Measuring nanoamps and below like a Ninja

[Vtile]

With my Marconi / Meratronic Meratester I'm in like a Flyn. Sensitivity (noticeable relative change) is somewhere in region of 50 fA. 

https://youtu.be/3HFX_nMm6D0

[branadic]

We do have one of those brown bricks, Keithley 617. Nice piece of kit. I ordered some Ohmite high value 1% metal oxide resistors (100meg, 1G, 10G) and build a small setup with the resistors mounted on teflon standoff. After all these years without being calibrated the readings on the meter are still consistent.

-branadic-

[TiN]

Accidentally bought myself Xmas gift... 



Source is Keithley 6221.

Oh well... I hope buying not broken modern equipment will not become a habit. 

[Inverted18650]

TiN, I am going to message you about the Keithley 417 PicoAmmeter w/ 4170 input head.

[TiN]

Thanks for offer, Inverted18650, but perhaps some other eager amp-nut would like that 417. I already have enough of backlogged projects. 

[PartialDischarge]

That is a beast, can handle fA like nothing wow, minor teardown at least coming?

[VintageNut]

I believe that I have shared these resistors on a different thread. For this thread, these resistors are intended to be used with an electrometer to check calibration points.

One end of each resistor has a BNC jack. The jack is for the voltage source of the electrometer. Any leakage from HI to LO through this jack is not measured. So, this jack can be a junk BNC with no negative effect on it intended use.

The other end of the resistor is a 3-lug triax jack. This jack is for the triax jack of the electrometer.

These resistors can also be used with a sourcemeter. Care has to be taken to account for any leakage (if any) associated with the BNC jack.   

I had these calibrated at a primary lab in 2017. I will be taking them back again for a cal in February of this year.

I have no idea about temp dependence of these. That info would be needed for any rigorous cal usage. 

[e61_phil]

Could you say some words about the calibration in the lab? How did they cal these resistors?

[VintageNut]

Could you say some words about the calibration in the lab? How did they cal these resistors?

I can sketch the methodology, but I do not have total command of the calibration procedure.

It was explained to me that the calibration system is a bridge. The bridge uses a very old collection of Keithley 5155 resistors that have a long calibration history. The bridge also has an old Keithley electrometer. The remainder of the bridge is unknown to me and the exact procedure is also unknown. I will try to find out more details when I visit in February.

Attached are the calibration reports

[VintageNut]

I double posted and missed one. Here is the missing one.

[VintageNut]

Since you asked about resistor calibration, attached is a calibration report for my Fluke 5450A. It was calibrated against two Measurements International bridges. The low ohms are calibrated with a constant current bridge and the high ohms are calibrated with a constant voltage bridge. You can see that the 100M resistor in the 5450A is measured with better than 10X uncertainty compared to the 100M stand-alone resistor that I built.

Every measurement on the MI bridge requires a soak time of about 30 minutes. Very time consuming.

[Inverted18650]

That is a beast, can handle fA like nothing wow, minor teardown at least coming?

We're you asking me or TiN? 

[PartialDischarge]

That is a beast, can handle fA like nothing wow, minor teardown at least coming?

We're you asking me or TiN? 

Oh I was asking TiN, that shiny new equipment calls for a teardown

[Echo88]

Question to the Electrometer-ninjas about the K617:

As far as i understand guarding is only usable when measuring voltage sources with high source resistance and when measuring high resistance DUTs with the electrometer-internal-current-source up to 200GR.
With the internal voltage source the K617 is capable of determining the resistance/leakage current of higher resistance DUTs like a reed contact, but without guarding it takes a great amount of measurement settling time.
Is there a way to get faster settling time with additional equipment like an external current source or would i need a better Electrometer?

Attached is the measurement of a reed contact MDSR-10, where the long settling time is visible. Cable used: 1.5m Keithley-7078-TRX

[guenthert]

Question to the Electrometer-ninjas about the K617:

As far as i understand guarding is only usable when measuring voltage sources with high source resistance and when measuring high resistance DUTs with the electrometer-internal-current-source up to 200GR.

I'd rather think of guarding being needed only when very low currents are to measured (you could have very low currents with voltage sources of low source resistance, but those would be very small voltages which then pose additional challenges).

With the internal voltage source the K617 is capable of determining the resistance/leakage current of higher resistance DUTs like a reed contact, but without guarding it takes a great amount of measurement settling time.
Is there a way to get faster settling time with additional equipment like an external current source or would i need a better Electrometer?

I don't quite follow.  Why can't guarding be used here and in which way would you want another instrument to be 'better'?

[Echo88]

The K617 manual (page 30) advises to not use guarding in Amps or Coloumbs-mode and it also doesnt show a how a guarded charge/current (when using the internal voltage source instead of the internal current source)-measurement would look like.
Thats why i thought the K617 isnt capable of guarding when measuring current/resistance using the internal voltage source.
The idea of "better Electrometer" meant that newer electrometers are capable of measuring higher resistance with their internal current source while providing guarding, which reduces the settling time.

[guenthert]

  Yeah, sorry, not sure what I was thinking there.  In current measurements, one can expect a very small difference in potential across low and high inputs (<1mV "burden voltage") which makes guarding typically unnecessary. 

  Do I understand the problem now correctly as being that the resistance of the DUT is expected to be so high (>200GOhm) that direct resistance measurements (where the 617 offers guarded operation) is not possible and for the U/I mode (which allows for notably higher resistances, but where there's no option to use the guard) measurements are slow (due to stray capacities)?

[Echo88]

Yep, thats the problem.

[MiDi]

The guard is driven from the preamp output.
In Volt and Ohm mode the preamp output is just the buffered input.
In Amp (V/I) and Coulumb mode it does not follow the input voltage.
As the input is held close to 0V, that should act as a guard to bootstrap input/stray capacitance.
See p. 116/6-4 in the manual.

[MiDi]

The last post is simply wrong, next try well-rested

For Amp (V/I) and Coulumb the inner shield is tied to signal gnd - for both normal and in guarding mode.
It would not make any difference, except for active guarding there has to be a return path to signal gnd outside of the triax cable.

[Alex Nikitin]

Attached is the measurement of a reed contact MDSR-10, where the long settling time is visible. Cable used: 1.5m Keithley-7078-TRX

I would advise you to repeat this measurement with just an air gap of roughly equivalent capacitance as the reed switch. You may find that the settling time is considerably reduced. If that is the case, the apparent settling time is the result of the dielectric absorption of the reed switch glass, so it is a real effect of the DUT, not of the electrometer used.

Cheers

Alex

[Echo88]

Thanks Alex.
I did the following tests:
-reed-contact-surface covered ~80% with aluminiumfoil and foil connected to V+ while sitting on a teflonplate
-reed contact airwired between electrometerinput and voltagesourceoutput and without sitting on teflonplate
-electrometerinput and voltagesource-output sitting on the teflonplate without reed contact

You were right: the settling comes directly from the DA of the reed-contact, since i can see only a very short settling time when no DUT is present and the teflon-plate isnt the DA/charge trapping-culprit.

Im a bit disappointed that reed-contacts arent behaving better and would have expected that the aluminiumfoil-cover would help.
Ive seen that you measured reed-contacts with same results regarding settling-time, any hints on what can be improved?

[MiDi]

When I tested the old brown relays of my unit, the better ones had settling times in the order of an hour for more than 100T
In comparison the measured Meder Relay took less than half an hour to reach 10P
Besides these measurements I have no experience, but I would guess that there are huge differences in DA for different relay types.

[Vtile]

Can these electrometers measure the leakage of typical small engine spark plug (dry&clean, but USED). I'm just wondering... "Metrology object". 

The backstory is that some time ago I was repairing two motorsaws of mine, which both had problems, one later was diagnosed a failed magnetocoil. The thing related to this threas was the idea to just quickly measure the difference between the really used and soothy plug and the new replacement with only a few tens of minutes runtime.

...It was not as easy as I thought, not a quick check of relative leakage current with my Meratester in series with 10Vdc source and dut (fA range). Well I did crank up my lab psu to full 60V ... nothing.

Not only after I changed to 1000Vdc source in series I had some form of reading. IIRC somewhere at Teraohm region.

Which made me wonder if I reached the limits of my setup, but obviously they must have a good insulators after all, but still.

[HighVoltage]

I have tried used the Keithley 617 and the 6517B to measure the insulation resistance of new spark plugs and that failed, because the pure Aluminium Oxid (Al2O3) insulator is better than the ATTO Amp resolution of the meter.

But the 6517B and also the 617 can be very well used to measure the insulation resistance of ignition coils.