Cold Cathode Gauge vs. MKS 901p Gauge

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Maxwell_Epstein
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Cold Cathode Gauge vs. MKS 901p Gauge

Post by Maxwell_Epstein »

Hello All,

I have recently acquired a new vacuum gauge for my fusor setup and am wondering if it will be at all useful for me or if I should return it in favor of an MKS 901p.

As background, the only other gauge that I have is a Varian 501 thermocouple with an 804-a controller which I believe is properly calibrated as I bought it calibrated from a neutron club member's fusor.

The new gauge+controller system that I bought (for comparable price to a 901p on eBay) is the MKS HPS Model 941 Cold Cathode Gauge which can measure from 10^-2 to 10^-9 torr. I bought it used from eBay, but the seller told me that it was calibrated at last use. I am fully aware that as a amateur fusioneer, I will never need (or be able to get) a vacuum of 10^-9 torr. However, my thermocouple only goes to 10^-3 and it is my impression that being able to meter my vacuum down to 10^-5 will be very beneficial.

Unfortunately, the gauge is logarithmic and analog, so my ability to measure vacuum this low will most likely be constrained to measuring orders of magnitude. Because of this, I have considered returning or reselling this gauge and buying an MKS 901p as was suggested to me after I had already bought the cold cathode gauge.

What would the best option be? Should I keep the cold cathode gauge that I already have or should I instead buy and build a controller for a 901p gauge?

Thank you!
Max E.

p.s. Attached are some pictures I took of the gauge. It looks a little dirty. If I do end up keeping it, what would be the best way to clean it? Thanks!
Attachments
PXL_20230210_123343605.jpg
PXL_20230210_123335017.jpg
PXL_20230210_123329994.jpg
PXL_20230210_123313856.jpg
See my needlessly verbose fusor blog here: https://highschool-fusioneer.medium.com/!
JoeBallantyne
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Re: Cold Cathode Gauge vs. MKS 901p Gauge

Post by JoeBallantyne »

Does the cold cathode gauge work?

The MKS901p is an excellent gauge. BUT, it is most useful if you get one with a built in digital display, or if you make or acquire one for it.

Reading a digital display is much easier than trying to read an analog display that is primarily about showing you what the exponent on the pressure is.

You are most likely not going to go very low in pressure, so why have a gauge that goes down to 10e-9? Most of the range on the analog display you won't ever even use.

The 901p will also put out an analog signal like the cold cathode gauge that you can feed into a cheap 3-4 digit voltmeter display that you can buy from China off ebay for a few bucks. (There are many different kinds available.)

But, you will need to get a DB15 connector that you can use to feed it power, and pull out the analog signal, and connect to the digital interfaces on the 901p if you want to build a digital display for it that reads the pressures and displays them.

I would make sure you get one with RS232 not RS485. That means the second digit in the model number should be a 1.

So either get a 11xxx, or a 21xxx. KF16 rs232, or KF25 rs232.

Nevada surplus is selling some of the 11040 models on ebay now, and has quite a lot of them, and you can probably make him an offer and get one for $75 or thereabouts. They are listed at $99 right now.

My 2 cents.

Joe Ballantyne
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Re: Cold Cathode Gauge vs. MKS 901p Gauge

Post by Jerry Biehler »

Nothing wrong with the log readout, you can read fine enough resolution for anything you will ever need to do. If you are getting 1x10-5 or just 3x10-5 it really does not matter for what you are doing.
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Re: Cold Cathode Gauge vs. MKS 901p Gauge

Post by JoeBallantyne »

In my experience, fusor operation and behavior can be extraordinarily sensitive to pressure. Especially when it comes to what kind of current it will draw for a given voltage. A pressure change of a micron or so can mean the difference between a current that remains stable for minutes on end, and a slowly ever climbing one that also causes further increases in pressure. In that case, the rise tends to be exponential, and at least with the current limited supply (Spellman DXM 70kV 8.5mA) that I am primarily using, without my intervention to reduce the pressure in the fusor, invariably ends with the power supply shutting down, and the plasma immediately extinguishing.

You are NEVER going to be able to tell the difference between 19 and 20 microns with that analog cold cathode gauge.

WIth the 901p and a proper digital readout of the actual pressure (not the cheap chinese voltmeter reading of the 901p analog output), you will know with a quick glance whether you are at 19 or 20, or 20.1 or 20.2 microns. I have found the submicron pressure resolution, and easy reading of the actual pressures to be invaluable when operating my fusor. Analog gauges just can't compete with that.

If anyone gave me the choice between an analog cold cathode gauge and a 901p with a digital display of the actual pressure, for the purpose of use with a fusor, I would take the 901p in a heartbeat.

Joe.
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Re: Cold Cathode Gauge vs. MKS 901p Gauge

Post by JoeBallantyne »

The problem is that 901p's with built in digital displays, don't show up on Ebay that often, and when they do, they tend to be quite pricey. At least 2x the price of ones with just the DB15 connector and no display.

Making a power supply and digital display unit that reads actual pressures over RS232 or RS485 from the 901p is non trivial.

I am working on building a few, but they are not done.

Joe.
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Richard Hull
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Re: Cold Cathode Gauge vs. MKS 901p Gauge

Post by Richard Hull »

Those cheap $6.95 Chicom digital meters found at harbor freight can rival a $200 more professional meter! I have several Weston primary reference cells and have done an older post comparing junky Chicom meters to my $250 fluke and they are a match to the thousandth of one volt level. The Weston cells hold their stock 1.019 volt output forever!

viewtopic.php?p=83225&hilit=Weston+Cell#p83225

Now for those who have a digital readout in the 901.... The meter in that is reading an analog voltage as the sensor in all of them produce an analog voltage! The digital readout 901s just, internally, have a simple digital voltmeter installed to read the analog voltage.

So hooking a Chicom digital reading meter to an analog output of a 901 with no meter is the same as what is done in the beautiful and pretty 901s with a built in digital meter. Same, same. no difference and the Chicom meter will read to a fractional micron just like the built in meter in a 901. Analog to digital conversion in both cases. The same goes for my $1400 Baratron! it puts out 0-10 volts linearly (analog) which I feed to a simple analog input meter that reads out in nice pretty LED bright numbers to show 2.34 microns when the gauge send out 2.34 millivolts!

All, as in 100% of all vacuum gauges produce only analog voltages at the sensor, itself!. A rare few have analog to digital meter readouts built into them which are typically no more accurate than a $6.95 harbor freight digital VOM.

No need to lord an internally metered 901 over the same 901 hooked to a cheap Chicom digital reading meter or a $250 Fluke digital reading meter.
Again, same,same.

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
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Re: Cold Cathode Gauge vs. MKS 901p Gauge

Post by JoeBallantyne »

The 901p has two different pressure sensors, and its analog output is used to report the absolute pressure as determined from both sensors.

The default analog output is 1V/decade of whatever unit of pressure the 901p is set to report. (Torr, mBar, or Pascal)

Bottom line is that you can't just read the voltage off your 4 digit DVM and think it corresponds to a straight pressure - like what you get on the digital screen.

The table for interpreting what the 1V/decade analog voltage means is in the manual, and I have replicated it here.

Torr/mbar Vout Torr/mbar Vout Torr/mbar Vout Torr/mbar Vout
1.0E-5 1.000 1.0E-3 3.000 1.0E-1 5.000 10 7.000
2.0E-5 1.301 2.0E-3 3.301 2.0E-1 5.301 20 7.301
3.0E-5 1.477 3.0E-3 3.477 3.0E-1 5.477 30 7.477
4.0E-5 1.602 4.0E-3 3.602 4.0E-1 5.602 40 7.602
5.0E-5 1.699 5.0E-3 3.699 5.0E-1 5.699 50 7.699
6.0E-5 1.778 6.0E-3 3.778 6.0E-1 5.778 60 7.778
7.0E-5 1.845 7.0E-3 3.845 7.0E-1 5.845 70 7.845
8.0E-5 1.903 8.0E-3 3.903 8.0E-1 5.903 80 7.903
9.0E-5 1.954 9.0E-3 3.954 9.0E-1 5.954 90 7.954
1.0E-4 2.000 1.0E-2 4.000 1.0 6.000 100 8.000
2.0E-4 2.301 2.0E-2 4.301 2.0 6.301 200 8.301
3.0E-4 2.477 3.0E-2 4.477 3.0 6.477 300 8.477
4.0E-4 2.602 4.0E-2 4.602 4.0 6.602 400 8.602
5.0E-4 2.699 5.0E-2 4.699 5.0 6.699 500 8.699
6.0E-4 2.778 6.0E-2 4.778 6.0 6.778 600 8.778
7.0E-4 2.845 7.0E-2 4.845 7.0 6.845 700 8.845
8.0E-4 2.903 8.0E-2 4.903 8.0 6.903 760 8.881
9.0E-4 2.954 9.0E-2 4.954 9.0 6.954 800 8.903

You want to know the actual pressure with a quick glance, you need a digital display showing you the actual pressure. A digital voltmeter telling you what the analog voltage output value is, won't cut it. Because then you have to figure out the mapping from that value, to the actual pressure value. Certainly not something I would like to have to do in my head, when I am trying to focus on fusor operation.

Joe.
Last edited by JoeBallantyne on Wed Feb 15, 2023 3:02 am, edited 4 times in total.
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Re: Cold Cathode Gauge vs. MKS 901p Gauge

Post by JoeBallantyne »

The 901p manual.

HPS901man-REVI.pdf
(2.42 MiB) Downloaded 153 times

Joe.
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Richard Hull
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Re: Cold Cathode Gauge vs. MKS 901p Gauge

Post by Richard Hull »

Ah so. The voltage scaling is like a TC gauge totally non-linear over the decades. I see from the manual the voltage out is a log-lin kind of voltage reporting. This would be something a small Arduino mini could do via a calculation log lookup table over the linear decade change and display the true pressure on an LCD display. Interesting. It would be a snap to program.

Of course like all such gauges it is gas sensitive and as such is non-linear in its response until very low pressures are reached.

I guess I am spoiled by the absolute linear voltage output across 4 decades of the baratron that can be read on a Chicom voltmeter and is free of the different gas non-linear reporting nature of the pirani and TC gauges.

Thanks for the manual and the info.

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
Rex Allers
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Re: Cold Cathode Gauge vs. MKS 901p Gauge

Post by Rex Allers »

True that for the 901p you could program a micro to read the analog output and translate that to a vacuum reading. But the electronics in the 901p already does that for you. Much easier to program the micro with a serial port to the 901p. Over that serial connection issue a command to the 901p and read the result. It returns the pressure in ascii text.

Via the serial port you can also send a command for the units you would like: Torr, mbar, pascal. And since the Pirani sensor responds a bit differently to different gasses, you can also send a command to set the response for: Nitrogen, Air, Argon, Helium, Hydrogen, H2O, Neon, CO2 or Xenon.

Much easier to let the 901p do the work and just talk to it with commands.

Some years back I found a really powerful serial "terminal" program for the pc. Called ScriptCommunicator, you can write scripts for it to do some amazing programming including formatted output. I wrote a set of scripts for the 901p. Here is the resulting screen for the Set Unit command and the window for displaying the auto-updating vacuum reading.

Set Unit Sequence.png
Set Unit Sequence.png (8.88 KiB) Viewed 1392 times

Welch 20 min.png
Rex Allers
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Richard Hull
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Re: Cold Cathode Gauge vs. MKS 901p Gauge

Post by Richard Hull »

Thanks Rex. I noticed in the manual that there was RS232 comm with the 901. Looks interesting. Should I ever get a 901 I would handle it via the com command system.

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
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Re: Cold Cathode Gauge vs. MKS 901p Gauge

Post by JoeBallantyne »

Rex -

Would you consider posting your scripts, and a link to where to get said serial program? (Or maybe even upload it to files on fusor.net if that is possible.)

Thanks.

Joe.
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Re: Cold Cathode Gauge vs. MKS 901p Gauge

Post by JoeBallantyne »

Rex Allers
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Re: Cold Cathode Gauge vs. MKS 901p Gauge

Post by Rex Allers »

Joe,

Yes, you found the main page for ScriptCommunicator [SC].

I have no problem with sharing the scripts I did for talking with the MKS 901p using SC -- in principle, that is. The difficulty is that SC is so powerful a tool and has so many features that I think I really need to also provide a fair bit of explanation on configuration and use of my scripts.

I did my version of SC add-ons in 2017, and started to share that here in 2018, but I never got to a point of documentation that I thought was detailed enough to be worth sharing, so I didn't post.

I've just set up a clean install of SC on a Win 10 PC and have been trying to make notes and begin to write up a guide to the parts I have used, some of which confused me for a while. I'll try to keep at it this time.

I'll post something on this forum when I think I have something worth sharing.

----
The ScriptCommunicator manual comes as part of the install process, but you can also get just the current manual version here:

https://github.com/szieke/ScriptCommuni ... umentation

That link shares the manual in its default PDF format, but also has an ODT version.

The manual is extensive but many aspects of the basic user interface are described in just a few words. I had to do some trial and error to understand what many configuration options are and how to use them.

Peruse the manual if you are so inclined. It is an amazing piece of software and shared for free.
Rex Allers
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Maxwell_Epstein
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Re: Cold Cathode Gauge vs. MKS 901p Gauge

Post by Maxwell_Epstein »

Thank you all for the very interesting responses and discussion! I've just had a chance to read through all of this and it seems like the consensus is that the 901p is by far the superior option. I plan on returning my other gauge and getting a 901p gauge to mess around with. Looking around the forum, especially at this post from a few years ago, it seems like there is a wealth of knowledge here on the operation of these systems. I'll definitely look into using SC for managing everything too!

Thanks again!
Max
See my needlessly verbose fusor blog here: https://highschool-fusioneer.medium.com/!
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