Slow Reading Fuel Gages

[Metalguy]

Just wondering how/why the fuel gages of many cars respond so slowly. As I understand things, the tank unit is just a variable resistor and the dash gage is a volktmeter or ammeter. SO, why the very slow response? I know WHY slow response is good; I just don't know how it's done.

 

[Retracnic]

Metalguy,

The easiest way I know of to slow the response of a DC circuit is to toss in some capacitors. Basically the capacitor acts as a noise filter. The delay is imparted by the capacitor's need to charge.

I'm sure there are other ways to do it. But that's a real simple and cheap way of doing it.
And as you know, the automakers like things cheap.

Regards
Bryan Carter

 

[GregLocock]

These days the fuel gauge is driven from the computer that controls the IP readouts, and has a time constant of the order of 1/4 hour (don't quote me). Reason is that otherwise the reading would change as you go up hills, or drive round corners. You can't normally mount the float in the centre of the tank, and the tanks are usually non-symmetric, so there is no easy way to eliminate these issues physically, even if you had antisurge baffles that worked.




Cheers

Greg Locock

 

[sunan]

The variable resistor in other system does not directly control the needle. Instead, in controls a bimetalic- mechanism which in turn averages the reading within certain period of time. This produces that slow response.
SUNAN

 

[zimbali]

What Greg said is exacly the reason. If I remember correctly from SAE standard, it should take about 60-90 seconds for the needle goes from empty to full when you connect the circuit. There are different methods to apply that:
1) using silicon oil inside the cage of fuel guage. This will damps the noises and fast movements of float in the tank as it doesn't let the inside magnet moves fast.
2) Using an integrator, usually a digital one to smooth the behaviour.
3) In some VDO models (or maybe others as well) the fuel tank is divided to two parts with a vertical plate, One of them is very small comparing to the other one. There is a very tiny hole at the bottom of this plate that lets the fuel flow from one side to another. Sensor is located in the small part and as you may guess, when there is a fast change in the big tank, smaller part doesn't FEEL it. This little hole plays the role of a delay and kind of integrator.

Sometimes there is a combination of two methods, which is usually methods 1 & 2.


Cheers


You can live in your car, but you can't drive your House!

 

[Metalguy]

Seems like the only slow gages I've seen were on newer, computer-brained cars. Thanks for the answers!