Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617

[JxR]

....lucky who lives in the us - low shipping costs and no import taxes...

Yeah, one of the only perks living here as far as I'm concerned.  When I finish up my schooling, me and the family are looking to move back overseas.  We both miss living in Japan, but I might try and find work in Germany since we both like it there.  My wife is a dual French/US citizen and misses Europe. Both her parents live in Germany so I've been there a few times, and we have her family to make the transition easier.

[balage]

My DIY low (tribuelectric) noise Triaxial cable made of PL75-23 and Lemo 001 101.
The Lemo cable has nearly same dimensions as Keithley SC-22, but has all teflon insulation and is much cheaper if I got it right (~20$/m vs. ~50$/m).

Hi MiDi,

Please share your experience on it, especially the resistance. I am interested if this cable as good as the Keithley SC-22.

Where have you bought that Lemo cable? Koax24.de?

[MiDi]

Please share your experience on it, especially the resistance. I am interested if this cable as good as the Keithley SC-22.

Where have you bought that Lemo cable? Koax24.de?

Yes, from koax24, min order qty. is 10m.

The Lemo should be better regarding resistance as it is PTFE against PE for SC-22.
But cable alone is useless, connectors and especially skills of builder add to it 

[balage]

But cable alone is useless, connectors and especially skills of builder add to it 

I have made several of them, and the least ones tended to be better, as I had more practice. But it is sure that rubber gloves must be worn.

Have you found the resistance specs of 001 101?

I was wondering if you would like to swap 1 meter of that Lemo cable to a few meters of G_02330_HT. For the first row I would love to try that, before ordering for more than 120EUR.

[MiDi]

No resistance specs for 001 101 found either.

Regarding swap: PM sent.

[balage]

I have asked Lemo about the resistence, and they have forwarded me to the local distributor. Will see.

However this spec "operating voltage = 0,75kV" does not say much, either. Especially because while resistance can be measured, but the voltage specification cannot be measured.

[MiDi]

As earlier stated there was an offset of ~280µV at preamp out with zero check on - in either amps or volt mode.
When adjusting the offset DAC to give ~0µV in amps, it showed a doubled offset in volts mode.

Deaper investigation revealed yet another layout flaw (Rev E):



The trace of preamp out near C311 was drilled open and a bodge from R304 to ribbon cable connector was made, not shure what could have been the issue to do that.
The preamp out is the common for the +-24V rails on T301 as well.
The current source (for Ohms range) is fed from this 24V and so are the op amp U304 (LM741) and the voltage reference VR301 (1N4577).
This gives ~4-5mA ~14mA return current through the bodge and the traces from ribbon cable to K301 and this weird looking star point nearby.

Could this be the source?
The measurement between R305/R308 (real preamp out) and ribbon cable (goes down to the ADC) gave ~280µV - what a bummer!
Why did Keithley "improved" it that way? This problem should have been recognized at least when doing the cal - unbelievable!

So how to fix it? Best point seemed to be the star point where trace from transformer common joins.
A look at the later revisions: they did exactly that 
This was a quick'n'easy fix, the bodge wire could be reused and the offset is now in single digit region (+-10µV) in either amps or volt mode.

[HighVoltage]

What a great thread of information ..

I am in the market of buying a 617 and would like to get a latest model.
- But how long did Keithley build these instruments and
- How can one detect from the outside, what is a latest model?

[JxR]

What a great thread of information ..

I am in the market of buying a 617 and would like to get a latest model.
- But how long did Keithley build these instruments and
- How can one detect from the outside, what is a latest model?

My 617-HIQ appears to have chips in it from 1996.  It wasn't until the 6517B I believe, until Keithley made another electrometer to have the same upper range for Coulomb measurements.  If going by the datasheet is any indication, the 6517A was already available when the 617-HIQ was still on the market.

The latest Rev-L boards have a date of 1984, and the latest manual I have seen (Rev-G), has a date of 1988.  Although I honestly have no idea when the instruments were actually discontinued.

There is the 6512 which has the same specs as the 617, but comes standard with a 3-Lug Triax.  Then the 6517A, and the 6517B that I believe is the latest model.

The serial number could probably be used as a rough estimate on getting a Rev-L model I would guess.

My serial number is: 0633900

[HighVoltage]

Thank you, very good info.

Your serial number (0633900) is the highest I have seen.

It seems very difficult to judge the functionality and quality of a 617 before buying.
And since most sellers are not capable to testing or do not even have a cable, its not easy.

[JxR]

Thank you, very good info.

Your serial number (0633900) is the highest I have seen.

It seems very difficult to judge the functionality and quality of a 617 before buying.
And since most sellers are not capable to testing or do not even have a cable, its not easy.

There are some Canadian sellers on ebay that ship worldwide and have a couple 6512s that are NIST Calibrated and come with a 90-day warranty.  Only downside is an increase in price, and I haven't seen a schematic for a 6512, although they provide a part list in the manual.  Color is a little weird for old Keithley gear...

They have a website as well: https://www.stratatek.com/

I've personally never bought from them, but they seem legit from first glance. 

[HighVoltage]

Interesting with the 6512, I was not really aware of that model.
I have a 614 and a 6517B, both in like new condition.

The 617 I want use for a client project were up to 100V for 10 GOhm measurements is just perfect and not too expensive.

Yesterday I bought a 617 in "unknown" condition.
Allegedly the seller does not have cables and/or capability to test.
It should arrive shortly.

[JxR]

Interesting with the 6512, I was not really aware of that model.
I have a 614 and a 6517B, both in like new condition.

The 617 I want use for a client project were up to 100V for 10 GOhm measurements is just perfect and not too expensive.

Yesterday I bought a 617 in "unknown" condition.
Allegedly the seller does not have cables and/or capability to test.
It should arrive shortly.

Gotcha.  I have to admit, I was a tad curious why you were first interested in a 617 since I knew you already had some really nice equipment.

The 617 can easily handle your intended needs once you perform the calibration procedures (which are fairly straight forward).  I think it can measure up to 10POhm in "V/I" mode.  I've gone as high as about 3.5POhm doing insulation testing while making the leads for my test fixture.

[MiDi]

After finishing my quick'n'dirty shielded triax box, it was time to do some measurements.
It is made of RF-case from shelf, BJ72-Triax (isolated outer shield), some SKS/Hirschmann AGF 1 clips, angled pinheader, safety banana jack, enameled copper wire (1.2/0.8mm) and some used solder wick.
Not a beauty and unsafe until properly enclosed, but seems to do the job for now:



As Reed relay K310 was worst of all, it became victim to do some tests if cleaning improves or even heals bad isolation resistance.
After soaking, cleaning and drying several hours at 50-60°C it was measured over several days - without significant changes in values - at ~10T\$\Omega\$ C-C and each C-ES at ~40T\$\Omega\$ - so no real improvement from cleaning  .

As this relay is dead, it was lead into its final designation - take it apart.
To remove case a heat-gun was used to make the resin gummy.
That worked good and it came apart in chunks quite quick:



Surprisingly it has a bit of magnetic shielding:



Trying to carefully separate glass-tube from coil, but finally the glass broke.
The contact from broken side does not seem to have any visible wearout:




There was no chance to slide it out as it is all glued together - so not the slightest chance to separate it without breaking something:







My conclusion is: if C-C resistance is lower than each C-ES, the problem has to be inside glass-tube of relay, because ES-Shield covers whole glass from outside with very thin glue in between.
Only other remaining leakage path could be direct on surface of outer glass, but that seems unlikely.
As the glue is really thin and seems not to provide high isolation resistance, this would affect the C-ES resistance as well.

To get a feeling what we are talking about, a MEDER HI05 Model 66 was ordered (spec'd with min isolation resistance of 1T\$\Omega\$) and measured at ~10P\$\Omega\$@~60%rH - impressive.
This was also a good verification, that the Box is suitable for this job.



Now waiting for the ordered RL-181 to arrive (spec'd min 100T\$\Omega\$) and we will see what they deliver
They are still awailable from Tektronix/Keithley as spare parts (web-form) and seemed to be the best option.

[Kleinstein]

The magnetic shield is not only a shield, but also provides a shorter return path for the driving field.  The shield thus also reduces the required current. This can make quite some difference when it comes to thermal EMF.
 

[HighVoltage]

Even the K200 reed relays in the Agilent DMMs have this kind of heavy metal shielding and then they are potted with some polyurethane.
Luckily these relays can be bought from Keysight.
May be all high end DMM have shielded relays?

But the Keithley 617 relays seem to be unobtainium.



 

[MiDi]

[Moved & Updated]
Investigation revealed that there are a couple of possible drop-in replacements for high insulation reed relays K307-K312 .
Even if there would be cheap salvaged parts awailable, better to be suspicious if they are untested and sold without any warranty.

-Original brown/black E-T R7526-1 (Keithley-Part# RL-70 - same as Coto 1240-0197, e.g. Keithley 617 < Rev L, 614): no source, no datasheet
Rcoil=600\$\Omega\$

-Coto 1240-0197 (Keithley-Part# RL-70, e.g. Keithley 617 >= Rev L, 199): Ebay (NOS) - 60$ (ask for stock & delivery outside us): Very high insulation resistance, 1FA, 350V, 0.35A/2.0A,10W Instrumentation Reed relay with an electrostatic shield and a 5V, Rcoil=625\$\Omega\$

No direct drop in:

-Coto 1203-0147 (Keithley-Part# RL-181 - same as ARI 300RCA - no datasheet found, Keithley 6514 & 6517 EM,  6521 & 6522 scanner cards): awailable from Keithley/Tektronix (spare-parts: web-form) - ~50$
spacing ES-Coil tighter

-ARI (American Relays?) SO051A 300RCA (Keithley-Part# RL-181 - no datasheet found, 6514 EM(at least from week 37 2008), Keithley 3761 scanner card): awailable from Keithley/Tektronix (spare-parts: web-form) - ~50$
Rcoil=300\$\Omega\$, spacing ES-Coil tighter

-Coto 1240-06-2104 (active part - confirmed by Coto, 1E14\$\Omega\$, 6V/5V compatible, RoHS): several sources, but ~100$
Rcoil=150\$\Omega\$


Other high insulation resistance reed relays:

-Okita TRY-105SV-15 (1E15/1E14\$\Omega\$, E/S-shield, 350V, 1/2.5A, 50W, 0.1pF, 50µV t-EMF) - source?, price?
Similar relays are used in Advantest R8340A Picoammeter (TRY-104S)

-Sanyu 95D-1A14N4 (1E14/1E13\$\Omega\$, E/S+M-shield, 350V, 0,5/2,8A, 50W) distributor, ~25$

-Standex-Meder HI-Series (mentioned by branadic, cheap: ~15$): no ES-shield (to be handcrafted - razberik), only 1E12\$\Omega\$ (Model 66) / 1E13\$\Omega\$ (Model 85) [Coto 1240-06-2104: 1E14\$\Omega\$], t-EMF? - preferred HI 05-1A85, but seems not to be awailable

[balage]

It is made of RF-case from shelf, BJ72-Triax (isolated outer shield), some SKS/Hirschmann AGF 1 clips, angled pinheader, safety banana jack, enameled copper wire (1.2/0.8mm) and some used solder wick.

Hi MiDi,

Why are you using isolated triax jack? I can see that a ground crocodile clip is tied to the case. But this clip is the same point as the outer shield of the triax, right?

[MiDi]

The case acts either as shield (~earth-potential) or guard (~input-potential).
With normal Triax-jack the outer shield of Triax-cable would always connect to case and so to earth-potential.
This would not work in guarded mode and not shure if it could cause problem in normal mode due to earth-loop formed by outer shield and inner shield.
So I decided to try it with an isolated triax which seems to be much simpler to get an universal box - have to figure out how it performs in guarded mode - so call it experimental

[JxR]

So I decided to try it with an isolated triax which seems to be much simpler to get an universal box - have to figure out how it performs in guarded mode - so call it experimental

Wow, I totally missed this.  Great find on the BJ72-Triax MiDi!  I made a single isolated Triax input on my test fixture using a PTFE washer covering the threads in PTFE heatshrink but this is much better!  I will have to get one of these in the future.

[balage]

Ah, that makes sense. And the isolated ones cost only a bit more than the BJ770.

[Vgkid]

Looks like I won the broken 617 on ebay.  Gosh the connectors/cables alone cost more than my most expensive meter.

[JxR]

Looks like I won the broken 617 on ebay.  Gosh the connectors/cables alone cost more than my most expensive meter.

Making your own is always an option, but that isn't exactly cheap either. 

I somehow got extremely lucky and eBay was saturated with used Keithley triax cables and accessories around the time I was looking to purchase.  All of my cables are used and cost 1/5 to 1/4 of what they cost new. Buying used was even cheaper than I would have spent making my own at that time.  I just glanced at eBay and don't really see many good deals now unfortunately.

It is cheaper to replace the 2-lug triax connector on the back with a new Keithley 3-lug than buying an adapter unless you get lucky.  So that is one place you can start.

[Vgkid]

Making your own is always an option, but that isn't exactly cheap either. 
/////////////////////
It is cheaper to replace the 2-lug triax connector on the back with a new Keithley 3-lug than buying an adapter unless you get lucky.  So that is one place you can start.

Thats the plan.
Huh, i always assumed that it was a 3 lug connector. Thanks for letting me know.

[MiDi]

Congrats, welcome to the club 
I really hope you have more luck with it than I have.

Last week there was a scary moment whilst preparing a measurement.
Wondered why the unit showed huge offset after couple of minutes after turning it on.
Then I smelled a bit of magic smoke.
Quick look under the hood and could not believe my eyes: glowing pcb near fixed fireworks-section