanks also tend not to empty completely despite the L/R Aux Fuel caption on the PFD. That said, you would notice this next time you filled the aux tanks.
Finally, are you using diesel or Jet and do you swap between the two?
The transducer does not give out more pulses. The extra pulses seen by the indicator are crap from the ignition. Worth reading my writeup. Sorry, writing in a rush.
Just as an aside here folks (if you don’t mind), how does one eliminate ignition “noise”? Yes I know the answers will be not simple but one has to start somewhere.
Dave,
The reason for the 65 gals is simple: For these tests I filled her completely ( to 76 gal as you say) but like to land with minimum 10 gal ;) So I measure the discrepancy of 5 gal on a used qty of some 65 gal.
I only use Jetfuel. Just once added 20 litres of Diesel.
I understand yr point on wiggling and making sure the tanks are full, but when I refill to completely full then things always seem to add up correctly: adding the remaining fuel left (as per the qty tranducers in the tanks) to fuel pumped in gives me a total of 76 gal. I also checked that for the individual tanks. So it’s not the refueller that screws me either 
. I also cross-checked the qty transducers when 10 gal left with the infamous manual measurement kit (you know which device I mean, the one that gets you soaked in fuel if you want measure properly )
She’s in annual inspection, so they’ll look at it too.
Thanks all..
So a fellow DA42 owner and myself did some further testing and we came to the same conclusion:
The FF meters seriously over-read at cruise power settings and under-read at low power settings.
See the following table, comparing the actual FF (as per the manual, very accurate) to the FF reported by the FF meter (X’d line)
Would be nice if others here flying a Conti CD135 (Thielert 1.7 or 2.0) would check whether what they see on their display matches this (gal/hr, per engine):
100% 7.4
90% 6.7
85% 6.4
80% 5.9
75% 5.4
70% 4.9
65% 4.6
60% 4.3
Ideas welcome.
I have practically the same situation as @aart with Thielert 2.0. The actual readings of FF-metar are lower than what manual specifies which results with lower quantity of fuel in tanks than totalizer shows. This has been confirmed across 400 flight hours and numerous refuelings. On my last trip to Sweden I manually tracked consumption according to values in manual which resulted in almost exact quantity shown by fuel gauges and what I refueled. I started with full tanks (76 USG) and landed after 6 hours with 8 USG in tanks, refuelling 68 USG which was consistent with what I calculated manually during the flight. The fuel gauges were showing 10 USG (5 in each tank) which I consider precise enough having in mind type of devices used for measuring. However, totalizer was showing 14 USG which is pretty much of difference.
Everything I recorded during this flight was consistent with what I usually notice when refueling i.e. when I refuel 40 to 50 USG the difference between actual quantity and what totalizer shows is 4 USG (totalizer shows more fuel left).
