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Fluke 115 and Nanofarad Capacitance Readings
Barry A. Waters:
I just bought a Fluke 115 for general hobbyist & household use to augment an old Radio Shack 22-181 from the 1980's. The specs say it should be capable of reading in nanofarads (nF) to a resolution of 1 nF over a range of 1000 nF, where it jumps to uF scale. Accuracy for that range is +/- ([% of Reading] + [Counts]) and is stated as 1.9% + 2.
When I attempt to measure a simple Ceramic Disc 104 capacitor (100000 pF) I expect to see somewhere around 100 nF depending on tolerance, quality, etc. and on my old Radio Shack meter I do. On the Fluke in both autoranging and manual nF settings it starts out around 370 nF and climbs to about 1.20 uF (1200 nF). I called Fluke support and they agreed that I should be seeing something closer to 100 nF and told me to take the meter back. I did and got a replacement and it does the same thing. I note that the meter appears to work in all other respects and is very accurate when testing electrolytic caps in the uF range (4.7, 10, 47, 100, 220, etc.).
Is there something I and the Fluke rep are not understanding here? Could anyone who has this meter check a 104 Ceramic Cap and let me know what they get? I'm baffled. If it's a calibration issue it may have been a batch because the serial numbers of the two units are only 42 apart.
Barry
bdunham7:
Can you check that capacitor for resistance and leave it connected for some time? If it is leaky--and yes ceramic discs can get leaky--then it might measure quite differently on meters that use different measuring methods.
gamalot:
--- Quote from: bdunham7 on April 15, 2023, 06:27:11 pm ---Can you check that capacitor for resistance and leave it connected for some time? If it is leaky--and yes ceramic discs can get leaky--then it might measure quite differently on meters that use different measuring methods.
--- End quote ---
I have a Fluke 117 and it measures 100nF ceramic disc capacitor without any problem. If I connect a 1 megohm resistor in parallel with the 100nF capacitor it will measure about 400nF.
Swainster:
I've got a 115 in front of me, but I dont have an equivalent ceramic cap to test your scenario. Best I can do is a 47nF film cap, which reads 46.5nF on a cheap component tester, and 48nF on the fluke. The largest ceramics that I have on hand are 1000pF which, out of a random selection of 7 pieces before I got bored, all read 1nF, and 2 of them in parallel read 2nF, so looks like the fluke is operating well within it's datasheet spec.
Swainster:
(Decided not to add this as an edit as it is new data)
The plot thickens! By rooting around in a few more boxes I managed to find some unused 104 marked ceramic disks (of the budget aliexpress component kit variety). And these only read 50 to 60nF (e.g. 50nF on the cheap component tester and 60nF on the Fluke).. I was not able to measure any parallel resistance but I'm not really set up for that measurement at home - best I can do is the 100M range on an Aneng 8009. I also had a go at measuring the leakage current with a 1V bias and an old Keithley 480 picoammeter, but without setting up some shielding, all I can say is that the ceramic appears to have more leakage than a film cap (in the 10s of picoamp range), but not really quantify it. I also put a 3M resistor in series with the cap to stop/reduce oscillation.
Incidentally, the Aneng capacitance measurement agrees with the fluke and the component tester, but I believe that they all use the same measurement technique so that is not too surprising. Tomorrow I will try to remember to bring the suspicious caps to the office, and try them out on an LCR meter. To be honest, my current theory is dodgy cheap capacitors, but it is possible that the multimeter style measurement technique is causing this issue.
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