I have a collection of 100 nF ceramic disk capacitors, which came from three completely different sources. They are all through-hole packages comprising brown disks and two leads, and marked only with the code 104.
My understanding is that, since there is no tolerance code, I should treat them as having a capacitance in the range 80–180 nF (-20% to +80%).
I have two instruments for measuring capacitance. One is a cheap digital multimeter, and the other is a cheap general purpose component tester that I bought as a DIY kit and soldered together. They give capacitance readings within about 10% of one another.
No matter what capacitor or instrument I try, the measured capacitance is about 45–75 nF, which is well below the tolerance range.
Are there any "obvious" (not obvious to me!) explanations for this phenomenon? Bad capacitors? Bad instruments? Some kind of resonance effect?