Another physics question

[Neil]

If you have a container and you put two vertical parallel wires in it, why can't you determine the quantiy of a liquid poured into that container based on the resistance measured between the two wires?

[The Natural Philosopher]

If the material is conductive.

You can also measure the capacitance if its an insulator

But modern oil gauges and height gauges prefer to use ultrasonic rangefinding on the top of the liquid.

Not always a good idea to have electrical conductors in flammable liquids..

[Martin Brown]

Only works if the resistance is in the right range. If the water is impure then it conducts too well and looks like a dead short. Tiny changes in the (im)purity of the water would affect the "level". That is dipping your hand in for instance providing more ionic salts.

There is a trick using a central wire and an outer metal tube as a variable capacitor with the central wire insulated that exploits the difference in relative permitivity between the liquid and air. Sort of an electronic version of the old manometer type oil tank measure.

These days most systems use an ultrasonic rangefinder to determine distance to liquid surface. They seem to last ages (unless the guy who delivers fuel sometimes replaces the battery unknown to me).

Regards, Martin Brown

[Ghostrecon]

between conductors and their area ... oh and the permitivity of the dielectric

[Gib Bogle]

All able to be calibrated.

[geoff]

In message , harry writes

Duh - The wires have a capacitance between them and the fluid (assuming its an insulator) acts as a dielectric between the wires

[geoff]

Not once its calibrated

[The Natural Philosopher]

until you change the fluid composition...

[geoff]

Well yes, I was assuming a certain level of common sense in my reply

[Steve Walker]

That's when you use intrinsically safe devices and either zener barriers or galvanic isolators - restricting the voltage (typically 28V dc max) and current (typically 20-odd milliamps) so that there is not enough energy supplied or stored to ignite any vapours.

SteveW

[geoff]

In message , harry writes

Any two conductors with an insulator in between form a capacitor

You are obviously out of your depth here - stop digging

[grimly4]

I'd understood the vapour concentration in a typical petrol tank was too high for explosion risk. Otoh, it's not entirely unknown for one to go up. Years ago I saw a pic of static discharges in an Avgas tank - slightly worrying.

[Tim Watts]

Surprised they don't have a vapour recovery system...

[grimly4]

I know. These were pics taken of the inside of a fuel tank *as it was being refuelled*, with anti-static precautions in place. It was remarkable how the whole lot didn't go up. I presume it was nothing unusual, but still...

[Tim Watts]

Same as the petrol station do when they recover it (which is rare in this country).

put it back in the storage tank?