Help please! Fuel pressure sensor under reading

[DavidRose]

I have a D120 & D100 for my Rotax 912uls. I've installed the 0-30psi fuel pressure sensor supplied with the rotax kit for the D120 as per the installation guide.

When I check the D120 fuel pressure reading against a calibrated gauge in the fuel system the Dynon is under reading.

When the actual fuel pressure is 0.15 bar the D120 displays 0.09 bar, when the actual fuel pressure is 0.21 bar the D120 displays 0.15 bar.

Is there any way to correct the D120 reading?

Thanks

 

[PhantomPholly]

Re: Help please! Fuel pressure sensor under readin

I may be having the same problem - reads about 1/2 psi when we expect about 5-10. Haven't checked mine with a calibrated gage yet.

 

[DavidRose]

Re: Help please! Fuel pressure sensor under readin

Hi Phantom,

As a side issue to our problem.

There's a lot of discussion about the fact that the sensor is 0-30psi when Jab and Rotax have max fuel pressures of about 5.8psi. Some doubt as to the sensitivity at the pressures we operate at.

cheers

 

[dynonsupport]

Re: Help please! Fuel pressure sensor under readin

There is no way to adjust the D120 for a particular sensor.

We have investigated other sensors, but we have yet to find one that is any better at low pressures than the one we sell. If anyone has suggestions of sensors to look at, please let us know.

We have asked for suggestions before, and just so you know, the UMA sensors won't work because of the way the sensor outputs (they can't drive against a resistor), and the stewart warner senders have been tested and are highly inaccurate.

It's one thing if you are off 20%, but if it's reading 1/2 PSI when you expect 5 PSI, that's a different problem.

 

[DavidRose]

Re: Help please! Fuel pressure sensor under readin

Hi Support,

As my D120 seems to be under reading by a constant 0.06bar do I have any options to add a resistor to the wire going to pin 8 on the harness? If so any idea what Ohms of resistance equate to 0.05bar?

Thanks

 

[dynonsupport]

Re: Help please! Fuel pressure sensor under readin

If you put a 5 ohm resistor in series with the sender it would read about .06 bar higher.

 

[DavidRose]

Re: Help please! Fuel pressure sensor under readin

Thanks! I'll let you know how I get on.

 

[DavidRose]

Re: Help please! Fuel pressure sensor under readin

Hi Support,

I've put a 4.7 Ohm resistor in series as you suggested and my D120 Fuel pressure indication is now spot on!

Thanks for your help.

 

[dbonnar]

Re: Help please! Fuel pressure sensor under readin

I installed the new sender that Dynon sold me, and still have low pressure readings. I'd like to try the ~5 ohm resistor. I want to maintain my S-LSA status. CZAW, the manufacturer of my SportCruiser probably won't respond. Can Dynon provide a mfgrs letter for this fix assuming that it works?

 

[dynonsupport]

Re: Help please! Fuel pressure sensor under readin

I'm sorry, but we can't issue a letter like that, and I'm not even sure it would be legal under the SLSA rules.

Our testing shows that our calibration is correct, and it's probably individual sensors that vary, so there is no specific resistor that can be added to fix all sensors. If there was, we'd update our cal tables. Unless you have hooked up a fully calibrated, absolutely tested pressure sender to your system and know the Dynon to be reading wrong, there is no way to be sure if it's the sender or the actual fuel system. Since you've now tried two senders, you should probably try an independent test.

 

[dbonnar]

Re: Help please! Fuel pressure sensor under readin

Agreed, thanks!
I have had trouble finding an accurate low pressure fuel gauge that I could know is accurate. I did buy a mechanical Pep-Boys 15 lb guage, but I just don't think it's fair to trust it at very low pressures - - - so caliberation might require a "commercial" grade gauge - - - which I haven't found yet. However, that is the right next step.

Once I do that, I can play with various resister values until I get an accurate mid-band reading. I realize that the sender is not perfectly straight line (can't spell linier?) but close in the mid-band I think is good enough.
Regards,
.dxb

 

[alan_milner]

Re: Help please! Fuel pressure sensor under readin

Just for the record I also has low fuel pressure readings during flight testing from my rotax 912S. I spent quite a while looking for problems and even temporarily plumbed in a mechanical pressure gauge.
Replacing the sender cured the problem and it reads very accurate, even at low rotax pressures. It's been running for about 2 years now without problems.
Alan

 

[dynonsupport]

Re: Help please! Fuel pressure sensor under readin

FYI - Sensors that are both inexpensive (sub $50) and perform exceptionally at very low pressures are somewhere between rare and nonexistent. The ones we currently sell aren't perfect but have tested better than anything else we've looked at. If anyone out there has a suggestion for a sensor that we've possibly overlooked, let us know.