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Is it safe ?

Discussion in 'Electronic Basics' started by mowhoong, Mar 12, 2008.

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  1. mowhoong

    mowhoong Guest

    I dismantled a fuel sensor from the petrol tank in my boat.
    To my surprise, the sensor made up of a roll of bare resistance wires
    in contact with a wiper arm.
    This sensor was exposed in the flammable environment, as the
    electrical flash will occur between the two contact points. It is very
    dangerous to install the sensor in such position.
    Do anyone have comment on this ? Thanks
  2. That depends on what petrol is.

    If the fuel is diesel oil, it might possibly cause ignition.
    This is because its low vapor pressure forms an air fuel
    mixture above the liquid that is in the explosive mixture range.

    If the fuel is for a spark ignited engine (called gasoline,
    in the U.S. Such highly volatile fuel produces so much
    vapor that little air remains in the tank to burn it. The
    vapor forms a non ignitable, over rich mixture. Many
    gasoline engine cars have electric fuel pumps in the tank
    (with carbon brushes and copper commutator bars) that run
    either submerged in gasoline, or dry, depending on how much
    fuel is in the tank. There is a risk of ignition only if
    the tank is not only low, but also dry, with just a little
    vapor remaining.
  3. Dave22

    Dave22 Guest

    While at first glance this does look bad, this is the way almost all
    of the fuel gauges on cars work.

    The obvious kicker is in the operation of the gauge itself. These use
    a thermocouple type of action that results in a very low current and
    voltage on the sensing wires you are looking at. Thus, due to the
    limited voltage and energy applied to the sensors there is no risk of
    a spark.

    This is also why most gauges are very slow to respond to changes in
    the tank level.

    Dave 22
    Master Electrician and JOAT
  4. Even aircraft use this type of sensors. The voltages and currents are so low
    that there is no danger for sparks. Seem to remember a case where such a low
    power signal line short circuited with a high(er) voltage wire and blew the
    main fuel tank. Some program like "Aircrash investigation".

    petrus bitbyter
  5. Gasoline composition varies widely, and depending temperature the vapor
    concentration at "full vapor pressure for the temperature" is often in the
    explosive range.

    As for diesel - this is usually not the case. Diesel produces less
    concentrated vapor than kerosene does, and the flash point of kerosene is
    above 100 degrees F (38 degrees C). However, this is something I would
    not like to bet my life on.

    - Don Klipstein ()
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