I recently flew an aircraft with a fuel totaliser.
It was an aircraft that was sometimes run on mogas and sometimes with avgas.
The instructor pointed out that the totaliser gave slightly difference readings with different fuels.
This is something that I hadn’t really thought about before (probably because I’d never flown an aircraft with a fuel totaliser before!).
I’d assumed that they were measuring volume and that wouldn’t change with the fuel. But I suppose they may be actually measuring weight/mass and using that to convert to volume.
So now I have the question – “Is this true for all GA fuel totalisers? Do they all give different readings with different fuel types?”.
They are measuring volume, not mass. It is a little turbine. It measures flow velocity which for a given dimensional config, is volume flow.
Mass flow is a lot more complicated.
But engine power is related to mass flow because it is produced by attaching x molecules of fuel to y molecules of oxygen 
Interesting stuff.
My engine monitor (G3X) has the option of calibrating the FF in flight, and therefore it’s working accuracy. Knowing the specific consumption, and having accurate fuel quantity readings will help
On another note concerning the use of different fuels, the fuel quantity readings might be affected too…
As an example my ship sports capacitance sensors. Those are nothing but 2 aluminum plates shaped like the tank ribs, one in the inner the other in the outer tank bay, installed on nylon standoffs, and wired together. A frequency (thru a small converter) being the output.
This system, light, cheap to manufacture, no moving parts, has proven to be quite precise. But to achieve this the calibration has to be done using the fuel which be used later, lest discrepancies will occur.
This must be a common issue with Rotax engines, where owners prefer to use Mogas or UL98 but often when away from base have to use avgas.
So if you are having to swap between different fuels depending on availability, is it impossible to have an accurate fuel totaliser?
dublinpilot wrote:
a common issue with Rotax engines, where owners prefer to use Mogas or UL98
Most Lycomings also prefer Mogas rather than Avgas, which I guess we already covered in some other thread here 
And so does mine, which I try to run exclusively on Mogas, or Cargas. Avgas will be used only if no other choice available.
dublinpilot wrote:
is it impossible to have an accurate fuel totaliser?
No, I cannot change the displayed fuel quantities. The only way would be to recalibrate the system using the “new” fuel. So I’m having to account for a loss of accuracy when using anything else than the Mogas 95 the system was calibrated with. But again, this is due to my fuel sensors being of capacitance type vs float type.
The FF is the indication which is in flight trim-able, up to + or – 10%. And my fuel calculations are primarily based on time and FF.
dublinpilot wrote:
So if you are having to swap between different fuels depending on availability, is it impossible to have an accurate fuel totaliser?
Dan was talking about the per-tank fuel level measures, not about a separate fuel flow measure. My understanding is that for a separate fuel flow measure, once calibrated it will be accurate by volume whatever liquid goes through it. But the fuel flow for a specific engine power / TAS / … may vary a bit depending on the fuel because they have different volumetric energy density.
https://hypertextbook.com/facts/2003/ArthurGolnik.shtml cites Nommensen, Arthur. List of common conversion factors (Engineering conversion factors). IOR Energy which gives 34.2 MJ/l for mogas and 33 MJ/l for avgas.
My O-360 drinks ether 100LL or 91UL ( when I can get it ) . He fuel flow numbers aer not significantly different and certainly not enough to be of any practical concern.
lionel wrote:
My understanding is that for a separate fuel flow measure, once calibrated it will be accurate by volume whatever liquid goes through it.
Not quite. Turbine flow sensor readings are affected by fluid viscosity (not much difference between avgas and mogas, but avgas and Jet-A certainly require different calibrations). What’s even worse, viscosity is affected by temperature.
Dan wrote:
As an example my ship sports capacitance sensors. Those are nothing but 2 aluminum plates shaped like the tank ribs, one in the inner the other in the outer tank bay, installed on nylon standoffs, and wired together. A frequency (thru a small converter) being the output.
This system, light, cheap to manufacture, no moving parts, has proven to be quite precise. But to achieve this the calibration has to be done using the fuel which be used later, lest discrepancies will occur.
It’s important to know that the presence of alcohols, ethers (e.g. ETBE) and especially dispersed water will have a bigger effect on a capacitive sensor than switching between different grades of hydrocarbon-only fuel.
Ultranomad wrote:
Not quite. Turbine flow sensor readings are affected by fluid viscosity. What’s even worse, viscosity is affected by temperature.
Thanks for the precision. I assume different grades of fuel have quite close viscosity?
lionel wrote:
I assume different grades of fuel have quite close viscosity?
Avgas and mogas are pretty close, avgas and Jet-A are quite different.
