T-34 Fuel Quantity Indicators

[Pags]

Does anyone know how the T-34's fuel quantity indicators work? I know that they are "capacitance type", but what exactly does that mean? I've heard it might involve a float...does anyone have any better info?

 

[Punk]

it involves the PFM circuit

 

[Pags]

i don't think that dog will hunt

 

[virtu050]

it's not in the t-34 natops?

 

[Rainman]

I think so. . I little dry, but it makes sense to me. . .

capacitance type meaning an AC signal is sent from one "plate" to another usually cocentric metal tubes in a probe. The "dielctric" is the fuel, or more accurate the fuel / air mixture (as fuel is burned becomes more air). Basic engineering (if you took any college physics) dictates that C = εA/d. Where ε is the permittivity of the dielectric. A is the area of the plates and d is the thickness between the plates. These are both constants in this setup.

As the amount of fuel in the probe decreases from burnoff, the amount of air in the probe increases, thus continuously changing the permittivity of the dielectric as a different electrical value. Capacitance type sending devices are typically less suspectible than float type devices (that are in your car for example) in regards to the movement or sloshing of fuel--so work better in a dyanmic aircraft environment. In general, the overall accuracy is really dependant on the consistency of the fuel (JP8 or JP5 work well, but civilian 100LL for example often contain different amount of additives that throw them off [changes the dielectric]) and more importantly the quality of the calibration that is done.

hope that helps. . . . . . .

 

[flashypants]

I think so. . I little dry, but it makes sense to me. . .

capacitance type meaning an AC signal is sent from one "plate" to another usually cocentric metal tubes in a probe. The "dielctric" is the fuel, or more accurate the fuel / air mixture (as fuel is burned becomes more air). Basic engineering (if you took any college physics) dictates that C = εA/d. Where ε is the permittivity of the dielectric. A is the area of the plates and d is the thickness between the plates. These are both constants in this setup.

As the amount of fuel in the probe decreases from burnoff, the amount of air in the probe increases, thus continuously changing the permittivity of the dielectric as a different electrical value. Capacitance type sending devices are typically less suspectible than float type devices (that are in your car for example) in regards to the movement or sloshing of fuel--so work better in a dyanmic aircraft environment. In general, the overall accuracy is really dependant on the consistency of the fuel (JP8 or JP5 work well, but civilian 100LL for example often contain different amount of additives that throw them off [changes the dielectric]) and more importantly the quality of the calibration that is done.

hope that helps. . . . . . .

"I believe, I believe!! Where's that damn button?!"

 

[Fly Navy]

Except it's not AC in a T-34.

 

[Rainman]

yeah good point. . the fuel indicating system is completely 28 VDC. .hmmm. . Obviously you can't send DC through a capacitor (you get an open circuit). . so not sure how it measures the capacitance from the changing fuel/air dielectric without some sort of AC signal. . We're talking about a very small current here. . doing a google search on different types of aircraft capacitance fuel signals . .on the order of less than 100 mA . .it could actually be al internal on the processing electronics (conversion to AC or whatever). .Other products out there do this.

hope it helps .. ..

 

[Eagle32]

Rainman,

Are you sure you are not a P-3 type??? Need to go take some Excedrin with bourbon now.

 

[Punk]

"I believe, I believe!! Where's that damn button?!"

I'm telling you, its connected to the PFM circuit

 

[Squid]

the fuel quantity indicators are on the same PFM bus as the FCU. So try looking there.

 

[Punk]

the fuel quantity indicators are on the same PFM bus as the FCU. So try looking there.

access for that would be right under the battery compartment

 

[ea6bflyr]

Great!! WHO LEFT THE DAMN DOOR OPEN? Now, as a lesson to you all, see what happens when you leave the door open? People just come walking in... Just kidding RAINMAN. Great technical explanation, but lets use little works as to no confuse the young ones. Just act like you are dealing with your Girlfiend or Wife (Speak S L O W L Y and make no SUDDEN MOVEMENTS)

Ok...Here's the easy man explanation: There are several TUBES that are made of a tube within a tube. Fuel level creates a Circuit load (More fuel = less resistance and Less Fuel = More resistance). Each load corresponds to a fuel reading. The Resistance loads are averaged out and viola you have a fuel reading on the gage. Hope this helps!!
ea6bflyr (I used to be an AE)

I think so. . I little dry, but it makes sense to me. . .

capacitance type meaning an AC signal is sent from one "plate" to another usually cocentric metal tubes in a probe. The "dielctric" is the fuel, or more accurate the fuel / air mixture (as fuel is burned becomes more air). Basic engineering (if you took any college physics) dictates that C = εA/d. Where ε is the permittivity of the dielectric. A is the area of the plates and d is the thickness between the plates. These are both constants in this setup.

As the amount of fuel in the probe decreases from burnoff, the amount of air in the probe increases, thus continuously changing the permittivity of the dielectric as a different electrical value. Capacitance type sending devices are typically less suspectible than float type devices (that are in your car for example) in regards to the movement or sloshing of fuel--so work better in a dyanmic aircraft environment. In general, the overall accuracy is really dependant on the consistency of the fuel (JP8 or JP5 work well, but civilian 100LL for example often contain different amount of additives that throw them off [changes the dielectric]) and more importantly the quality of the calibration that is done.

hope that helps. . . . . . .

 

[goplay234]

You know what? It really doesn't matter...Just remember to tap the gauges prior to a cruise check...You might find yourself up to fifty pounds low...Trust me, it happens...Scared the crap out of me...I personally think there is a sadistic hamster controlling it all, but I could be wrong.

 

[Pags]

it kinda does matter when your on-wing is asking you, "how do they work?", and the best answer you can give is, "um, they're capacitance type." but, thanks for the help, y'all.