When I measure the capacitance of two cables I get very volatile values if I go to lower frequencies like 500 Hz. The capacitance is about 50 nF. Why does this happen?

  • 3
    \$\begingroup\$ Describe your measurement procedure and equipment \$\endgroup\$
    – AndrejaKo
    Sep 27 '12 at 19:30
  • 1
    \$\begingroup\$ What do you mean by "volatile"? Do the values fluctuate at a fixed frequency or change drastically as the frequency changes? \$\endgroup\$
    – Barry
    Sep 27 '12 at 21:25
  • \$\begingroup\$ Measuring 50 nF with 500 Hz doesn't "compute". A 50 nF capacitor looks pretty much like an open circuit to 500 Hz. \$\endgroup\$
    – Kaz
    Sep 28 '12 at 2:14

If this device measures capacitance by measuring the current at a given frequency, the current drops as the frequency goes down. This is due to capacitive reactance being proportional to frequency. (X = 2*pi*f*C) At low frequency, the current is very small and hard to measure reliably.

  • \$\begingroup\$ Xc = 1 / (2*pi*f*c) \$\endgroup\$
    – Macuser
    Sep 19 at 15:57

@gbarry has the right idea, plus also consider the fact that you're moving closer to mains frequencies (60 Hz, 50 Hz) and the fact that 50 pF is easily coupled into with stray fields. It looks like you're just more susceptible to the noise around you at the time that the instrument is needing to be more sensitive.


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