Keithley picoammeter manuals?

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Rich Feldman
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Keithley picoammeter manuals?

Post by Rich Feldman » Tue Mar 24, 2020 6:38 pm

Anybody got a manual for Keithley 26000 (or 26xxx) Logarithmic Picoammeter?
I found an old comment on another forum, that there were 9 variants. (pos,neg,bipolar) x (1e-13,1e-12,1e-11 bottom of scale).

Or a digital successor, Keithley model 480?
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Re: Keithley picoammeter manuals?

Post by Chris Mullins » Wed Mar 25, 2020 12:25 am


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Rich Feldman
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Re: Keithley picoammeter manuals?

Post by Rich Feldman » Wed Mar 25, 2020 7:19 am

Thanks, Chris.
The Model 480 manual (I've seen it somewhere before) explained the blinking 0.00 I was seeing on this old specimen.
It indicates over-range, in what I think is a Keithley-designed dual slope A to D.

Anybody looked under the aluminum plate behind the range switches?
Somewhere I got the idea that the push buttons operate some special low-leakage contact system, which might need cleaning or burnishing.
Pushing a range button also operates an ordinary switch, to set the decimal point etc., but sometimes it needs to be pushed farther and held there, or wiggled just right, to get a sensible reading.
On some ranges it's hard to make it stick in sensible-reading mode.
My test source is an old 9V battery (carbon-zinc!) and a 5000 megohm resistor to get about +/- 2 nA. I think the battery won't notice if this load is left connected for days at a time.
pa.jpg
Let's see if the model 480 can get the job done, before we play with the model 26000.

Note to noobs: high megohm resistors like the one in picture are not suitable for kilovolt metering, even if the theoretical power dissipation is modest.
Like many plain old resistors with bands that end in green or blue, the voltage limit gets you long before the power dissipation limit.
All models are wrong; some models are useful. -- George Box

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Rich Feldman
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Re: Keithley picoammeter manuals?

Post by Rich Feldman » Wed Mar 25, 2020 8:18 am

Wasn't hard to find a couple of teardown video on youtube! For example,
https://www.youtube.com/watch?v=zDcl0-t7ceY
pa2.jpg
.
Nice to know that two of the ranges, 1 nA and 10 nA, don't depend on the special gold contacts.

[edit] I spy some zero-ohm resistors, a thing they don't teach about in EE school. They can be had in most common resistor shapes and sizes.
All models are wrong; some models are useful. -- George Box

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Re: Keithley picoammeter manuals?

Post by Rich Feldman » Sat Mar 28, 2020 11:33 pm

While sheltering at home, with weekend immune system, took the opportunity to fire up the other picoammeter without benefit of a manual.
Same current reference: 9V battery and 5000 megohm resistor, which indicated as + or - a little less than 2 nA on the model 480.
k26000_2.jpg
With no current, the meter indicated off-scale low.
WIth negative current, negative light came on and reading was about 1e-9.
WIth positive current, positive light came on and reading was about 6e-10.

Perhaps the burden voltage is significant and not balanced.
Looks like the unit could use some adjusting, but that had better wait for a manual.
The cal low / cal high switch does move the pointer down and up.
It would be telling to see what happens with some better-known reference currents over a range of magnitudes.

The interior is mostly air, as in many instruments (even modern ones) where the value added is in some tiny subassembly filled with finesse and/or unobtainium. Current input is BNC conector on rear bulkhead.
k26000_1.jpg
While lid was off, I shook the unit upside down to remove what was rattling around. Couple of rat poops. Cost of having lab in garage or shed.
All models are wrong; some models are useful. -- George Box

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Re: Keithley picoammeter manuals?

Post by Richard Hull » Sun Mar 29, 2020 9:25 pm

It is interesting the marginal difference considering the analog needle uncertainty. I always loved the error displayed in a 5 gig resistor. Glass sealed resistors of the type shown were at best +/- 5% so the error range in value in this case would be +/- 250 million ohms for the resistor to be in tolerance. Sounds terrible doesn't it, but this error is nothing in the low end of measurement. (still 5% in any measurement) We are just not used to errors in resistance tolerances being that large, but then we are not used to talking in nano and picoamps either. Picoamps flow from our bodies as we pass by a filing cabinet in winter. Fempto-amps flow in gentle zephyrs of indoor air as air moves over metal objects. The world is an electrical place. I use electrometers to track such things. Picoammeters dip into this region as well. I have a great old Keithley picoammeter from the army's Harry Diamond Ordinance Laboratory. I bought it at a hamfest, of course! What would they be doing with it??? They would be testing current loops that might flow in a circuit of detonators to see what might make them explode prematurely with lightning strikes, RF signals, etc.

Richard Hull
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