Electrometers and Picoammeters

For the design and construction details of ion guns, necessary for more advanced designs and lower vacuums.
ian_krase
Posts: 386
Joined: Mon Nov 28, 2016 7:48 am
Real name: Ian Krase

Electrometers and Picoammeters

Post by ian_krase » Sun Dec 31, 2017 3:21 am

I'm looking for an electrometer or picoammeter for the next stage of my mass spectrometer project.

One option of course is to just DIY a picoammeter with a precision electrometer grade op-amp and a very high value resistor, like Thomas Rapp did.

However, I see that a number of rather old but apparently functional Keithley electrometers are for sale on Ebay. And these have the benefits of multiple ranges, built-in readouts, doing both current and voltage (and charge), protection circuitry, etc.




I'm wondering if anybody has any suggestions. Richard, I seem to remember that you had lots of experience with your lots of electrometers.

User avatar
Richard Hull
Site Admin
Posts: 10581
Joined: Fri Jun 15, 2001 1:44 pm
Real name: Richard Hull

Re: Electrometers and Picoammeters

Post by Richard Hull » Sun Dec 31, 2017 7:15 am

Yes, I still have a large number of electrometers, several picoammeters and nanovoltmeters, (mainly Keithleys).

I tend to prefer the older 610 series with 5886 special dual electrometer tube front ends, though they are a bear to keep clean over long periods of non-use. This was mainly because I used them around extremely high fields and voltages. Transistor front ends are more stable but can be zapped in a heart-beat and a replacement matched pair from keithley are very big bucks.

If you buy from e-bay, make sure if they are of the more modern transistor front end vintage that the front ends are good. The vast majority of sellers haven't got a clue, of course. The best model ever was the 610C, (transistor front end). The 610A and 610B are vacuum tube units and you can't kill those front ends.

The big problem is nulling and stability in the 10e-13 amp range. With a good electrometer, you will find out the world is a very electrical place. Hanging 4 foot piece of wire on the electrometer, I could see a full half scale jittery deflection as my cat walked by about 6 feet away. Tapping my toe on the floor 3 feet away had the meter rocking back and forth. I tend to use the 0 center scale to see positive and negative transitions of totally unexpected charge exchanges in the environment.

Most interesting is my observations during a nasty summer thunder storm with the keithley 610C hooked to a sapphire insulated 10 foot longwire in the wooden ceiling of my lab. The zero centered needle very slowly moves in one direction and ultimately pegs left or right. Then suddenly the needle slams across the entire face pegging in the opposite direction. I start counting and in a second or two a loud thunder clap is heard. The local air charge is slowly built up as charge accumulates. When the bolt hits the earth, all that charge is sucked away leaving a relative opposite indicating charge, only to build up again. Really cool. Only electrometers can see these near fempto ampere currents. Nano amps are huge currents in an electrometer. Only pico amp currents are important and fempto amp currents can be seen in a tightly controlled system.

Vibrating reed electrometers commonly can see current differentials of +/- 100 electrons. I have only two vibrating reed instruments.

There are a number of 8 pin DIPs that have bias currents in the 4-8 fempto amp range. At 13 bucks a pop you can make a good electrometer if need be. Blowing one of those would not break the bank to replace. I have a number of 100 + gigaohm electrometer resistors. A fellow electronic engineer, who I went to college with, refers to them as calibrated, precision and warranted open circuits. It is amazing to realize that a 5% 250 gigaohm resistor is intolerance if it has a 12.5 billion ohm, (12,500 megaohm), error in it.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

John Futter
Posts: 1229
Joined: Thu Apr 22, 2004 2:29 am
Real name:
Contact:

Re: Electrometers and Picoammeters

Post by John Futter » Sun Dec 31, 2017 8:08 am

Analog Devices have an electrometer on a chip
we use it down to 10^-15amps FSD with no problem
you have to isolate the input lead to a teflon insulated post with appropriate guarding.
a heater resistor keeping water away is a must

ian_krase
Posts: 386
Joined: Mon Nov 28, 2016 7:48 am
Real name: Ian Krase

Re: Electrometers and Picoammeters

Post by ian_krase » Sun Dec 31, 2017 11:26 pm

What makes the tubes get dirty? Is it literal dirt sticking to them?


Also, what's the longevity of the tube electrometers like? And availability of replacement front end tube pairs?


Would you consider it viable to replace a dead frontend with a cheap modern electrometer op amp?



I was primarily looking at the Keithley 616.

User avatar
Richard Hull
Site Admin
Posts: 10581
Joined: Fri Jun 15, 2001 1:44 pm
Real name: Richard Hull

Re: Electrometers and Picoammeters

Post by Richard Hull » Mon Jan 01, 2018 7:46 pm

I have a 616 which has a digital readout. I generally prefer the big analog meters of the 610 series.

The 5886 tubes in a moist or dirty environment, over the years, can have grime collect on the base of the tube. 5886 tubes are flying lead type miniatures and I have never seen one go bad. I was able to snap up a box of nearly 100 NOS 5886 tubes at an old bric-a-brac shop for $20.00 some years ago. You can generally buy them on line for $30.00 to $60.00 each. I have sold about 20 of my stash at $25.00 each. These were used in a lot of early helium leak detectors, as well.

To restore, (clean the tubes), I use absolute alcohol on a Q tip around the base of the tubes and a fine alcohol soaked tissue between the wires exiting the base.

You could replace an old front end with a modern one, but why not just roll your own electrometer from scratch with modern components..

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

ian_krase
Posts: 386
Joined: Mon Nov 28, 2016 7:48 am
Real name: Ian Krase

Re: Electrometers and Picoammeters

Post by ian_krase » Tue Jan 02, 2018 3:19 am

Took a look at the 610A (i think? It just said 610) manual. "Micromicrofarads" were mentioned. :)


I'm a bit leery of vacuum tube equipment because of my concerns about the reliability of the vacuum tubes -- though I know that most have fairly long lifetimes and I would not be running the equipment that often.

Still, this makes it look like the 610A is a viable option as well.



WRT "fet front ends" -- one thing I had thought of was to build a modern electrometer in an old electrometer's case, taking advantage of the rare custom Teflon-insulated ultra low leakage current rotary range switch (or matrix of ultra low leakage relays).

User avatar
Richard Hull
Site Admin
Posts: 10581
Joined: Fri Jun 15, 2001 1:44 pm
Real name: Richard Hull

Re: Electrometers and Picoammeters

Post by Richard Hull » Tue Jan 02, 2018 7:14 am

That is a good idea, swapping out the front end. Remember, the 610C is all solid state but looks exactly like the 610A and 610B. If you are in an environment of unstable currents the big 6" analog meter can be very valuable. The world in the fempto amp range is never stable without a lot of controls in place. Keithly used triaxial BNC cables on all their electrometers with the 610c and its immediate successor the 616. (expensive) I tend to just use BNC cables for easy work, but have an official Keithly triaxial for critical situations.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

ian_krase
Posts: 386
Joined: Mon Nov 28, 2016 7:48 am
Real name: Ian Krase

Re: Electrometers and Picoammeters

Post by ian_krase » Tue Jan 02, 2018 7:55 am

Fortunately I think I have a source of cheap triaxial cable.


What would you go for: a 616 or a 610A, knowing that in both cases the seller claims that they "work" and power on, but gives little evidence of knowing what an electrometer does?

John Futter
Posts: 1229
Joined: Thu Apr 22, 2004 2:29 am
Real name:
Contact:

Re: Electrometers and Picoammeters

Post by John Futter » Tue Jan 02, 2018 6:46 pm

Ian
It is not the triax cable that is hard to get, it is the eye wateringly expensive cord plug
you can get them from Pasternak

ian_krase
Posts: 386
Joined: Mon Nov 28, 2016 7:48 am
Real name: Ian Krase

Re: Electrometers and Picoammeters

Post by ian_krase » Tue Jan 02, 2018 8:08 pm

I have a potential source for those too.

Post Reply