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### Topic: Fuel level sending unit (Read 1 time)previous topic - next topic

#### EmilyJane

##### Aug 08, 2011, 03:32 am
The gas gauge on my Jeep has been broken for about 10 years. I decide when it's time to refill based on the trip odometer. The sending unit is mounted from the top and you have to remove the tank to get at it so I haven't bothered to have it replaced.

The way I understand that it works is that a float on the end of an arm moves a rheostat through its range as the fuel level changes and this change in resistance is transformed into the gas gauge needle movement by various means. If I should ever replace the sending unit it might be interesting to improve on the design, as most gauges are notoriously nonlinear.

My main question is how is an explosion in the tank prevented with all that potential for sparks around? The images I have found for sending units seem to have the rheostat inside the tank.

#1
##### Aug 08, 2011, 06:48 pm

There is a small amount of current going through the rheostat so, that it will be unlikely to make arcing.
Then, the LEL-lower explosive limit, UEL- upper explosive limit have to be met for the conditions of fire.
Basically, you have to have the correct air-fuel ratio to create a fire.

There are probably other factors, those are the two I can think of,
Mark
General Arduion tutorials = http://tronixstuff.wordpress.com
http://www.gammon.com.au/forum/bbshowpost.php?bbtopic_id=123

#### EmilyJane

#2
##### Aug 08, 2011, 07:48 pm
Quote
There is a small amount of current going through the rheostat so, that it will be unlikely to make arcing.

It looks like the current can go to about 0.5A at one end of the travel. It just seems counterintuitive to me that a short circuit wouldn't produce enough heat to ignite the mixture. I realize that MOST of the time the gasoline/air ratio is above the explosive limit but is it guaranteed to be?

#### MarkT

#3
##### Aug 08, 2011, 07:55 pm
Most of the time petrol tanks contain no air at all - so explosion isn't possible.  Petrol gives off huge amounts of vapour, as the fuel slowly drains it is replaced by vapour.  Also the designers of fuel-level guages have done their homework, one assumes - I wouldn't recommend a home-made approach if that's what you're thinking!

It may be that the sliding contact is usually flooded, which would prevent ignition.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

#### flyboy

#4
##### Aug 08, 2011, 08:08 pm
There is one gauge that I might suggest.  There is a guy that used to write for kitplanes magazine and I saw this:
http://www.rst-engr.com/kitplanes/KP0006/KP0006.htm
http://www.rst-engr.com/kitplanes/KP0007/KP0007.htm
http://www.rst-engr.com/kitplanes/KP0008/KP0008.htm

I think it's a very nice solution, AND, you won't have any chance of ignition if you follow their suggestion and coat the plates.  Have a look.

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