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'How control Liquid Level with PIC16F84 ?'
1998\11\08@112138 by Alessandro Marzini

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Hello!

I would like to build a level meter to misure the Liquid level (propeller,
benzine, etc) in my car using a PIc16f84.

which pricipe can I use? Is it dangerous putting some metal sensor in the
liquid?

Can you help me?

Thanks!

Alessandro Marzini

1998\11\08@143557 by Brian Striggow

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Its not the metal in the tank that's dangerous, but the available
electrical energy.  I believe most fuel level sensors use some sort of
intrinsically safe approach to limit the available spark energy to 1mJ
or less for gasoline.  Often this is done by using a high series
resistance in the sensor circuit to limit potential ground fault current.

If I had lots of time to tinker with this, I'd try an ultrasonic
approach.  Look for recent article in Nuts & Volts using PIC to drive and
interpret an ultrasonic sonar system.


On Sat, 7 Nov 1998, Alessandro Marzini wrote:

{Quote hidden}

1998\11\08@154727 by paulb

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Brian Striggow wrote:

> Its not the metal in the tank that's dangerous, but the available
> electrical energy.  I believe most fuel level sensors use some sort of
> intrinsically safe approach to limit the available spark energy to 1mJ
> or less for gasoline.

 Curiously enough, it's not that dangerous at all.  The commonly used
"idiot light" on the dashboard to tell you your fuel is low uses a non-
linear NTC thermistor dipped in the fuel itself as the sensor.

>  Often this is done by using a high series resistance in the sensor
> circuit to limit potential ground fault current.

 This thermistor gets quite hot when not submerged at which point its
resistance drops low enought to turn the indicator light (1W) on (but
the resistance doesn't of course go so low it doesn't stay warm).  You
notice this light never goes on or off rapidly.

 I'm not sure the temperature is likely to exceed the flashpoint of the
petrol, but the safety of the mechanism *actually* relies on the absence
of *air* in the tank.  No air, no explosions.

 Petrol only explodes when there is much *more* air than petrol vapour.
This can only occur in an empty tank, so those cars in the movies which
explode on (or before) impact do so because of the dynamite, not the
petrol tank!
--
 Cheers,
       Paul B.

1998\11\09@110453 by Anthony Wilson

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There is an example design like this on the Microchip web page.
Check it out!

Anthony

1998\11\11@161948 by enpassant

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Is there something wrong with the list?
All of sudden I stopped getting posting????

Alessandro Marzini wrote:

{Quote hidden}

1998\11\11@162000 by Markus Rohe

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>
> Its not the metal in the tank that's dangerous, but the available
> electrical energy.  I believe most fuel level sensors use some sort of
> intrinsically safe approach to limit the available spark energy to 1mJ
> or less for gasoline.  Often this is done by using a high series
> resistance in the sensor circuit to limit potential ground fault current.
>
> If I had lots of time to tinker with this, I'd try an ultrasonic
> approach.  Look for recent article in Nuts & Volts using PIC to drive

<SNIP>

Well this is a little late answer but unfortunatelly I came across to
read my mail today. I also didn't see the application on microchips
webpage. But...

Why don't you use a conservative method? Think about how a capacitor
works. Two isolated metal plates with a dielectricum inbetween.
Now for your application take two well isolated metal stripes or sticks
(for the plates; place them in a distance of a few millimeters) and your
dielectricum is the liquid (water, fuel, etc.). If you have alook at the
formula of the capacity of a capacitor, you'll see that the capacity is
proportional to your filling level. So if you use a low-voltage
oscillator (CMOS) there sould be no problem with dangerous sparks. A few
weeks ago there was a schematic for a frequncy counter based on a 16C84
on the list. Now you just calculate the filling level from the
frequncy...


well this was just an idea. It should also work in oil tanks and water
wells...

--
Friendly regards

Markus Rohe

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Markus Rohe
spam_OUTmaro0019TakeThisOuTspamstud.uni-sb.de

Universitaet des Saarlandes
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