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Bob, I use Shadin DigiData systems in several aircraft and each exhibits their own small, but predictable error. In every case it errs on the "safe" side (says more fuel was consumed than actually was). The worst case unit indicates about 6% too high of a few burn. So when I fill it up, it takes a little less fuel than the Shadin predicts.
This should be a simple thing to calibrate - although I've never taken the trouble to do it. The fuel flow transducer has a "K-Factor" printed on its tag - and this number gets entered into the Shadin through a setup menu. The K-Factor is the number of pulses per gallon that the fuel transductor will output. As I recall, one of my units had a 28,500 K-Factor. To make the 6% correction I should be able to tell the Shadin that the K-Factor is 6% higher than the tags says. In other words, if I enter 30,210 into the Shadin it should cancel out the error.
I have no idea whether this is legal. You'd have to check with your avionics shop or Shadin to see what the rules are.
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