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how do you determine..and fuel levels

N333M

Active Member
How do you determine if you have resistive float senders, or capacitive senders in your tanks?

I am beginning to calibrate the fuel levels in my RV9A for a GRT EIS system, which I didnt build, with a bit of difficulty.
On my attempt yesterday, I filled the right tank one gallon at a time, noting the reading on the EIS after each. The empty reading was 4.2 It began to change on the 4th gallon.
Then, it stopped changing at the 7th gallon, and read 5.7 each gallon there after. this isnt going to work..
Don
 
Resistive usually are just a flat thin metal plate on the outside of the tank and capacitive are usually 1/2" thick and plastic with epoxy filled center. The gauges normally do not read right to the top and if the sensor was not installed all the way to the bottom it might not read down to one gallon.
 
Restive senders have only one wire, airframe is used as ground to complete the circuit.

Capacitive units that I have seen will have two wires, sometimes coax, and use a converter box.
 
OK, As far as I know, I have but one wire.
But..I didnt trace it to the tank.
The craft had the standard Vans 2 1/4 round fuel gauges in it. they seemed to work ok in the 75 hours I flew it before I started the GRT upgrade..
IF I understand this GRT EIS fuel level calibration correctly, They recommend using the reading for full tanks as the level where the float sender stops changing..and the empty reading as the level it stops when going empty..in my case, it seems that I will have an empty reading at 3 gallons, and a full reading at 7. hmm..as I said, thats not going to work for a 16 gallon tank.
I went thru the math several times on the GRT worksheet.
Im scratching my head. If the sender output stopped increasing after the 7th gallon added...I guess it could be faulty..hard to believe it would only have a 3 to 7 gallon range of motion. (if its a float)
aircraft was level sitting in the hanger.
 
The craft had the standard Vans 2 1/4 round fuel gauges in it. they seemed to work ok in the 75 hours I flew it before I started the GRT upgrade..

If the senders were working correctly before switching from Vans gauges to the GRT----------my money is on the calibration process of the GRT.

I remember this being a royal pain in the backside, but sorry, I do not remember the actual process step-by-step.

There are two different parameters you need to get right, offset and scale factor.

http://grtavionics.com/media/User-Manual-Rev-M.-with-figures.pdf
 
First, check the wiring. You should have a wire from the regulated power (4.7 volts, I think) going to a resistor (470 ohms). The other side of the resistor is connected to both the EIS input for fuel level, and the center terminal on the float gauge. The other side of the gauge should be grounded locally.
Turn off the aircraft master switch (power off).
Disconnect the wire from the float gauge center terminal. Put an ohmmeter on the center terminal, and measure resistance to ground. It should be somewhere between 30 and 250 ohms.
If all this works, there are several ways to trouble shoot. My guess is on the wiring.
With the wire still disconnected, but not shorted to ground, turn the power on. Put the voltmeter on the disconnected wire and measure the voltage (other probe on a good ground). It should be 4.7 volts. If not, there's a wiring problem. With the same setup, carefully hold a 470 ohm resistor between the probes (while still in contact with the wire). You should see the voltage drop in half, to, 2.3 volts or so. If grossly off, you don't have a 470 ohm resistor on the other end. You'll need to find it and replace it.
Finally, if you suspect the sender:
Drain the fuel, then start adding it back into the tank. Measure the resistance each time you add fuel. You should see it smoothly go from about 250 ohms to about 30 ohms (probably stop changing as you get with 3 or 4 gallons of full). (I might have it backwards, 30 to 250? I forget.)
 
... If the 470 ohm resistors were not installed, but presumably everything else in this system was installed correctly, would changing the fuel level still be reflected on the instrument fuel quantity readings? Ie, would you still see some indication change as fueled or refueled?
 
... If the 470 ohm resistors were not installed, but presumably everything else in this system was installed correctly, would changing the fuel level still be reflected on the instrument fuel quantity readings? Ie, would you still see some indication change as fueled or refueled?

Hard to say, maybe yes, maybe no. For ideal circuits the answer is no.
If there are no resistors but the 4.7 volt source is connected directly to the EIS and float gauge, it may or not not be able to supply enough current when the float is at 30 ohms, e.g., the voltage may sag.
If the 4.7 volt source is not connected, there may be some bias voltage from the input circuity of the EIS which will be a bit affected by the changing resistance of the fuel level detector. I'd expect pretty small changes.

What I was really looking for was the resistor being grossly off in value, or hooked to 12 volts by mistake, so that the voltage is going over the range the EIS is designed to accept.
 
Bob, I was with clam most of the day today (and last weekend) wrestling with this, so I am also trying to figure it out. The GRT documentation or lack thereof does not help.

Our understanding, flawed as it probably is, is that there are two ways to connect the float based sensors in a scenario where you have a single HXR EFIS and a single EIS. Option one is to connect them directly to the EIS and then convey that info via serial port to the EFIS. Option two is to connect them directly to the EFIS via the aux ports. Option two is never mentioned in the HXR install manual...in fact, the word "fuel" is never mentioned in the HXR install manual, which I find curious. But we've heard that you can do a direct connect to the HXR...is that true? In either option, you'd need the 470 ohm resistor, right? And in option two, since there's no excitation current available from the HXR, would you have to get it from the EIS?
 
YEs..the Blue 4.8 v excitation wire is tied to a resistor installed parallel to the signal wire from the sender. The video from GRT calls it a "resistor tree" you need to build. I did that, but I did NOT check to see what the voltage was after installation.
I will do that next.
 
GRT EIS Fuel Senders

GRT has a document that can be downloaded from their website that explains/details how to connect and calibrate resistive fuel senders with the EIS. It is located in the "EIS Engine Monitor" area in the "Support" section on the GRT website. The title of the document is:

"Calibrating Resistive Float-Type Fuel Senders EIS and Sport EFIS Manual Supplement"

There is another GRT document that explains how to connect resistive fuel senders directly to the Hx or Hxr EFIS. The title of this document is:

"Calibrating Resistive Float-Type Fuel Senders with the Horizon EFIS"

It doesn't appear on the GRT website anymore but can be found by just doing a Google search on the interweb. When connecting the fuel sender(s) directly to one of the analog inputs of an Hx or Hxr display the external pull up resistor isn't needed.
 
When connecting the fuel sender(s) directly to one of the analog inputs of an Hx or Hxr display the external pull up resistor isn't needed.

That?s because the Hx/HXr have software selectable internal pull up resistors. Be sure to turn them on.
 
Sport EX and EIS 4000

I am installing the Sport EX, with the EIS 4000 Hmm..I wonder if the Sport EX hooks up the same way?.. i have already downloaded several "other than " sport EX manuals to figure out how to do this, as there are not any for it yet.
Don
 
That?s because the Hx/HXr have software selectable internal pull up resistors. Be sure to turn them on.

Thanks for the feedback... I have both of the manuals (EIS connection and HXr connection). I originally installed per EIS method, but then changed my mind when I installed the HXr and thus removed the resistors and kept the internal resistors selected on.

My issue is that I see no change in fuel quantity real time when adding fuel. Almost like the senders aren't moving. However, if I then go back into the analog settings, change a few things back and forth (sensing, scaling, etc), I will see a new value on fuel quantity, and not just a scaled version of the previous reading. So I believe senders are moving. Plus, this is a new installation and the senders checked fine at installation a couple of months ago.

Ironically, I received guidance from GRT last week for a third option: excitation from EIS, wire in the resistors, and then connect to HXr. No mention of the internal resistors in that exchange.

I will give GRT a phone call and discuss. I probably gooned up the wiring somewhere but we having been diligently retracing the system with no joy yet.

Last tidbit for the GRT users... my strategy was intending to use the HXr to display fuel quantity from the senders and the EIS to display fuel flow based purely on fuel flow. Just an attempt to provide some cross-checking.
Cheers
 
I am installing the Sport EX, with the EIS 4000 Hmm..I wonder if the Sport EX hooks up the same way?.. i have already downloaded several "other than " sport EX manuals to figure out how to do this, as there are not any for it yet.
Don


Don, I assume you have seen paragraph 2.4 of the "Calibrating Resistive Float Type Fuel Senders EIS and Sport EFIS Supplement"? It's available as Don mentions. Summarizing...

2.4 Sport EFIS
*sport does not have analog inputs
*must wire senders to EIS aux inputs
*however, do not have to calibrate via EIS method
*you can send raw voltage data over the serial line to the EFIS
*this method eliminates the math, but keeps EIS aux readouts in raw format
*recommended only for users with blind mounted EIS
*after wiring the senders per EIS method, refer to the Horizon EFIS supplement
 
... sorry, as James mentions ....


Don, I assume you have seen paragraph 2.4 of the "Calibrating Resistive Float Type Fuel Senders EIS and Sport EFIS Supplement"? It's available as Don mentions. Summarizing...

2.4 Sport EFIS
*sport does not have analog inputs
*must wire senders to EIS aux inputs
*however, do not have to calibrate via EIS method
*you can send raw voltage data over the serial line to the EFIS
*this method eliminates the math, but keeps EIS aux readouts in raw format
*recommended only for users with blind mounted EIS
*after wiring the senders per EIS method, refer to the Horizon EFIS supplement
 
RE: GRT Fuel Levels

According to the HS Sport Sx/Ex manual, the Ex version has 4 analog inputs that can be used to measure voltage inputs (like the Hx and Hxr) and fuel level is mentioned as a possible use for them.

My two cents (for what its worth) using the EIS 4.8V excitation output and external resistors should provide a little more precision. Internal pull up resistors are typically just used for discrete (on or off) type inputs rather than accurate analog voltage measurements.

Once calibrated, the EIS calculation of fuel remaining (fuel used) using the fuel flow measurement is within a few tenths of a gallon of actual fuel used so I trust it way more than what the fuel tank measurement is indicating.
 
Ironically, I received guidance from GRT last week for a third option: excitation from EIS, wire in the resistors, and then connect to HXr. No mention of the internal resistors in that exchange.

I will give GRT a phone call and discuss. I probably gooned up the wiring somewhere but we having been diligently retracing the system with no joy yet.

Last tidbit for the GRT users... my strategy was intending to use the HXr to display fuel quantity from the senders and the EIS to display fuel flow based purely on fuel flow. Just an attempt to provide some cross-checking.
Cheers

If you use the EIS 4.7 volts with 470 ohm resistors, then you need the internal pull ups off. This probably is more accurate than using the internal pullups, as they are something like 10K ohms - pretty high for this application.
The HX (so I presume tbe others too) can display total fuel remaining from the floats, and from the fuel flow, on a single bar graph - very easy way to cross check the two methods.
Everyone should know the shortcut to resetting the EIS fuel level based on flow - push and hold the left and right buttons at the same time.
 
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