Zj-27 ionisation gauge controller for the people

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Robert Clarke
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Re: Zj-27 ionisation gauge controller for the people

Post by Robert Clarke »

Thank you for those links. I will read them as soon as I am at my desktop.
In the meantime I revisited the basics of the opamp circuit for sensing. And I now understand what you said about the measurement being in current only.
As the circuit is a transimpedance amplifier, the voltage I was so concerned about develops across the feedback resistor. Silly me.

So the feedback resistor is chosen based on expected current and desired output voltage.

Does this sound correct?
Aleksa Djokic
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Re: Zj-27 ionisation gauge controller for the people

Post by Aleksa Djokic »

Yes, that is correct.

Start with something in the range of 10 to 100M ohm, that will give you some room for experimentation.

Just to mention a little situation I had with measuring low currents, when I was cleaning my leak detector, it had a metal-ceramic feedthrough that carried the collector current to the outside world, directly above sits 5889 electrometer tube with a glass sealed 1G ohm resistor. After degreasing, acid clean, water rinsing in lab grade demineralized water for half hour, acetone and IPA, I dried the whole detector cell in a dry nitrogen environment at 60c. When I reassembled the whole unit, of course it had problems, the output from the electrometer was always pegging the needle, this was the result of not cleaning the ceramic feedthrough enough(imagine not being "good enough" after all that I mentioned above). The solution was to again heat up the whole cell, dry it under vacuum and in the chamber where the tube lives, purge it with freon R114 that eliminated all the moisture that would eventually come back and affect the measurement. This story should give you some idea of how cleanliness is very important, and that low levels of current flow in very delicate way. Keep in mind this is picoAmps range, so not that dramatic, imagine going femto..
Robert Clarke
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Re: Zj-27 ionisation gauge controller for the people

Post by Robert Clarke »

Aleksa,
I could imagine going femto must be quite the trip!
Last night I salvaged a filthy 10 Mega Ohm bleed resistor from a microwave oven capacitor,
10MOhm.jpg
It was covered in some mysterious oil which I scrubbed off with IPA. When I installed this in my LMC662 TIA circuit it immediately became very sensitive to me touching the croc clip jumper wire. This is how I am currently connecting the 'collectrode' to the opamp. I realise this is the most horrible situation in these circumstances. Especially that it is part of a rats nest of wires in my prototype. Induced currents a go-go, I'm sure.
At 10Meg my results have been variable. The clearest indication I am seeing is a difference of 50mV on my output. This being most obvious when I disconnect power to the tube. It jumps back up to the 'off' voltage.
So, my thoughts are: This is induced current in my jumper wire OR it is working and the tube is at high vacuum and I should be using 100meg or Gig Ohm resistors.
Pesky work got in the way over the last few days. I will order some high value resistors and see what I get.
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Rich Feldman
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Re: Zj-27 ionisation gauge controller for the people

Post by Rich Feldman »

Nice resistor recovery, Robert! A resistor with an honest kilovolt rating. Does your picture also show if MO capacitors are designed to break connection between terminals and the foil roll, if lid bulges from overpressure?

The talk about femtoamps, and Richards' comment:
>>My cat walking across the room 8 feet away with a 7 inch antenna on the input makes it go rail-to-rail.
reminded me of Wikipedia page about "Electrostatics".
Cat_demonstrating_static_cling_with_styrofoam_peanuts.jpg
Cat_demonstrating_static_cling_with_styrofoam_peanuts.jpg (53.3 KiB) Viewed 5253 times
All models are wrong; some models are useful. -- George Box
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Richard Hull
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Re: Zj-27 ionisation gauge controller for the people

Post by Richard Hull »

My transimpedance amp in image below. It is built in a tiny Pamona BNC component box using the LMC 6041. Data sheet URL in my post above. This is the item and its stub antenna mentioned that is hyper sensitive to the outside world of charge exchange between it and my cat at range and the tapping of my toe on the carpeted floor 6 feet away. Charge exchange in motion in a world filled with electric fields we can't sense.

I built this little guy back in 1995. I have been on this for years as just another one of the HEAS experimental demos and sidelines in electronics. It makes one think long and deep. Nothing beats the doing, and then some deep thinking. All to weigh and consider. It can be considered a cool toy by those with little depth of thought not use to stopping to smell the roses.

Open up a new world for $2.64 from digi-key

https://www.digikey.com/en/products/det ... OPB/364306

Hook the thing up with 4 coin cells +/- 6 volts to ground as a simple unity gain amp use no other components, no IC socket no pc board, float in air with antenna hooked to the positive op amp terminal. Amazing!

Note: a zeroing attempt is a futile gesture if ultimate sensitivity and a true study of the outside world is the goal. If you must, install a highly insulated antenna input to ground shorting button to temporarily zero. (this effort on your part will be a miracle in itself). Needless to say never touch the antenna unless you first touch the circuit grounding first...(battery supply center point....usually case metal shell, if you enclose this thing like my little Pamona box.)

Place this hooked to a scope via a long enough cable to isolate it from large grounded objects. Use a lunch box type O'scope with an LCD display that retains it trace image as a fading trial on .2-.5sec/division and marvel in experiment.

Richard Hull
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LMC 6041 based transimpedance system opens up the world of electrostatics and charge exchange events.
LMC 6041 based transimpedance system opens up the world of electrostatics and charge exchange events.
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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
Robert Clarke
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Re: Zj-27 ionisation gauge controller for the people

Post by Robert Clarke »

Rich and Richard you can rest assured my cat will be tested with the electrometer once I have it constructed. No more secrets Alberta.I await 100meg and 1 gig resistors.

Would I be correct in assuming that a single rail power supply will be ok for Ion current sensing as it can't go negative...or is that positive...I'm starting to mix up conventional current, electron current and Ion current and photoelectric current!
Last edited by Robert Clarke on Fri Jan 27, 2023 5:15 pm, edited 1 time in total.
Robert Clarke
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Re: Zj-27 ionisation gauge controller for the people

Post by Robert Clarke »

Rich,
Regarding the MO capacitor. The lugs in the photo have a tiny spot welds to the tabs of the foil in the dielectric roll. The lid came away without any pulling or distortion on the tabs so I assume they are 'fuses' of sorts. Is it even possible spot weld aluminium foil to iron? Maybe there is more going on here than I can see!
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Richard Hull
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Re: Zj-27 ionisation gauge controller for the people

Post by Richard Hull »

Current is current regardless of any classy names we ascribe to it as regards to the mechanism from which it springs into measurable existence. Polarity and direction of current flow is also just a handy way to try and understand a special case. Current flows whenever an extant electric field is disturbed via some form of motion either in air or a conductor. All charge in this marco world be it in a battery or between fields is strictly due to the electrons establishing a field condition either by an over abundance at a point or a forced lack thereof at another point. Chemistry is 100% electronic in nature.

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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
Robert Clarke
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Re: Zj-27 ionisation gauge controller for the people

Post by Robert Clarke »

Hi Folks!
Here is a little update of progress. My high Ohm resistors arrived so here is my circuit in progress...
IMG_20230204_191701.jpg
This will eventually be housed in a metal surround, probably a copper pipe. The glass feedthrough in the picture is from a bulb that was over the bathroom sink. I hope this will isolate the signal enough.
In the meantime I have built a manual adjustment for electron current.
IMG_20230204_191736.jpg
This is just the existing trimmer VR from the 3.3V power supply brought out to the front od the enclosure. There is a nichrome wire resistor in series with the filament of the ionisation gauge tube.

I know this is very ham fisted but until I learn more this will have to do for now!
Robert Clarke
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Re: Zj-27 ionisation gauge controller for the people

Post by Robert Clarke »

A small progress report. This is my electrometer circuit as of this evening
Electrometer Faraday cage
Electrometer Faraday cage
The casing was cast by myself and friends over the weekend. The model was a 3d print.

Let's see if it works! More to follow
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Re: Zj-27 ionisation gauge controller for the people

Post by Aleksa Djokic »

The glass feed through is a bit of an overkill, a typical BNC or other similar connector would suffice. A typical RG58 coax and alligator clips at the and are good for single digits pA. I would expect the ZJ27 is sealed somewhere between 10-5 or maybe 10-6mbar, in that case, the range is in nA/pA, so 100M might be too much, it may overload the input and cause the output to swing the power rail, so keep that in mind if you see such behavior.
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Richard Hull
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Re: Zj-27 ionisation gauge controller for the people

Post by Richard Hull »

I wonder...........Is this latest posting about building an electrometer to look at the electrical world around you? If so that is fabulous.

You original post was about a vacuum gauge issue and a replacement for a formal controller.

Work for a purely informative electrometer of extreme sensitivity, (Fempto ampere work), would best be posted in the neutron/radiation detection forum as a very sensitive electrometer is the basis for an amateur construction of a very sensitive ion chamber, radiation detection system where 10e-14 ampere currents are the norm.

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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
Robert Clarke
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Re: Zj-27 ionisation gauge controller for the people

Post by Robert Clarke »

Thanks both for the replies,

Aleksa, thank you for the guidance on what to expect from the output. I have yet to power it up and connect the ZJ27. I have 12dc available with the enclosure so I will use that. TBH I'm not sure what I'm doing with respect to voltages. Its a little frustrating that the only 'variation' I have is when the tube heats up or I fiddle with the filament power. I will report back.
The glass feedthrough is partly a philosophical choice. We are surrounded by such wonders in our everyday objects. These Dumet wire seals in bulbs are amazing. I suppose I wanted to celebrate that a little. Glass to metal seals are very curious to me.

Richard, I am certainly inspired by your stand alone electrometer. And my enclosure is a homage no doubt. But primarily its just to try ensure isolation. The one in the image above is currently mounted within the plastic enclosure seen furthur above. I'll post an image when I have it tidied up. I intend to reuse the pattern and cast another enclosure for a hand held version. Seems like a fascinating device to play with in the wild. It is deliberately scaled to allow for CR2032s or so as a battery pack like the one you describe above.
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Richard Hull
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Re: Zj-27 ionisation gauge controller for the people

Post by Richard Hull »

All the best in this project. You will be amazed if the amp you are using has a bias in the fempto-ampere range. Pico amps are fine, but nothing shows the electrical world as being alive like a hyper low bias amp. I like the glass insulator, make sure to wipe it off with alcohol to remove grease from hands. Do not use an input resistor. Just let the input lead float to the insulator. You are looking for sensitivity not a calibrated response offered by a teraohm resistor effectively shorting out the 5-10 teraohm natural input of the FET gate.

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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
Robert Clarke
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Re: Zj-27 ionisation gauge controller for the people

Post by Robert Clarke »

In the interim and for the enrichment of the community I post a photo I took of the electrometer part of the Varian 880 gauge controller. If work stops getting in the way, I will progress my build some more.
Varian 880 electrometer.jpg
That transistor is a 2n3904 of all things. Curiously with the base and collector tied to that post. (you can't see from that angle but there are two legs there)
If there are any relevant observations, feel free!
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Re: Zj-27 ionisation gauge controller for the people

Post by Aleksa Djokic »

Yes, that indeed looks like a typical ion gauge electrometer design. The transistor is there just as a protection, its used as a diode, the input of the OpAmp is very sensitive to ESD. AD545 is a very good device with only 1pA input bias current, impressive for the time...
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Re: Zj-27 ionisation gauge controller for the people

Post by Aleksa Djokic »

Typical voltage that gets out of the ion gauge collector is up to 10v DC, so the 2n3904 is used as a zener and reverse protection, in zener direction it clamps somewhere between 8 to 9V and in the other direction a typical 0.6V,
Robert Clarke
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Re: Zj-27 ionisation gauge controller for the people

Post by Robert Clarke »

*Another aside*
This zener effect is very interesting. A circuit I have used a few times in work has been this:
http://cappels.org/dproj/simplest_LED_f ... rcuit.html

Am I right in saying it uses the same effect in the transistor?

Aesthetically this flasher is very pleasant, analogue looking. Giving a nice fast dimming, with no PWM, which is good for camera.
I will be back on topic next post, I realise this is meandering.
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