edit: Did you exclude the resistance of the leads of your multimeter? Use the REL feature if your multimeter has one.
See PA4TIM's blog post about ESR, hopefully it can explain more about it in simpler terms:
http://www.pa4tim.nl/?p=3775Also see these pdf's besides the one I attached:
http://www.low-esr.com/QT_LowESR.pdfIntroduction to capacitors, the terms, formulas etc:
http://www.vishay.com/docs/28356/alucapsintroduction.pdfYou don't see the effect of the ESR because you're working at low frequencies.
ESR is the DC resistance of the inner parts of an electrolytic capacitor - the lead wires, the aluminum foil, the electrolyte (the electrolyte is not the dielectric).
Impedance is the overall resistance of the part to AC current, of which ESR is a part.
Impedance is the vector sum of the ESR (DC) and the
reactance (capacitive reactance minus the parasitic inductive reactance of the leads and foils).
______________
Z= ? (ESR2 + (Xl-Xc)2
At low frequencies (e.g. 100Hz or 120Hz) the capacitive reactance dominates so ESR is irrelevant;
In the 10s of KHz the ESR dominates;
In the 100s of KHz the inductive reactance becomes relevant and begins to dominate. You can see this effect in the impedance vs. frequency charts in the datasheets of many/most low impedance electrolytic capacitors.
Switching power supplies run at 40-60 kHz, some work at up to 200-400 kHz. Either way, they usually work above what the human ear can hear, which is around 22kHz.
Dissipation factor (tan ?)or DF is defined as the ratio of the ESR and capacitive reactance. It is the
Dissipation factor is also known as the tangent of the loss angle and is commonly expressed in percent.
Recognized standards express the dissipation factor at specific frequencies typically 120Hz for Aluminum electrolytic and Tantalum capacitors while for film capacitors is 1 kHz and 1 MHz for ceramic capacitors.
DF=ESR/Xc or tan ?
Equivalent Series Resistance or ESR for short is the sum of the ohmic losses of the dielectric, materials and connections used in the construction of the capacitor.
ESR=DF*Xc=DF/(2* ?*f*C)
ESR is normally expressed as a maximum value at specified frequencies, 120 Hz and 100kHz for aluminum electrolytic and tantalum capacitors and 100kHz for film capacitors.
Impedance is the total resistance the capacitor represents to alternating waveforms. This includes the
inductive and resistive components.
______________
Z= ? (ESR2 + (Xl-Xc)2
An important observation is the Fr parameter. Fr is the self-resonant frequency. Defined as the frequency
where Xl and Xc are equal.
_____
Fr=1/[2*?*? (L*C)]
At this frequency the impedance is equal to the ESR.
Below self-resonance the Xc component is dominant and the capacitor behaves like a capacitor. Above the self-resonant frequency the inductive component is dominant and the capacitor behaves more like an inductor.
Aluminum electrolytic and film capacitors have the impedance specified as a maximum at a specific frequency normally at 100 kHz and 20°C for aluminum electrolytic and 70°C for film capacitors.
note: fragments of texts above are copy pasted from forum posts and pdf's I saved locally and I don't have the sources anymore. Better than typing from memory.