Is ESR value different between on Circuit and off Circuit?

[chth96]

A few days ago, I purchased MESR-100 ESR meter, and When I measured a SMD capacitor on circuit,MESR-100 showed ESR value of 30, But I managed to desolder this SMD capacitor from circuit and measured it then MESR-100 showed ESR value of 18.
I also measured it with Mega328 Transistor Tester, and It showed ESR value of 17.
Is my MESR-100 defective, if the ESR value is different between on Circuit and off Circuit?

[Per Hansson]

Re: Is ESR value different between on Circuit and off Circuit?

There are several things to keep in mind when measuring capacitors in circuit:
If there are two capacitors in parallel you will have no idea if one of them is bad.
If there are resistors in parallel this will show an artificial high reading.
If there are really sensitive semiconductors in parallel these could in theory be switched on by the low testing voltage and affect the reading.
Another thing to keep in mind is that ESR is highly temperature defendant (just refer to the datasheets).
So when you desolder a capacitor you heat it up: this gives it a much better ESR value until it has had time to cool down internally.
And if you heat it up allot you can even boil the electrolyte which could have both bad and good results depending on pure chance.