Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Capacitive level sensor for liquids

  1. Mar 17, 2010 #1
    1. The problem statement, all variables and given/known data

    The quantity of liquid available in its storage tank is often monitored by a capacitive level sensor. This sensor is a vertically aligned cylindrical capacitor with outer and inner conductor radii Ra and Rb, whose length L spans the height of the tank. When a nonconducting liquid fills the tank to a height (h less than or equal to L) from the tank's bottom, the dielectric in the lower and upper region between the cylindrical conductors is the liquid (Kliq) and its vapor (Kv) respectively.

    a) Determine a formula for the fraction F of the tank filled by liquid in terms of the level-sensor capacitance C
    b) By connecting a capacitance-measuring instrument to the level sensor, F can be monitored. If L = 2.0m, Ra = 5.0mm, Rb = 4.5mm, Kliq = 1.4 and Kv = 1.0, what values of C (in pF) will correspond to the tank being completely full and completely empty?

    2. Relevant equations

    Capacitance for a cylinder: C = (2piεL)/ln(Ra/Rb)

    and ε= Kε0

    C=QV, Q is charge on the capacitor and V is potential difference between them

    3. The attempt at a solution

    I figured to treat the capacitor as 2 different cylindrical capacitors in parallel with different dielectrics in them.

    C = Cliq + Cv
    = (2piKliqε0h)/ln(Ra/Rb) + (2piKvε0(L-h)/ln(Ra/Rb)
    after some simplification:
    = (2piε0(hKliq - hKv + LKv)/ln(Ra/Rb)

    I don't know how to get from an equation with h and L to F, since F is h/L.
  2. jcsd
  3. Mar 17, 2010 #2


    User Avatar
    Homework Helper

    What if you divide both sides by L?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook