Fuel level goes down during radio transmission

[dpbarnes]

I have a problem that I have lived with since my first flight about 15 months ago.  I have a lancair with a fiberglass wet wing that holds 50 gallons on each side.  The wing has an 8 foot capacitance probe to sense the fuel level.  I have this hooked to dynon's capacitance to voltage converter then to skyview.  Everything works great until I transmit on either radio.  Then I go from 50 gallons to 0 in three seconds and get the announcement about low fuel.  After transmitting in about 10 seconds it goes to normal.  After talking to tech support and some troubleshooting it appears that the probe is acting like an antenna and causing the capacitance to voltage module to go to zero.  Tech support does not have a solution for me.  I know EI with their mvp-50 has a transmit input to freeze the fuel level until after the transmission.  It hooks to the push to talk switch.  Tech support says I am the only one to ever have this problem.  So if anyone else is living with this speak up now so our voices can be heard.

I thought maybe another work around would be to slow how fast dynon responds to fuel changes.  Changing 50 gallons in 3 seconds seems faster then anyone would need.

 

[kurtfly]

I have the same "feature" with my Glasair but not to the degree you are experiencing. I see a few gal's less when transmitting but I just ignore. Does not cause an alarm. I am using the Dynon C/V converter and EI 10ft probe in the main wing tank. Also have a homemade alum plates in the header tank.
Before my Skyview upgrade I had the same probes but had the E/I C/ converters and E/I gage. This combination would go to zero when I keyed a transmitter. Skyview is much better for my installation.
I was unaware that anyone had a suppress during transmit, but that seems like a great idea!

 

[jdubner]

I was unaware that anyone had a suppress during transmit, but that seems like a great idea!

There's a long-time precedent for that.  The old (now defunct) Navaid autopilot has an optional connection to the radio's PTT connection to prevent the autopilot doing something untoward during transmissions.  (I think the A/P simply maintained straight-and-level while the PTT was depressed.)

Changing 50 gallons in 3 seconds seems faster then anyone would need.

I agree -- 50 gallons in 3 seconds is 1000 gallons per minute or 60,000 GPH.  Yikes!

Having a slower "slew rate" in the fuel indication would also help toward a problem I experience whenever one fuel tank gets close to the alarm level.  With the current slew rate and no hysteresis, I'm bothered by nuisance alarms as the fuel level sloshes below and above the alarm level.

--
Joe

 

[dynonsupport]

Slower reactions to fuel level changes is something we are currently beta testing, so that may just work well for everyone. We were mostly doing it because of planes with little or no baffling in the tanks where any yaw would cause changes.