Cap question

[Mark Hammer]

So, I was trying to match some caps for a semi-parametric EQ.  I put some "greenies" in the meter, and their value stayed stable very shortly after insertion.  I put some small ceramics in, and the values started low and just climbed and climbed and climbed.  It's not hot indoors.  I wasn't touching them post-insertion.

So why is the one stable and the other not?  DoI need to wait for the meter to come to rest before assuming the actual value of the ceramics?

[Gus]

What brand, model number meter?
What type of ceramic? This matters a lot.

[armdnrdy]

My Fluke 87V doesn't "like" capacitors in the pf range. 

[Mark Hammer]

What brand, model number meter?
What type of ceramic? This matters a lot.

Fair point and fair questions.
It is a "generic" 50v, +80/-20% disc, about 3mm across, with a "y5V" temperature coefficient.  In this case, 15nf.  The DMM is an older 3-1/2 digit Micronta (Radio Shack).

[dschwartz]

Y5V ... I use X7R on my builds with no issues

[duck_arse]

I had a 2n2 ceramic disc, from a bag of 100 "the same", in a 7555 doing some non-critical timing. the one in was drifting everytime I switched on, others from the bag were stable, and much closer to the marked value. very ordinary caps indeed.

[Gus]

I don't know how the RS meter measures capacitance.  Does it have DC on the leads http://www.nandblog.com/avoid-y5v-capacitors/
some links from a search for "y5b ceramic"  look at the voltage and temp curves
http://www.johansondielectrics.com/basics-of-ceramic-chip-capacitors
http://www.niccomp.com/help/VoltageCoefficientofCapacitors-032012-R1.pdf

[snap]

Maybe you get a more consistent reading, if you shorten (discharge) the leads of the cap before measuring? Might be some "static" in them, that fools the meter?

[12Bass]

Apparently your capacitance meter is a cork sniffer....   

[dschwartz]

Question..is the cap in question being measured in the circuit or by itself?

You can't measure capacitance if the cap is soldered or connected to something else..it gives random readings

[Mark Hammer]

The cap isn't installed in anything other than the socket on the meter where you insert the leads.