[ASK] Standard Capacitor ....

[BravoV]

Which capacitor type is considered "good enough" as a standard capacitor ? Also curious what are other types that are used as in metrological standard ?

The words "good enough" here I mean for enthusiast grade people like my self. 

Ok, for me, maybe not a metrological standard grade to be honest, but more like stable and predictable enough for used as hobbyist reference caps. My reason is quite simple, I own few LCR meters, starting from cheap handheld up to benchtop type that can measure and sweep the DUT up to 1 Mhz with adjustable DC bias feature.

As enthusiast, for example sending my benchtop LCR to calibration is way too overkill for my budget also my need.

What I want is to have few caps that are known to be stable in years, once measured and profiled by other calibrated high end LCR meter, these will be my own reference for checking my LCR meters in the future whether they're still in spec, exactly like those voltage standards that are sold for checking DMM.

Any comments regarding long term stable capacitor type and also it's discussion are welcome.

Thanks in advance.

[Vgkid]

Silver mica caps are used for standards. They should be pretty stable, but im not sure on the TC of commercial ones.

[lowimpedance]

Fused silica is a popular dielectric for Metrology grade capacitors.
for example:
http://www.andeen-hagerling.com/ah11a.htm

[quarks]

Fused silica is a popular dielectric for Metrology grade capacitors.
for example:
http://www.andeen-hagerling.com/ah11a.htm

Very interesting
Until now I thought the GenRad 1404 is the very best Standard.
http://www.ietlabs.com/1404.html

before I got the metrology-/voltnut-/ppm-virus, I used this kind of capacitors (see att. pic. 1)
I have them since early to mid 80s and they are still very stable and usefull for hobbiest references (but by no means Standard capacitors)
If I would search for replacements I would look for Glass Capacitors and good NPO (like this blue ones in pic. 2)

[manganin]

Which capacitor type is considered "good enough" as a standard capacitor ? Also curious what are other types that are used as in metrological standard ?

Any comments regarding long term stable capacitor type and also it's discussion are welcome.

Gas dielectric capacitors

The capacitance is proportional to the area of the electrodes and inversely proportional to their distance. That is why mechanical rigidity and low thermal expansion coefficient of the electrodes and their support structures (insulators) are the main design parameters. In the before mentioned General Radio 1404 the electrodes and support screws are made of extremely low expansion alloy Invar (FeNi36). The mechanical rigidity can never be overlooked: when turned upside down, the capacitance of the 1404 slightly changes due to gravity.

The capacitance is also related to the permittivity of the dielectric gas which unfortunately depends on temperature, pressure and moisture. The capacitance element of the 1404 is closed inside a rigid hermetic brass container filled with dry nitrogen. The oxygen and moisture free construction is also important to avoid corrosion. Which basically means that conducting metal electodes slowly turn into insulating oxide which affects the capacitance by reducing the thickness of the electrodes and altering the permittivity of the dielectric.

The supporting insulator material forms a part of the dielectric and also a path for the losses. However in 3-terminal capacitors this problem can be avoided if there are no supporting insulators between the electrodes but only between the electrodes and the guard terminal.

The frequency dependence of the gas dielectric capacitors is very low because the permittivity of gases is relatively constant up to several hundred MHz. In real life the parasitic inductance of the connections sets the limit.





Solid dielectric capacitors

Using solid dielectric is usually the only practical way for making capacitance standard above a few nF. The solid dielectric allows the electrodes and the dielectric to be very thin but still mechanically stable (again the capacitance is proportional to the area of the electrodes and inversely proportional to their distance). The electrodes can even be vaporized on the surface of a thin dielectric. The permittivity of the solid dielectrics is also higher than gases which increases the capacitance. The most useable dielectrics in capacitance standards are mica and fused silica.

The frequency dependence of the solid dielectrics is much worse than gasses and so are the losses.

[BravoV]

before I got the metrology-/voltnut-/ppm-virus, I used this kind of capacitors (see att. pic. 1)
I have them since early to mid 80s and they are still very stable and usefull for hobbiest references (but by no means Standard capacitors)
If I would search for replacements I would look for Glass Capacitors and good NPO (like this blue ones in pic. 2)

Same here, although mine are mostly silver micas.

Bought few of this NPO recently, going to use it as reference cap, with tempco of 30ppm and virtually doesn't age as Kemet claimed. 

What made me interested at this cap is apart from its a NPO, it has relatively big capacitance (100nF) compared to most silver micas I have.

[BravoV]

Solid dielectric capacitors

Using solid dielectric is usually the only practical way for making capacitance standard above a few nF. The solid dielectric allows the electrodes and the dielectric to be very thin but still mechanically stable (again the capacitance is proportional to the area of the electrodes and inversely proportional to their distance). The electrodes can even be vaporized on the surface of a thin dielectric. The permittivity of the solid dielectrics is also higher than gases which increases the capacitance. The most useable dielectrics in capacitance standards are mica and fused silica.

@Manganin, thanks for the insight of the big boys standards, its amazing to see how it was build, to my eyes its more like a piece of fine art.

As you pointed above on solid dielectric, fused silica is definitely out of question, compared to mica, how about the 100nF NPO at my previous post ? Is it good enough as a hobbyist reference cap ? Also reading Kemet's literatures, the NPO's tempco is quite linear and predictable.

It has relatively huge capacitance compared to common mica caps which usually only max out at few thousands pF.

[manganin]

how about the 100nF NPO at my previous post ? Is it good enough as a hobbyist reference cap ? Also reading Kemet's literatures, the NPO's tempco is quite linear and predictable. It has relatively huge capacitance compared to common mica caps which usually only max out at few thousands pF.

Capacitance standards constructed from NP0 SMD ceramic capacitors have succesfully been used even in primary impedance metrology. But as far as I know for short term transfer only. There are application like high frequency and large capacitance where the stability requirements are not as high. But I am still a little suspicious if there are any types stable enough for other than transfer use. On the other hand there has been a lot of development in the materials and manufacturing of the ceramic capacitors recently...

The NP0 capacitors have many good properties. The dissipation factor equal or lower than mica capacitors. And the high frequency behaviour is good, mostly because of the small size. The temperature coefficient is too high as-is, but it is easy to make an oven for a small chip.

The transfer standards that I have seen were constructed from a large number of a few nF range capacitors. Usually higher capacitance in the same size chip means more compromises.

[TimFox]

In the old General Radio standard capacitors and decade capacitors, silver mica was recommended for high frequencies, but polystyrene for lower frequencies.  The manuals had examples of change in effective capacitance for mica at low frequencies, which they blamed on polarization effects (presumably of the water of crystallization in the mica).  Polystyrene has a higher tempco, however.

[BravoV]

When in old General Radio's era, apart from the proven mica, just wonder if the development of NPO material at that time already matured as now ?

[ManateeMafia]

There have been several different Russian NOS silver mica capacitors for sale.
Search for the SSG mica capacitors on ebay. They appear to be hermetically sealed and many are sold in lots.

[TimFox]

General Radio and Cardwell made air-spaced standard capacitors up to about 1000 pF, with low-expansion glass mounting plate and spacers.  They were contained in coffee-mug sized cylinders with two male banana plugs (one to the case).

[BravoV]

There have been several different Russian NOS silver mica capacitors for sale.
Search for the SSG mica capacitors on ebay. They appear to be hermetically sealed and many are sold in lots.

Cool, didn't aware of those thingy, I will be watching ebay on those caps, thanks.

General Radio and Cardwell made air-spaced standard capacitors up to about 1000 pF, with low-expansion glass mounting plate and spacers.  They were contained in coffee-mug sized cylinders with two male banana plugs (one to the case).

Yep, now I have an idea on how to hermetically seal these reference caps, thanks.

[amspire]

I have a pile of Russian 50,000pF 0.5% SSG capacitors. I see you can get them on eBay for about US$10 for 4 capacitors.

They are enclosed in a metal body and they appear to be hermetically sealed with glass seals around the leads. Mechanically very stable - squeezing the body causes less then 1ppm change. I will try and do some tests including a comparison with a GenRad type 1409 Silver Mica standard capacitor. I suspect they will perform very well.

I also have some Russian plastic case 360pF 5% Mica caps that I will test as well.

I will have to set up a little shielded box for 3 wire measurement since measuring changes in a 360pF capacitor down to 1ppm means the bridge is sensitive to capacitances of 360 atto Farads (360 x 10^-18 Farads). Without a shielded box, walking into the room will change the capacitance.

[BravoV]

I have a pile of Russian 50,000pF 0.5% SSG capacitors. I see you can get them on eBay for about US$10 for 4 capacitors.

They are enclosed in a metal body and they appear to be hermetically sealed with glass seals around the leads. Mechanically very stable - squeezing the body causes less then 1ppm change. I will try and do some tests including a comparison with a GenRad type 1409 Silver Mica standard capacitor. I suspect they will perform very well.

I also have some Russian plastic case 360pF 5% Mica caps that I will test as well.

I will have to set up a little shielded box for 3 wire measurement since measuring changes in a 360pF capacitor down to 1ppm means the bridge is sensitive to capacitances of 360 atto Farads (360 x 10^-18 Farads). Without a shielded box, walking into the room will change the capacitance.

Hi Richard, welcome back, its been a while since your last post ! 

Looking forward to see that little box of yours and how it performs. 

[acbern]

So how do you verify a 1ppm change? The IET1689 digibridge, although one of the standards in metrology industry, will not resolve this.
Just a 1616 bridge I guess or an andeen hagerling. If you have one of these, I am jaleous.

[Vgkid]

The genrad 1620/1615 gives 1ppm resolution as well. Though if i remember correctly you can use the 1689 to give the fifference in  values.

[amspire]

Something similar to the Genrad transformer bridge. I have an ESI 707b monster that is also a transformer bridge, but it is a different design to the Genrad. The Genrad needs a self calibration step to calibrate all the range capacitors from the inbuilt standard capacitor. The ESI references everything back to a single 1nF Invar steel dry nitrogen dielectric hermetic standard capacitor. The bridge main range goes to 5 places, but it has a second Difference section that goes to 1ppm. It can resolve down to 10 atto Farads. Made about 1967.

[acbern]

This one (just as the 1615 and 1616 bridges) need a high voltage oscillator and a selective microvoltmeter. For curiosity, what do you use for these? I would think, if you are not going into a range lower than say 10pF, a 10V generator may actualy even do it.

[amspire]

I actually have the ESI 701 Capacitance Measuring System that is a hardly liftable box including the 707B bridge and the 861A AC Generator Detector. This generates 20hz to 20kHz at up to 240v RMS, and has a null Detector with a tracking bandpass filter, and a 0.25 microvolt noise level. All valves, but it still works great.

[acbern]

So from your experience, is it as good as it looks on paper?

[amspire]

I will get some photo's when I do the tests - I have just been making up some leads and making a little shielded box out of brass shim sheet. I am going to do some simple temperature tests on the Genrad capacitor and the Russian capacitor. All Genrad/IET say is that type 1409 capacitors at 23deg +/- 5 deg C, they should be within specifications. For the Genrad caps, that was often 0.05% and the newer IET 1409 capacitors, you can get 0.02%. No temperature coefficient curves are published, so I am curious.

I think the specifications are pretty conservative. It came from the factory with the standard capacitor adjusted to 0.001% and probably has never been adjusted since. In spite of this, I am pretty sure it is still within the 0.01% accuracy.

The quality is fantastic. The custom made switches have a light quality feel and are designed so they should never need maintenance. They still seem to be perfect after 50 years. The last two digits is a vernier dial connected to a custom 1.4 ohm resistive slider across transformer winding and they are constructed to be accurate to 1%. It is quite good having the last two digits as a dial as it is better then switches for tweaking. The bridge can get pretty clear nulls even down to the 1pp level. Readings seem very stable and repetative.

The Genrad bridges are masterpieces of design and you would probably still go for one first, but the ESI is very impressive.

The huge problem with all these transformer bridges is just they are so incredibly big and heavy. It sits in the garage most of the time, and occasionally comes out. I do love the concept of precision transformer bridges though. Basically the transformer cannot lose accuracy. The oscillator does not have enough power to ever damage the bridge windings, so as long as the switches are good, all I need is one accurate reference capacitor and I can calibrate the bridge with one adjustment. There is a small hex screw on the front panel that allows the standard capacitor to be adjusted +/- 0.03%. Strait after calibration, it is probably accurate to near 0.001%.

Concerning the calibration, I am trying to come up with a plan for a quadrature bridge, so I can precisely calibrate a capacitor from a resistor. It should be possible to make a quadrature bridge with a 10ppm precision. I need two sinewaves of exactly the same amplitude (within a few ppm) and exactly 90 degrees phase difference - I think I have worked out a plan to do that. Unfortunately, I may be forced to use a FPGA to realize my concept. I hope to find a way to avoid that.

[acbern]

Very interesting. I have ordered a 707 bridge, do not have it yet. I still need a generator and a detector, I plan to use a 8904A DDS generator and a lock in amplifier as detector. These I have, so no additional spendings. l have a set of Genrad capacitors from 0.1pF to 10 nF, a homemade 100nF, and an ESI SC1000 hermetic standard cap which I will use as my CAL standard. That will be calibrated next week and will be the basis for deriving all the rest. The concept of decade switchable transformers, not needing any calibration, is great. That way really everything can be derived from one capacitor. I had done some initial tests building up a capacitive bridge based on a Dekatran 7 digit ratio tranformer, and got pretty good results. A finished bridge though has the advantage that it has a defined accuracy as specified by the manufatcurer, versus a self-built bridge which needs to be validated (so you need the set of calibrated capacitors, not cost efficient).

Interestingly enough, I had also planned to build a quadrature bridge. I started out with a precise reference capacitor, a precise coaxial resistor and again the dekatran bridge. My results were not so good, from what I can say, not knowing exactly the value of the standard I used at that time. That will change now with the SC1000 comming back from calibration. As generators I used an Agilent 16 bit DDS (81150A) and I measured (verified) the phase between the two outputs with an Agilent dual channel counter, which is very precise. Levels with a 3458A, pretty precise too (SYNC), especially if you just need to know the voltage relation of the two channels, rather than the absolute value. So I believe that the signal quality/accuracy were not the problem, but rather the setup. I worked with 10kHz, and I think going to 1kHz is the first change to be implemented. Also the overall coaxial setup needs improvement, so I had ordered isolation transformers. Quadrature bridges as used in national metrology labs are pretty complex, so I started out with a much simpler approach (see below, last page).
http://home.mit.bme.hu/~kollar/IMEKO-procfiles-for-web/tc4/TC4-11st-Lisbon-2001/Imeko%202001%20Full%20Papers%20Directory/imeko-105.pdf
What configuration do you use?

[amspire]

You need lots of voltage for the ESI 707B bridge, but the easy way to do that is with a transformer - if you can find something suitable.

When you are measuring 1uF, the voltage across the 1uF is 1000 times less then the voltage across the standard capacitor. So to even get 0.2v across the 1uF cap, you need 200V across the standard capacitor. Also to measure at full resolution on the 10pF range, you need 200V across the measured cap to get 1V across the standard capacitor. The bridge works perfectly well at lower voltages, but the minimum for the 707B is 100V. The maximum voltage is 350V I think.  The oscillator in my bridge it only 1W power but it can output 200V at 10K output impedance, 20V at 100 ohms impedance, 2V at 1ohm and 0.2v at 0.01 ohms. Distortion is 2% max. The transformer output helps in meeting the isolation requirements needed by the bridge. It is important that the bridge is earthed at one point only - the input to the detector.

I assume you have the manual? I have the PDF if you need.

[amspire]

My quadrature bridge is much simpler - I have two very stable sinewave generators that I first null at a 1:1 resistive divider. It is dead easy getting a 1:1 divider that is as accurate as you need. This gives two digitally generated sinewaves that are exactly the same amplitude and exactly 180 degrees out of phase. I then exactly shift one oscillator by exactly 90 degrees (easy if it is digitally generated) and apply it to the reference C-R pair. With a few extra refinements I can make sure the load on the sinewaves is identical in both the calibrate and measure stages, and handling capacitor dissipation factor, etc can also be managed. Distortion of the sinewave may not be critical, but it has to have a very stable short term amplitude. It doesn't have to be a general purpose tool, as its main job is to be able to calibrate at least one capacitor value very accurately from a resistor. I have ways that even a simple 8 bit microcontroller can possibly be generating the oscillator source waveforms, but I can do better with a FPGA. Most high end oscillators/synthesizers are unlikely to have the stability and phase accuracy I need.

Then all I need is a way to get my resistor standards calibrated.

[acbern]

So looks as if the 8904 alone, delivering only 10V will not do it on the extreme ends. I was execting this, but do not yet have an HV amplifier. A standard amplifier and a 100V transformer should do it, the only problem may be the capacitance to earth and between wingdings. I need to see. According to the 707 manual, 100V should be sufficient. Also, the lock-in amplifier is much more senstive than the ESI detector, that should compensate too. I will see.

Re. the qudarature bridge, if you want to determine the capacitance of your standard, either you change the resistance (diffcult for a coaxial resistor), the amplitude ratio (dekatran) or the frequency (changes the amplitude relationship too). So the best (and thats what the national labs do) is the ratio with a dekatran. 

[BravoV]

 

[amspire]

Should finish my tests of the Genrad Standard capacitor versus the eBay Russian Silver Mica capacitors. The results are looking really interesting and a little surprising. I should be able to post some results and some photos today.

[amspire]

I did some testing on the Russian SSG capacitors, and also a Genrad 1409-G Standard Mica capacitor. I saw the same capacitor that I am using was available on eBay at $10 for 4 capacitors. The Genrads often sell for over US$100.

My test setup was a ESI 701 Capacitance Measuring System, with  a small foam esky to act as my temperature chamber.

I put two 50nF Russian SSG capacitors in parallel is a small shielded box to make a 0.1uF capacitor (it measured 0.10001 uF).

The Genrad was a custom value type 1409-g. This is normally a 2nF +/- 0.1% part, but it was obviously customized by Genrad before sealing to be 1.9865 +/- 0.05%. This is probably important to the following tests. It measured at 1.9875 nF.

Here is the Capacitance variation versus frequency. The Russian capacitors were clearly better then my Genrad.

Now for the capacitance variation versus temperature at 1kHz test frequency.

The Russian capacitor is brilliant, but what is happening to the Genrad? The Genrad is meant to be +25 to +45 ppm/C between 13 and 33 degrees, but instead it is rubbish.

If you look at the dissipation factor, same story. The Russian capacitor dissipation factor was 0.00007 at 12 degrees to 0.00012 at 39 degrees. Again brilliant!

 The Genrad is meant to be better then 0.0003. The dissipation factor was 0.0003 at 10 degrees, about 0.006 at 23 degrees and 0.0013 at 33 degrees! Something is very wrong.

What I think has happened is that to get 1.9865 uF, they took an unsealed 2nF from the production line and added a 0.27uF cap in series. The trouble is that the 0.27 uF would not have been mica. It was built about 1959, and they could have even put a paper capacitor in. It would explain the poor curves now. Also, the Genrad capacitors are not hermetically sealed. They put in a packet of Silica Gel and seal it with something like Butyl Rubber (Mastic). After over 50 years, the silica gel may have lost the ability to absorb any more moisture. The Russian capacitor is hermetically sealed.

The new type 1409 caps from IET have a coefficient of +20ppm/C and this Russian cap is about +11ppm/C.

I have only tested one, but I think that the Russian SSG caps are about half the drift and dissipation of the IET Type 1409 Standard Capacitors. The SSG's look very stable on the bridge and I think with a more accurate temperature curve characterization, it would be very easy to work out the capacitance to 10ppm given the temperature.

The other message is: avoid custom value Genrad Standard caps.

[manganin]

The Genrad was a custom value type 1409-g. This is normally a 2nF +/- 0.1% part, but it was obviously customized by Genrad before sealing to be 1.9865 +/- 0.05%. This is probably important to the following tests. It measured at 1.9875 nF.

That is not a custom value (which would have been engraved on the name plate), but it has simply drifted out of the specs. That is very common. The 1409 is extremely sensitive to non-optimal storage conditions and many (if not most) of the used ones have problems.

The capacitor was factory adjusted within 500 ppm of the nominal value at 1 kHz according to the specifications, but within 100 ppm in practice. Except some of the early ones which had larger margins. If the capacitor is good it should still be within the one year specification.

What I think has happened is that to get 1.9865 uF, they took an unsealed 2nF from the production line and added a 0.27uF cap in series. The trouble is that the 0.27 uF would not have been mica. It was built about 1959, and they could have even put a paper capacitor in.

Every capacitor was hand made, there was no "production line". All you will find inside is a small factory made mica capacitor used for the final adjustment and sometimes not even that.

[amspire]

You could be right. The rear panel does have an old printed label that says: "Adjustment Accuracy +/- 0.05%" but I cannot be sure that the label came from Genrad, or that the back plate even originally came from this standard. The back plate is removable.

So is it possible that moisture or contamination can get into the standard enough to not only to cause a 0.7% error, but also totally change the temperature coefficient? If the sealant is that bad, it is really not much of a standard capacitor.

I might stick it in the oven at 65 degC (the maximum storage temperature) for a while and see if the capacitance or dissipation factor changes.

Doesn't look like it has any use as a standard capacitor anymore.

[manganin]

So is it possible that moisture or contamination can get into the standard enough to not only to cause a 0.7% error, but also totally change the temperature coefficient? If the sealant is that bad, it is really not much of a standard capacitor.

The moisture gets easily in but it is practically impossible to get it out. The first sign of the problem is the dissipation factor going up and when the damage progresses it will also change the capacitance.

Another but rare fault mechanism is with the clamp holding the stack together, which can change the capacitance a lot, but that usually doesn't much affect the dissipation factor.

I might stick it in the oven at 65 degC (the maximum storage temperature) for a while and see if the capacitance or dissipation factor changes.

Doesn't look like it has any use as a standard capacitor anymore.

The process is irreversible. A good candidate for teardown...

[BravoV]

Thanks amspire & manganin ! 

You guys inspired me, pulled trigger on eBay -> http://www.ebay.com/itm/321715093338 , 5 pairs. 

[manganin]

You guys inspired me, pulled trigger on eBay -> http://www.ebay.com/itm/321715093338 , 5 pairs. 

The long term stability is still the big question.

The SSG capacitors that I measured many years ago showed both systematic and random drift way too high for metrology use. Material purity issue?

[BravoV]

You guys inspired me, pulled trigger on eBay -> http://www.ebay.com/itm/321715093338 , 5 pairs. 

The long term stability is still the big question.

The SSG capacitors that I measured many years ago showed both systematic and random drift way too high for metrology use. Material purity issue?

Only for transfer standard, is it good enough ? Also does this type of cap sensitive to mechanical shock ?

[amspire]

I decided to find out a bit more about the SSG capacitors, and luckily I found all the details I could possible want from an Audio site:

These have such feathery, extended highs that almost make Mundorf gold/silver seem a bit less open up top. Not bad for a $5 capacitor, so what's the caveat? Unfortunately for the SSG-3, I've been listening to some of the best Teflon capacitors of late, and compared to the good Teflons, the silver mica's don't quite have the stop-dead-in-your tracks resolution and definition, especially in the midrange. Compared to Teflons, SSG-3 is a tad more laid-back and softer in the midrange, and the bass, while impactful, is not quite bounce-rain-off-the-drumskin tight. Still, I can see a lot of people being quite happy with these silver mica's, especially if used in equipment that leans in opposite sonic directions or if your tastes cotton to the open, airy, feathery, smooth sound.

So there you go - a good capacitor for a transfer standard, as long as you are happy with the feathery sound.

I can probably give some stability information. I have had my capacitors for about 10 years, and I think I measured the value for one of them. If I can dig up the details, I will measure it again.

[amspire]

Found the old measurement and repeated it. Over about 10 years, the SSG-1 50000pF capacitor drift against my ESI bridge is less the 2pF - that is less then 0.004%.

The original measurement was only a 2 wire measurement, so I cannot be precise about this error. It doesn't take much to cause a 2-wire error of a few pF when you do not have a proper testing jig. I just tried to measure it the same way I did before. Luckily, the room has the same temperature as the old measurement - 17 degrees C.

I have no idea of the long term stability of my bridge standard capacitor, but its calibration adjustment range is +/- 0.03%, so obviously ESI do not expect it to drift much during its life. 

A one-off sample, but again I am impressed. It is not enough to make conclusions about all SSG-1 capacitors, but it looks like there are some good ones out there.

Richard

[acbern]

It should be kept in mind that the genrad 1403 air capacitors may drift up to 0.05% per year, and even if they drift less, it is still the value to be taken when using them as a standard (unless one has made a statistical analysis showing it is better). Most silver mica capacitors are not worse. I will do some measurements once my gear is back from calibration in  2 weeks timeframe on 10pF to 100nF capacitors.

[amspire]

The new IET 1409 capacitors are rated at 0.01%/year maximum drift.

I don't think the original 1409 back in the 1950's had a yearly drift spec, but in-house tests over 3 years had shown less then 0.01% drift, and 0.01% was also the limit to Genrad's own in-house standards.

[amspire]

I might stick it in the oven at 65 degC (the maximum storage temperature) for a while and see if the capacitance or dissipation factor changes.

Doesn't look like it has any use as a standard capacitor anymore.

The process is irreversible. A good candidate for teardown...

Well, we currently have dry cool weather. I thought I would try baking my sick Genrad 2nF 0.1% standard cap.

Did it for 2 hours and the dissipation factor dropped from .00057 to .00040. Promising. The capacitance dropped from 1.9875nF to 1.9867nF that was closer to the value marked on the capacitor.

OK. I did another 8 hours of baking.

Dissipation factor is now 0.0003 which just meets the specifications. Capacitance has increased to 1.9988nf. It has gone from 0.7% below the engraved 2nF capacitance to 0.06% out. The original accuracy on the label was 0.1%, so the capacitor is now back in specification. Interesting thing is I have had this for about 8 years and it has always been stable and pretty close to 1.9865nF. It looks like once moisture gets in, it will not leave until the temperature is high enough. The fact it only took 8 hours is damning on the state of the sealant.

I am leaving it out in the sun all day today and keep baking it until there is no more change - I want to dry out the internal Silica Gel pack. Then I will add a new layer of sealant. We do not get too much humid weather, so I am pretty sure it will be stable after this.

Can it be trusted again as a Standard? I wouldn't use it commercially, but for use at home, you make the most out of what you have.

[manganin]

OK. I did another 8 hours of baking.

Dissipation factor is now 0.0003 which just meets the specifications. Capacitance has increased to 1.9988nf. It has gone from 0.7% below the engraved 2nF capacitance to 0.06% out. The original accuracy on the label was 0.1%, so the capacitor is now back in specification. Interesting thing is I have had this for about 8 years and it has always been stable and pretty close to 1.9865nF. It looks like once moisture gets in, it will not leave until the temperature is high enough. The fact it only took 8 hours is damning on the state of the sealant.

Very interesting findings!

One of the capacitors (0.5uF) that I tried to cure was baked several months in 60 C. It slowly affacted the dissipation factor but the capacitance stayed practically the same. The original error was smaller than yours though. My guess was that the humidity had corroded the thin silver layer

Because the capacitance changed so fast, could it be just a contact issue. One of the silver layers lost a contact because of the mechanical force by the expanded silica gel. Or the pressure of the expanded silica gel working against the stack spring. Hard to believe that the capacitor itself dries so fast.

Can you see any obvious slots or cracks in the sealant, when the bottom lid has been removed?

Can it be trusted again as a Standard? I wouldn't use it commercially, but for use at home, you make the most out of what you have.

It would be interesting to see how it reacts if exposed to humidity again. And if the change of the capacitance is smooth or sudden.

[BravoV]

Abit OT, about the precision mica caps, presumably closely matched, why they're packed in pair from factory ?

[woodchips]

On measuring capacitors, has anyone had a play with the Pacitor aircraft measuring systems?

These are used to measure the fuel remaining in the tanks and rely on the dielectric difference between air and fuel to vary the measured capacitance. The test sets have a clock type dial reading down to single pF. This is driven by an oscillator in some type of bridge arrangement.

Another manufacturer of precision capacitors is Muirhead, air spaced, quartz supports, loss angle 1 micro radian. Comes in a large wood box, looks really nice!

[Vtile]

When in old General Radio's era, apart from the proven mica, just wonder if the development of NPO material at that time already matured as now ?

NP0 is just plain ceramic, and yes they had them, but not in surface mount [obviously].

Mica is a very old material, and over the millions of years it has already done whatever it is going to do-- so, that's one of the main reasons that it is used in capacitors and resistors.  Mica *is* affected by humidity though.

A vacuum capacitor made of silver-plated Invar plates and Sapphire spacers would probably be quite stable with time and temperature, and it would have very low [almost zero] dissipation factor up into UHF.  There have been some experiments at NIST, but I can't remember the papers nor do I have any links.  I think everyone went with fused-silica capacitors, because they would be more robust during shipping than any capacitor made of interdigitated plates.

This is really old topic, I just ended up here by gugle.. When talking about Mica is it a nicname for special type of mica product or just general term referring to all types of mica sheets etc. (which some have resin like micanite and some are pure sheets)

On measuring capacitors, has anyone had a play with the Pacitor aircraft measuring systems?

These are used to measure the fuel remaining in the tanks and rely on the dielectric difference between air and fuel to vary the measured capacitance. The test sets have a clock type dial reading down to single pF. This is driven by an oscillator in some type of bridge arrangement.

Another manufacturer of precision capacitors is Muirhead, air spaced, quartz supports, loss angle 1 micro radian. Comes in a large wood box, looks really nice!

Capacitive level sensing is 'pretty common' in industrial settings.

[amspire]

This is really old topic, I just ended up here by gugle.. When talking about Mica is it a nicname for special type of mica product or just general term referring to all types of mica sheets etc. (which some have resin like micanite and some are pure sheets)

Mica is a rock that you mine. It cleaves perfectly in one plane and so that is how the sheets are made.

[Vtile]

This is really old topic, I just ended up here by gugle.. When talking about Mica is it a nicname for special type of mica product or just general term referring to all types of mica sheets etc. (which some have resin like micanite and some are pure sheets)

Mica is a rock that you mine. It cleaves perfectly in one plane and so that is how the sheets are made.

Yes, but there is also products that are some form of compressed mica laminates etc. composites.

PS. What I tried to ask is, when people are talking about mica capacitors are they referring to any Mica product or specific type of mica ie. natural 100% mica sheets.

[Conrad Hoffman]

I've only had one GR mica cap apart, but it had a desiccant pack, a cover plate and was potted in rosin. I think I replaced or baked the desiccant pack, probably a low temp bake of the whole thing, then potted the back with 2-part RTV. Sometimes over time those rosin jobs can pull away from the case and admit moisture. The rosin can be a nightmare to get out- I'm still trying to get inside a big GR inductor that went open.

[BravoV]

Yet another Russia silver mica capacitor, SSG-3 series 200nF 0.3% 350V rated.

I'm guessing this SSG-3 series is also as good as the SSG-1 as the one Amspire tested on ESI 701 above ?

Few shots, measured result and size comparison with the smaller SSG-1 series, a common thru hole Cornel Dubilier silver mica, and a TO220 as reference size.

[HighVoltage]

My first "standard" capacitor, bought as NOS and really looks like it was never used before.

But, what does "BELOW 750 KC" mean on the label?

[BravoV]

Olde calling ... KC = Kilo Cycle = Kilo Hertz

[HighVoltage]

Thanks, that makes sense.

First test passed, showing 10.004 nF at 1kHz on my PM6306

[BravoV]

Congratz on the score  , really envy you. 

[SeanB]

Mica sheet does vary from mine to mine, and from actual mined area in that mine as well, due to contaminants in the material. There is a little town here around 800km away from me called Mica, which I was very familiar with, mostly through passing through it in a cutting near the mine, which has a lovely cut through the mineral formation that gives it it's name. The best capacitors are made from selected cuts from the mines, and command a premium, as they are very pure. Cheaper sheets are made from other product, and I have a sheet of that that is used to cut new waveguide covers for microwave ovens from, or you can use it to make formers to hold resistance wire to make heaters or high power resistors.

[GregDunn]

OK, while people are viewing this thread... I need an accurate 10nF capacitor to use as a sanity check for meter calibration (it's specifically the value called for in the manual).  The best I can find on commercial sites is 0.5% accuracy.  Does anyone have a recommendation for where I could find a more accurate one for a somewhat reasonable price?  Trying to hunt down more accurate caps on evilBay returns a lot of useless hits...

[HighVoltage]

A quick search on ebay reveals:

GENERAL RADIO 0.01 uf TYPE 505-L STANDARD CAPACITOR
US $ 19.99
I am not sure how good they are.

[GregDunn]

That looks pretty good, actually - apparently I was using the wrong search criteria,  I may buy a couple of those caps.

[The Electrician]

OK, while people are viewing this thread... I need an accurate 10nF capacitor to use as a sanity check for meter calibration (it's specifically the value called for in the manual).  The best I can find on commercial sites is 0.5% accuracy.  Does anyone have a recommendation for where I could find a more accurate one for a somewhat reasonable price?  Trying to hunt down more accurate caps on evilBay returns a lot of useless hits...

Would a capacitor somewhat larger than 10 nF, but accurately known do the job?  For example, would an accurately known 11.2nF capacitor serve your need?

[GregDunn]

It probably would be OK.  I assume they picked .01µF just to have a standard value for the calibration setting - it may have worked out .09, .11 or something else and they decided the linearity curve would be close enough with that as the set point.

This is not a NIST calibration by any means.     Someone pointed me to a .01 GenRad standard cap (without cert) for a reasonable $20, so that's the neighborhood I'm living in right now.   

[Henrik_V]

Here are some 1nF 1404 standard cap cans (not genuine GR) I dumpster dived..
They are all prone to wisker, thats why 
See second pic
A future project will be a rework. Polish and gold plate the plates (and standoffs) , remove the tin wisker plant ground , refill with dry N.
Decapsulation was done with a torch, first tried 20 min with a hot air gun .... 
If it doesn't work out the first time I still have some more .. 

[doktor pyta]

was it from IET Labs ?
now I see. Quadtech.

[Conrad Hoffman]

Wow, wish I had a dumpster like that! I thought the plates were supposed to be invar, which shouldn't have a whisker issue. Or is that just the big reference caps and the ones in their bridges?

[Henrik_V]

HI Conrad,  you are right, the plates are INVAR and don't show wisker.
The the housing (can) and the mounting base are tin plated and are prone to wisker. We had short cuts to the feedthrough and we found wisker parts flying around in the can.. 
I regulary check my caps isolation resistances.. and I had wiskers (one plate some kOhm to housing) even with genuine GR 1404s ,  applied a 400V pulse to burn the free.. got luck the value didn't change (recal rigth after) 

The INVAR plates look horrible ... OK, due to my rough torch desoldering they migth get smoked a little    but surface and edges of the genuine GR part look better.
however we want to give it a try and polish (and gold plate) them ..
Two units are open .. I bet after remouting , N refill , and tempering (this time without a torch ) it will cost me one unit for our Cap-lab people 
And I need a way to get rid of the tin plating.....

[HighVoltage]

  applied a 400V pulse to burn the free.. got luck the value didn't change (recal rigth after) 

Wow, that is some kind of brute force on a metrology device.
What a great dumpster you have.

Very interesting with the whiskers.
Do all original GR 1404s have whisker issues or did they fix that may be at a later state by using different methods with the can?

[Henrik_V]

  applied a 400V pulse to burn the free.. got luck the value didn't change (recal rigth after) 

Wow, that is some kind of brute force on a metrology device.
What a great dumpster you have.

Very interesting with the whiskers.
Do all original GR 1404s have whisker issues or did they fix that may be at a later state by using different methods with the can?

All ? I don't know. They all (AFAIK) have the tin plated cans .. maybe small variances in the solder composition matter ??
A lot of information on whisker is collected here: https://nepp.nasa.gov/whisker/

I wasn't shure what will happen with my GR 1404 .. Its rated for higher voltage  ... what could I loose?  It performes fine since that 'burn out'    ~6 years ago.

[branadic]

The INVAR plates look horrible ... OK, due to my rough torch desoldering they migth get smoked a little    but surface and edges of the genuine GR part look better.
however we want to give it a try and polish (and gold plate) them ..
Two units are open .. I bet after remouting , N refill , and tempering (this time without a torch ) it will cost me one unit for our Cap-lab people 
And I need a way to get rid of the tin plating.....

That's gonna be a long journey. First you need to strip the tin, then you need to galvanically add nickel and gold. Maybe you can give PVD a try, but I'm not sure how good that works with the aspect ratios of the can. But why a gold finish? Wouldn't nickel be good enough?

-branadic-

[Henrik_V]

The INVAR plates look horrible ... OK, due to my rough torch desoldering they migth get smoked a little    but surface and edges of the genuine GR part look better.
however we want to give it a try and polish (and gold plate) them ..
Two units are open .. I bet after remouting , N refill , and tempering (this time without a torch ) it will cost me one unit for our Cap-lab people 
And I need a way to get rid of the tin plating.....

That's gonna be a long journey. First you need to strip the tin, then you need to galvanically add nickel and gold. Maybe you can give PVD a try, but I'm not sure how good that works with the aspect ratios of the can. But why a gold finish? Wouldn't nickel be good enough?

-branadic-

The INVAR plates to be polished and plated are not tin plated   Our galvanisation lab will know how to treat them and what bounding layers migth be needed.

Removing the tin from the can and mounting block?  Cover the feedthroughs and cap mounting holes and try sandblasting  ?? 
One nasa paper reports a coating with ?? ... let's  see   maybe end of next year

[gamalot]

I got this from Japan.

[Conrad Hoffman]

FWIW, invar is miserable to plate and I wouldn't do it. It's known for poor adhesion even with various intermediate layers. Plating can be  prone to bubbles and that would change the capacitance. I'd polish and leave it at that. Not sure if it can be electropolished, but that might worth a look.

[seebeck]

The GR1404 is a better standard than general capacitor standards . GR was bought by the IET Lab (https://www.ietlabs.com/) in 2000 and to continue to produce and market LCR standards. I shared the A2LA certificated report for IETLab company as attached file.
BTW, the ESI（also an old brand company for standard resistor such as SR104 ）,it was bought by the IET Lab in 2006. The other part as meter axies for ESI was bought by TEGAM . TEGAM is a very well-know company for the microwave standard area such as power sensor calibration products , it associated ESI、Keithley、Tektronix、Weinschel...
The best physical capacitor standard is AH10/11 from Andeen-Hagerling around the world!

[iisak]

before I got the metrology-/voltnut-/ppm-virus, I used this kind of capacitors (see att. pic. 1)
I have them since early to mid 80s and they are still very stable and usefull for hobbiest references (but by no means Standard capacitors)
If I would search for replacements I would look for Glass Capacitors and good NPO (like this blue ones in pic. 2)

Same here, although mine are mostly silver micas.

Bought few of this NPO recently, going to use it as reference cap, with tempco of 30ppm and virtually doesn't age as Kemet claimed. 

What made me interested at this cap is apart from its a NPO, it has relatively big capacitance (100nF) compared to most silver micas I have.

Have these KEMET GoldMax capacitors been stable? Have you measured them over these years?

I would be interested to have a set of stable capacitors from 10 pF to 10 nF. I understood that MICA or some good ceramic would be good. Some 10 ppm/year would be nice. I wonder would that be realistic.

I have found these, for example:

Cornell Dubilier  10 pF MICA (see Digikey part 338-1068-ND)
KEMET GoldMax 10 pF caramic COG (see Digikey part C315C100GAG5TA-ND)

[cdev]

A standard to me most of all demands stability. Meaning physical stability, so a standard capacitor wouldnt necessarily handle thousands of volts, but it would be built like a rock in a way so it would not be prone to change due to any external factor, like air pressure or humidity. So it might be sealed? I dont know. I'm not an expert on this at all.

The music from the video is nice and suggests precision well, mentally.

Is there anything worth seeing in Boonton, NJ?

I know they made innovative RF measurement equipment, a long time ago.