Lab standard capacitors - Page 1
Forum section: electronics / general

author: DeepLink
author_rank: Regular Contributor
timestamp: March 10, 2025, 05:25:16 pm
content: Which type of capacitors are used for calibration?Playing with the capacitor function in a DMM vs an LCR bridge, I see quite some difference, which is expectedThe DMM uses constant current and measures the time between to voltage levelsMy LCR is a auto balancing bridge (I'm using 120Hz / 1kHz and 1 VAC)Reading the calibration manual for the DMM, they recommend a Fluke 5520 for calibrationI tried several different capacitor types, and found that film types had the most equal capacitance when measured on both the DMM and the LCR bridgeBut film capacitor in 10mF (10000uF) seems quite largeSo which types are used in calibrators?

author: TimFox
author_rank: Super Contributor
timestamp: March 10, 2025, 05:32:56 pm
content: Very large capacitors often suffer from dielectric absorption, which probably causes higher errors in current-timing measurements than in AC bridges. Film capacitors can have good dielectric absorption, but I have never seen one so large as 10 mF: one could build a parallel capacitor bank with careful construction.Dielectric absorption is often treated with a model using parasitic internal resistors, but it is really a non-linear effect analogous to ferromagnetism. An electret is an extreme example of ferroelectricity.

author: manganin
author_rank: Regular Contributor
timestamp: March 10, 2025, 06:40:42 pm
content: Quote from: DeepLink on March 10, 2025, 05:25:16 pmElectronically simulated capacitor.Before modern op-amps a smaller capacitor with autotransformers.https://www.ietlabs.com/pdf/Manuals/1417_im.pdf(Chapter 4)

author: TimFox
author_rank: Super Contributor
timestamp: March 10, 2025, 06:50:27 pm
content: Quote from: manganin on March 10, 2025, 06:40:42 pmThat unit is a beautiful example of General Radio design for calibration.Couple of details:Ratio transformers are cool. I own several Wayne-Kerr bridges for different frequency ranges.Since it uses ratio transformers with practical capacitors, it does not act as a capacitor at DC.The dissipation factor (D= 1/Q) is degraded on purpose to a positive value (with resistors) to give a calibrated value ofDto compare with the external bridge being checked.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: March 10, 2025, 06:58:38 pm
content: As above, the master reference in most cal labs was the 1000 pF GR air capacitor. That would be used to calibrate a GR 1615-A capacitance bridge which used ratio transformers, allowing complete calibration from a single standard. The calibration of everything else flowed down from that. Today a "better" 1615 is made by Andeen-Hagerling:https://www.andeen-hagerling.com/ah2500a.htmThey also have ovenized fused silica standards:https://www.andeen-hagerling.com/ah11a.htmNaturally the cost of all that stuff is a bit beyond hobbyist level!

author: bsdphk
author_rank: Regular Contributor
timestamp: March 10, 2025, 07:14:42 pm
content: The really heavy duty calibration is done using "A Thompson-Lambert calculable capacitor" which is more of a physics experiment than a capacitor.It's basically a capacitor consisting of four cylindrical metal rods in a square, where you vary the "electrical length" with two "guard electrodes" in the center of the square, one fixed, one moveable.The net result is that you can calculate the capacitance starting from laws of nature.The major problem is that you end up with less than 1pF of capacitance, and laws of nature being what they are, there's not a lot you can do about that.

author: Henrik_V
author_rank: Regular Contributor
timestamp: March 11, 2025, 12:16:49 pm
content: Quote from: Conrad Hoffman on March 10, 2025, 06:58:38 pmGR 1404 A/B/C (1000/100/10 pF)https://www.ietlabs.com/1404.htmlbut can have whisker problems, the can is tin plated

author: zrq
author_rank: Frequent Contributor
timestamp: March 11, 2025, 09:11:45 pm
content: For low frequencies (small kHz) the capacitance (or any complex impedance) can be simulated digitally (with a "lab grade" sound card, PXI-4461)https://ieeexplore.ieee.org/document/6251114/

author: MarkT
author_rank: Frequent Contributor
timestamp: March 26, 2025, 09:47:02 pm
content: Quote from: TimFox on March 10, 2025, 05:32:56 pmAn electret has a permanent dipole, ferro-electricity is about materials whole dipoles respond in concert to external fields. Not sure those are the same thing, although they are clearly related. Its like comparing a permanent magnet (hard magnetic material) to a piece of soft iron (soft magnetic material).

author: TimFox
author_rank: Super Contributor
timestamp: March 26, 2025, 10:01:47 pm
content: Ferromagnetism and ferroelectricism are analogous.In both cases, a polarization remains after the original excitation is removed.In the case of an electret, this is exploited as a permanent voltage across the device due to the remaining polarization and its voltage gradient.