Zj-27 ionisation gauge controller for the people

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Robert Clarke
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Zj-27 ionisation gauge controller for the people

Post by Robert Clarke »

The ZJ-27 is an ionisation gauge tube which is readily and cheaply available from Chinese suppliers. This conversation is an attempt to gather information and ultimately to establish a basic design for an easily reproducible, open source controller.
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Re: Zj-27 ionisation gauge controller for the people

Post by Robert Clarke »

I would love if any folk with experience in this subject could give an overview of what it takes to drive this kind of gauge tube.
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Re: Zj-27 ionisation gauge controller for the people

Post by Bob Reite »

Look at the spec sheet., you will find the filament requirements and other parameters there.
The more reactive the materials, the more spectacular the failures.
The testing isn't over until the prototype is destroyed.
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Re: Zj-27 ionisation gauge controller for the people

Post by Robert Clarke »

Hi Bob. I have those details to hand. I am just attempting to get a narrative going in the thread. Starting with an overview and perhaps some general comments form someone who has built a similar bit of hardware.
If there are no replies in a couple of days I'll copy the full specs here and hopefully get some good ideas going
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Re: Zj-27 ionisation gauge controller for the people

Post by Aleksa Djokic »

First, the basics, this is a good start: https://avs.scitation.org/doi/pdf/10.1116/1.5025060

Seems like not that many people know what it takes to drive and read the ion gauge tube.

When I first started with BAG tubes I had no clue how they works or how to drive them. Unfortunately, there is zero information on the web about how to drive them or read the current output. I was lucky enough to find some very old BAG controllers and take a look how they did it.

The first and most simple task is to make a circuit that controls the constant emission of 0.1 to 10mA, so lets start from this point.

In my first design, I've used a simple constant current circuit with LM358 and a LED diode for the output, that LED was a part of TRIAC/DIAC/Photo resistor network that drove the filament supply. The set point was adjusted with a trim pot and a TL432 reference, the current was read from a point on the filament (shown in the provided link). By using a Triac to drive the transformer, I had some issues that were solved in a not so good way, first, there were a lot of oscillation because there was no hysteresis, so the filament output would start jumping all over the place until it settled down on the setpoint, after some time, it would break again, the solution was simple, add a capacitor in the feedback loop to slow down the reaction time, this worked, but the output, when first energized, overshoots and undershoots several times before it settles and stayed on the setpoint. A better way is to control the filament with DC, the output of the OPamp should go to a transistor that controls the filament. If done this way, the circuit must be isolated from the rest of the tube, it will sit at 30v bias voltage.

As I've posted the schematic in the previous thread, here is another one that is the same: https://i.stack.imgur.com/BtnzA.png

This circuit is for a constant current source for laser diodes. Instead of the diode, use the filament. The supply voltage (V3) should be set with an external power supply to the maximum voltage that the filament can be run at, just as a safety, in case the circuit doesn't regulate the output. The reference diode should be tied to the supply rail of the OPamp. The + input on the op amp is the setpoint input, this will be adjusted to give a constant emission current of 1mA, the (-) input of the op amp should be tied to the shunt resistor on the filament. The shunt resistor should be sized to give a voltage in the range of 1V/1mA, or there about(meaning the setpoint at the + input should also be in that range).

It may sound confusing at first, but when you get the hang of it, it will not be so complicated.
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Re: Zj-27 ionisation gauge controller for the people

Post by Gustavo Cervantes »

I was alerted by Aleksa on this topic, there is a spanish forum which has a lot of good content regarding vacuum work, not sure how good the translation on google part would be, but here is a hot ionization gauge made y one of its members:

http://www.cientificosaficionados.com/f ... 5y5yNreK-g

Also someone made a cold cathode gauge:

http://www.cientificosaficionados.com/f ... hp?t=19438

There is a few missing images cause the threads are a bit old and the forum moved not long ago
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Re: Zj-27 ionisation gauge controller for the people

Post by Robert Clarke »

Hi all,
Below is a quick high level comment on a matter which confused me in the past.

I began my vacuum journey with John Strong's Procedures in Experimental Physics. In this awesome book the ionisation gauge tube is described as a modified triode tube in which the glass bulb is connect to the apparatus to be measured. Easy right?

So what's all this talk of BA tubes? Well, there comes a point at very low pressures where the physical design of a modified triode tube causes a problem. Around 1x 10-5 the tube begins to create X-rays. The new charged particles created by the x-rays look to the 'collectrode' as the ionised gas current it expects to see; therefore at and below these levels of vacuum the readings are erroneous.

The solution? Turn the triode inside out! This is what Bayard and Alpert did circa 1950. The problem with the Olde world Triode was that it had a large surface area cylindrical collectrode. While this was great for collecting the ionised charges we deliberately create, it is also a huge target for the more random x-ray induced charged particles.
So Bayard and Alpert made the collectrode like a pin. This pin still had the bias charge which made it attractive to the ionised gas molecules being measured but was a much much smaller target for the x-ray induced charges. And so the gauge could continue to be useful at lower pressures.

So for clarity the ZJ-27 is in the older and now surpassed modified Triode construction. It is not a Bayard Alpert Gauge tube strictly speaking. However for our controller project I don't believe this makes any practical difference.
Last edited by Robert Clarke on Tue Jan 10, 2023 8:04 am, edited 1 time in total.
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Re: Zj-27 ionisation gauge controller for the people

Post by Robert Clarke »

Hi all,
May I start by thanking you all for your responses so far.
In an attempt to keep my solutions as simple as possible (off the shelf) may I ask if the following is feasible.

Dealing only with adjustment of Filament power supply for a moment. The old literature has a variable power supply to filament and the mA gauge on the grid/anode. No active feedback at all!

Can one simply pump down to 10-3 or whatever is safe, and while watching the grid current gauge, just trim the voltage and so current through the filament?
Example: I have a 3.3v 6A SMPS. These are very common and easy to find. It has a small degree of adjustment possible with a trim pot on the front. Can I use a power resistor to get close to the 1.3v across the filament and tweek the SMPS that until I see 1mA on my grid gauge?

I realise this is horribly wasteful in terms of Wattage but these parts come from China and any waste I should be worried about has already happened flying them halfway around the world!
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Re: Zj-27 ionisation gauge controller for the people

Post by Aleksa Djokic »

Safe vacuum is in the range 10-4mBar and below, these tubes probably use pure tungsten filaments, if they were yttria coated iridium then you would be able to go up to 10-1mBar.

You could control emission current like that, but you will immediately see why you need a circuit to do it for you. Just be careful with the voltage, try not to go above what's specified in the datasheet.
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Re: Zj-27 ionisation gauge controller for the people

Post by Robert Clarke »

I accept I may have to crawl before I run!
The Zj-27 is described as having a Nobel metal oxide cathode so hopefully I have not already burnt it out.
Aleksa, I do not doubt your advice is correct. And I'm sure I will ultimately use active feedback control but my learning is slow and takes much energy. Every little success pushes me on to try a better solution.
Tomorrow I will check anode current with my 'voltage divider' plan. See how it varies with changes to the SMPS trim pot. I will report back.
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Re: Zj-27 ionisation gauge controller for the people

Post by Robert Clarke »

I just tested with my Zj-27 still sealed in it's glass tube. I was under the impression they were shipped at a very high vacuum but it seems this may not be so.
Regardless I pumped full wack through the voltage divider as described above. I saw 30mA on my analogue gauge attached in the 200v anode/grid circuit. EEk!
I trimmed it and at 1.03V across filament I was seeing 1.5 mA on my analogue gauge. At this voltage the filament was dull red.

Official specs of the ZJ-27 are as follows:

Filament 1.3V @2.5A
Filament to ground 25V
Gate/Grid/Anode to ground 225V/200V (depending on the source of info')
'Collectrode' to Ground 0V

What I assume is de-gas 10V/3A

More to follow...
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Re: Zj-27 ionisation gauge controller for the people

Post by Robert Clarke »

Moving on to the 'Collectrode' end of the gauge, previously Aleksa suggested the LMC622 as a suitable OpAmp to sense the ion current.
This OpAmp has an Input Bias Current of 2-4 pA, if I understand the datasheet correctly. Aleksa tells me this is the critical parameter to look for in this situation.
Also a high quality resistor 100m to 1Gohm is required I hope to find more details on this requirement shortly. I will order am OpAmp or two and see what I can see.
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Re: Zj-27 ionisation gauge controller for the people

Post by Alex Aitken »

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Re: Zj-27 ionisation gauge controller for the people

Post by Robert Clarke »

Alex,
Thanks for that link I had not seen it.
They state that the ZJ-27 is shipped at 10-5 to 10-6mBar. If that is consistent with all the tubes manufactured then I think testing with the tube still sealed should be safe.
The pdfs of their circuits also have great information. I will prototype with the Lmc662 TIA style circuit for now to assist my learning. Once I have some clearer understanding of how the straight forward circuit works I will revisit the pdf and see how they implement it.
I intend to aim for somewhere 5v to 0v output from the opamp to hopefully interface with an Arduino ADC or even a moving coil meter.
I think the Lmc662s will not arrive until early next week so in the meantime I will see if I can see ripple in my high voltage smps.
I will also rebuild my power resistor to a slightly higher resistance
Thank you all again for assisting my project
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Re: Zj-27 ionisation gauge controller for the people

Post by Robert Clarke »

Aleksa may I ask you some questions regarding your description of the laser diode CC supply circuit?
Is this adjustability necessary because once the tube is installed conditions might change and emission current will be different?

I have redrawn the circuit of the laser diode. if you have time could you tell me if it looks correct to you?
IMG_20230116_123301.jpg
Many thanks
Rob
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Re: Zj-27 ionisation gauge controller for the people

Post by Aleksa Djokic »

The first thing you learn when starting with high vacuum is how much everything is unstable and outgasses like you wouldn't believe. The stability of the emission current will never be stable on its own, it will always move around depending on many factors. The only thing, for now, what you need to worry about is the filament temperature variation and gas load. The schematic you have drawn is indeed just a constant current source that takes feedback from the shunt resistor. The feedback point is not from that resistor but from a resistor between the anode and cathode. What the BAG tube needs is a constant emission current with a varying filament current that adjusts it self for stable emission. So the feedback point needs to be between the cathode and anode. Take a closer look at the schematic of the BAG tube.
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Re: Zj-27 ionisation gauge controller for the people

Post by Robert Clarke »

Aah! I did not realise there was should be a shunt resistor between Anode and Cathode on the BAG tube. TBH the only thing I have seen in my research between the Cathode and Anode is 180v-200v! I'm sorry for asking silly questions, for you this must be very obvious.
I have been looking at the Github repository and the Ionpak circuit diagram but I can not see where the anode/cathode shunt feedback comes from. I am trying to piece together information from various places to learn.
Perhaps this project is just beyond my understanding at the moment. Tomorrow is a new day. Thank you Aleksa.
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Re: Zj-27 ionisation gauge controller for the people

Post by Aleksa Djokic »

For a beginner, the IonPak is very complicated, personally, I don't like how they did the layout, very confusing and hard to understand. If you have the power supplies on hand, and you have made the CC circuit, just don't connect the feedback point to the shunt on the schematic, instead, connect it to a shunt resistor between the anode and ground. Keep in mind that you must make isolated power supplies, they cannot share the same ground reference. Filament power should be provided from a high quality transformer(good isolation), 180-200v can be supplied from a SMPS, 30v filament bias is not critical. If I saved my files from proteus, I would have gladly posted the schematic...
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Re: Zj-27 ionisation gauge controller for the people

Post by Aleksa Djokic »

Here is a quick sketch, the component values are not real, don't use them as a reference. The concept should be ok, this is the most simplest way I could think of. If you get any oscillation, you will need to use a capacitor in the feedback (integrate).
Attachments
ion.jpg
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Re: Zj-27 ionisation gauge controller for the people

Post by Robert Clarke »

Many, many thanks Aleksa. I'm so grateful for the time you put into explaining this for me and doing the sketch. I will take some time and study it. I think part of my problem in understanding this situation is that I have not reconciled in my mind how the voltages and their respective currents exist in the circuit and tube.
I have been reading so much which just refers to current without mention of the voltages involved, which is OK until I need to figure out values for components!
Is it safe to think of the filament-grid electron flow as an equivalent resistor? like 180/0.001=180,000Ohm?
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Re: Zj-27 ionisation gauge controller for the people

Post by Robert Clarke »

Also I must think of this in the terms of the isolated power supplies. I have been basing my understanding on very simplistic power supply diagrams. TBH isolated power supplies within the one circuit is new to me! So I will think about this information for a day or two and breadboard something.
BTW my god the LMC662 is sensitive! I did a quick circuit and I could not move in my seat but it would register! Fantastic!
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Re: Zj-27 ionisation gauge controller for the people

Post by Richard Hull »

I have used the LMC 6041 for many years. 2 femptoamp bias.
My cat walking across the room 8 feet away with a 7 inch antenna on the input makes it go rail-to-rail.

https://www.ti.com/lit/ds/symlink/lmc6041.pdf

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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
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Re: Zj-27 ionisation gauge controller for the people

Post by Aleksa Djokic »

I would strongly recommend checking out some of these YT channels in order to get some deeper understanding of how the electrons flow in a vacuum environment.

Applied science, glasslinger, the thought emporium, mr carlson lab, Eric Dalgetty, jdflyback, Signal Ditch...

Also don't forget to do some reading: http://www.tubebooks.org/

The thing about isolation, as you can see from the sketch, there is a return path from a separate power supply that will introduce 30v bias in to the feedback of the op-amp, I have avoided this problem by running the filament from a transformer(AC), that gave me the needed isolation. So, in theory, the schematic I drew may not work correct, instead, there should be a SMPS driving the filament and feedback would just adjust the PWM, the transformer would provide the isolation and allow for biasing. Try it anyhow, just limit the input voltage to the filament...

Electrometers are a lot of fun, they can be used for many interesting experiments and measurements. Keep in mind that when dealing with such low currents, there is a whole science behind shielding and guarding the path that needs to be measured, surface leakage currents, material leakage, loading impedance, filtering, etc....
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Re: Zj-27 ionisation gauge controller for the people

Post by Robert Clarke »

Thank you both!
I just read the first half of 'Inside the Vacuum Tube' by John F. Rider. It was very entertaining. I got the book for this very project. I got as far as Pentodes and thought that's as much as I can handle for now!
I keep thinking back to what is the simplest solution for me to get this working. So I might start with an analogue meter for electron current with manual trimming of the filament power supply and an LMC662 collector circuit.
I realise this is very clumsy but small wins help keep the momentum for me.
How would one estimate the ion current in order to determine the correct components?
Am I correct in assuming the voltage of the ion current is the bias voltage?
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Re: Zj-27 ionisation gauge controller for the people

Post by Aleksa Djokic »

The output of the collector is measured in current only, voltage is not measured. If you are using the tube as a it arrived sealed, if it is sealed under high vacuum, expect the current to be in the range of nA to pA. For example, 10-4mbar is 1uA, 10-5mbar is 100nA, 10-6mbar is 1nA etc...

Here are some more good reading materials...

https://sci-hub.se/https://doi.org/10.1063/1.4869873

https://physics.mff.cuni.cz/kfpp/skript ... ratura.pdf
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