Fuel level configuration

[RV711AC]

During my conditional inspection I am attempting to parallel the fuel level senders to both the Dynon and the Vans steam gauges. With the Dynon inputs set as unused there seems to be no effect on the Van's gauges and there is still a reasonable output from the Dynon as indicated by the sensor diagnostic screen. As soon as the inputs are configured as level sensors there is loading on the Vans gauges that decreases their reading. Is there a way to configure these inputs as a higher impedance?
Ralph

 

[dynonsupport]

If you are looking for redundancy, it is not possible to parallel connections. The gauge powers the senders, so having two gauges (SkyView and mechanical) means there is twice the power expected and neither reads right. You can configure SkyView as a capacitive input, which will get rid of the power from SkyView, but now you are 100% reliant on the Vans gauge working perfectly. Any failure in the vans gauge will cause SkyView to read wrong too.

Is there a reason that redundant fuel level is important? Most people consider the loss of their SkyView to be a good reason to land, and you know your fuel state at the moment of the failure.

 

[RV711AC]

Redundancy was not the issue. The Van's gauges are on the far right side of the panel. I was trying to get them front and center while not reworking the right panel. I thought a reading the voltage to the Van's gauge and displaying it as a level would not be too difficult but the input would need to be high Z with no pull-up or down, just a straight dc input from the Van's guage to the A to D converter. Scaled in SW to read correctly, which I assumed was done via the gauge calibration drill.
Just puttin the mental in Experimental!!
Ralph

 

[dynonsupport]

We can calibrate any curve, and as I said, you can choose capacitive as your input type which will turn off the pull up and just read the voltage.

I think the second issue is that the Vans gauge puts 12V on the sender, while we can only read 5V. Also, that 12V may not be regulated, it may be aircraft voltage, so your fuel level may change depending on the engine running. In general, we don't support that configuration so we don't have great data about how to make it work. It's definitely possible with a but of work.

 

[RV711AC]

I'm not sure about regulation but the max voltage on the resistive sensor is less than 5 V. I would think there must be regulation for the very same reason. Autmotive instruments have had regulated supplies for many years.
I will be out of town for a couple of weeks but I'll give the capacitive sensor set-up a try and see where it takes me.
Thanks for the prompt reply. I'll post my findings good or bad.
Ralph

 

[mmarien]

On the subject of fuel level probes, I was looking at change my float type sender to a capacitive fuel lever sender for a couple of reasons and was looking for some advice. The float type sender I have doesn't read the bottom 40 litres. At 30 litres per hour that leaves over an hour of unusable fuel. From what I read, the capacitive type fuel lever senders can be cut to fit the tank which would solve the problem.

In the SV manual it says the capacitive fuel sender needs to output 0-5 volts. Some senders specify ohms which I suspect won't work. Others specify that they output 0-5 volts. And some need calibration in addition to the Skyview calibration.

Can someone suggest a capacitive type fuel sender available from Aircraft Spruce that would work with Skyview without requiring two calibrations.

 

[dynonsupport]

Just to be clear - it needs to output a variable voltage that goes no higher than 5 or lower than 0. It doesn't need to use the full range. That's what our cal procedure is full. The 2 set point ones from Princeton should work well: http://www.aircraftspruce.com/catalog/inpages/princefuelprobes.php. I don't remember whether or not they "require" a calibration before they output voltage, though.

 

[mmarien]

Thanks for the suggestion. I did look at the Princeton and it does output 0-5V. I also looked the calibration. The instructions from one source was to set the empty tank in open air (0V) and the full tank (5V) with the probe fully submerged in fuel. This would give it linear readings. The Dynon fuel calibration will compute the non linear volume curve. Solves the problem of filling the tank twice.

The other thing that was mentioned was the +12V they need must be regulated. Any fluctuations in the voltage would result in fluctuations in the readings. Is the +12V from the SV-EMS-220 regulated? That brings up the other question. How much load can I put on the SV-EMS-220 +12V and +5V pins. So far +12V pin will have two fuel flow sensors and possibly the fuel level. The +5V pin will have the MAP sensor plus a Kavlico fuel pressure sensor.

 

[dynonsupport]

<snip>

The other thing that was mentioned was the +12V they need must be regulated. Any fluctuations in the voltage would result in fluctuations in the readings. Is the +12V from the SV-EMS-220 regulated? That brings up the other question. How much load can I put on the SV-EMS-220 +12V and +5V pins. So far +12V pin will have two fuel flow sensors and possibly the fuel level. The +5V pin will have the MAP sensor plus a Kavlico fuel pressure sensor.

Yes, the 12V output (Pin 15) of the SV-EMS-220 is regulated. It's a reasonable concern of how many sensors can be connected to the SV-EMS-220 12V output, and as of our SkyView System Installation Guide, Rev. K for firmware v3.3*, we offer the following guidance (Page 7-10):

SV-EMS-220 output voltage: 12V
Output current limit: 80 mA
Typical sensors used:
FloScan Fuel Flow Sensor (100403-001): 50mA
EI Fuel Flow Sensor (100403-003): 15mA
UMA Differential Fuel Pressure: 10mA
Rotax Honeywell Oil Pressure Sensor: 20mA

You'll have to determine from the Princeton data sheet if their requirements fit within the output current limit of the SV-EMS-220's 12V output. If not, you'll have to add a regulated 12V supply.

For completeness, here's the corresponding info for the 5V line
SV-EMS-220 output voltage: 5V
Output current limit: 300 mA
Typical sensors used:
Kavlico pressure sensors: 5mA
3rd party trim and flaps position: 5mA
Manifold pressure sensor (100434-000): 10mA

* To be released "soon".

 

[mmarien]

I checked with Todd Stehouwer at Princton and he said that the probe draws about 0.02 amps at 12V.

They also have one with 1 set point for computer based displays. This is new from the last time I looked. The set points are to calibrate the fuel level. I assumed that that meant the calibration is done on the Skyview rather than on the probe itself. Heck of an idea.

http://www.aircraftspruce.ca/catalog/inpages/princefuelprobes.php

I ordered one. 

 

[mmarien]

Just an update on the Princeton capacitive fuel level I ordered. They sent a five set point one but we got that figured out and they replaced it with a one set point. The one set point means that there is no calibration for the fuel probe. Just empty the tank and press the button to calibrate the device. The Skyview does require calibration but you only have to do it once.

When I calibrated my tank, the difference in voltage between calibration points was only a few hundreds of a volt. Empty was 0.11 volts and full (130 litres) was 0.59 volts. Skyview complained that it didn't see a difference between the add points (10 litres) but calibrated successfully. After filling the tank I removed sixty liters for test flight configuration and the fuel gauge was within a litre.

That brings up the question. One litre is about 0.004 volts. I seen some discussion on this forum about the resolution that Skyview can measure but can't find it. Does anyone know what the resolution is? If it's only hundredths of a volt, my fuel resolution will be about 3 litres.

 

[Gordona23]

Re: Fuel level gauge backup

I've got a slightly different idea. On a Skyview in a Zenith 750, I'd like to "parallel" an input to a PIC microcontroller's ADC, to read the resistive fuel level sender. I will then use the PIC to drive any backup gauge (actually an LED bargraph). I don't think the PIC ADC port will "load" your line but I'd like to hear what you think. Thanks.

 

[dynonsupport]

The resistive sender is powered by SkyView. If SkyView powers off, the voltage goes to zero and your "backup" gauge stops working. Thus, this is not much of a backup.

 

[jnmeade]

I'm joining this discussion late. Do I understand that if there is an odd shaped tank, the Skyview can be programmed to provide the right fuel level reading?
I'm toying with the idea of adding a capacitive fuel gauge system to my airplane.

 

[Carl_Froehlich]

For what it's worth - I've used the standard float senders for two RVs. With the Dynon, calibration of the senders is dirt simple and the output is accurate to within a gallon. Keep in mind when you install the sender most will not have an arm movement wide enough to go from empty to full. Install the sender such that it still has room to move with the tank empty as that is where you are most interested in an accurate level reading. On the RV-10 (30 gallon tanks) the senders read from 0-24 gallons. When the tanks are filled above this range the SkyView reads 24+.
The fuel tank senders (either float or capacitive) will never be as accurate as the fuel flow meter, but they provide a valuable cross check to guard against a sump valve leaking, unbalanced tanks or such that will not be caught by the fuel flow meter. Duplicating this function, I offer, provides little gain.
SkyView made this simple - no need to reinvent it.
Carl

 

[Gordona23]

The resistive sender is powered by SkyView. If SkyView powers off, the voltage goes to zero and your "backup" gauge stops working. Thus, this is not much of a backup.

Sorry for the confusion. I don't intend this as a backup but rather a UI issue. I like having a fuel quantity display right at the fuel selector knob. My original question is will this cause any problems for your circuit ? Thanks.

 

[dynonsupport]

Gordon,
As long as your input is high impedance, then it shouldn't cause any issues. This is a 5V signal, so if you use a 3.3V PIC, that may be an issue. You can probably get away with a voltage divider if it's 50K impedance or higher.

Jnmeade,
All Dynon engine monitors are calibrated for your specific fuel tank, over multiple points per tank. Even with odd shaped tanks, we can generally be within a gallon at all points.

 

[Gordona23]

Thanks. Good info.

 

[Dynon]

The one caveat here is that we can't measure fuel we can't read. When the sensor hits the top of the tank - which frequently happens with a few gallons to go - that's the max fuel we'll display. So if you have a tank that holds 10 gals but the sensor stops moving at 8, your display will say "8+" until the sensor comes off the peg and starts to move.

 

[mmarien]

The capacitive fuel level sender doesn't have any moving parts so reading to the top (or bottom) of the tank isn't a problem. I've landed showing five litres and drained six so I'm confident it's accurate where it needs to be. It does read different descending or climbing so I leveled the airplane to cruise attitude to do my fuel calibration.