MKS 901p

Every fusor and fusion system seems to need a vacuum. This area is for detailed discussion of vacuum systems, materials, gauging, etc. related to fusor or fusion research.
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Richard Hull
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Re: MKS 901p

Post by Richard Hull » Wed Jul 26, 2017 5:00 am

I bread boarded a very simple circuit to get only 5 volts into the arduino until the sensor's output voltage drops down below 5 volts.

You place a 470 ohm resistor in series with a 4.8 volt precision zener diode. Feed a variable 9 volt to 0 volt positive voltage from a supply to the open end of the 470 ohm resistor and ground or return of the supply to the open end of the diode. (make sure the diode polarity is correct). Place a voltmeter between the ground point and the junction of the diode and resistor. Set the supply to 9 volts. You will read ~4.8 volts. As you lower the voltage to 7 volts. The meter will still read 4.8 volts, safe for the arduino. As you continue to lower the voltage, below 4.8 volts, you hit the "zener knee". At this point on to zero volts, from the supply, the meter reads exactly what the supply reads.

Thus, you have had 4.8 volts out of the this circuit regardless of input voltage until you go below 4.8 volts applied, at which time any lower voltage just passes by into the Arduino, safely, until you hit zero.

Note, you have obviated and failed to register the the first four decades of the device and can read the pressure with the arduino from about 4.6 volts down to zero. This jams more bits and resolution into the range that you need at fusion pressures.

Caveat!!**** Nothing is easy....Always remember that in electronics.... There is almost always a way to skin a cat, but that doesn't mean you won't get scratched doing it.

It turns out that at the zener knee there is a small "loopy zone" (leakage region) of about .4 volts where things and the voltages monkey about a bit. But once you are well below the zener voltage the lower voltage exactly tracks the input voltage ~4.6v or so to zero. You could use a precision zener at 5 volts which the Arduino would be happy with, but I like a safety zone afforded by a 4.8 volt zener.

If you do not follow this discussion perhaps you might go with the serial suggestions. This is just how I would do this if I was pressed to use the Arduino as an experimental effort and exercise for self-satisfaction.

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.

Pablo Llaguno
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Re: MKS 901p

Post by Pablo Llaguno » Thu Jul 27, 2017 11:34 pm

Checked the voltage with a voltmeter and it said 9.6Volts, that doesn't sound good to me so I think the multimeter is wrong. Now I need to do some serial communication to see what the real pressure is. Talking about serial communication, zTerm is not free, I looked for other serial apps and found some but most aren't free and only give you a free trial. Any tips on what app to get?

Jerry Biehler
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Re: MKS 901p

Post by Jerry Biehler » Thu Jul 27, 2017 11:42 pm

ZTerm is shareware, pay if you want.

The chances of the meter being wrong are almost zero. It just may need an atmospheric adjustment.

Pablo Llaguno
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Re: MKS 901p

Post by Pablo Llaguno » Fri Jul 28, 2017 6:04 am

Nice news!

Managed to do serial communication, got 720 Torr at atmospheric. Using my Robinair VacuMaster I managed to hit 150microns, problem is, I know the pump is capable of more vacuum than that. I am using refrigeration copper pipe (1/4")with flares to connect it to my chamber, my chamber has needle valves and Hysol1C in all fittings, except the NPT adapter to the KF16 flange of the transducer. I am also using the MAKE design (I know it is terrible), which means I "sandwiched" a glass jar between two plates, and to seal it I am using Nitrile gaskets.

I would think that my system is leaking because of the small diameter copper pipe, but I would like to hear your thoughts.

John Futter
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Re: MKS 901p

Post by John Futter » Fri Jul 28, 2017 6:45 am

look up
vacuum conductance

Rex Allers
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Re: MKS 901p

Post by Rex Allers » Fri Jul 28, 2017 6:49 am


I was typing a message and just as I was ready to send I noticed you just replied with new status -- so here two parts.

--- My original message; may still something helpful here --
Does your 901P show a solid green light when you have it powered? If not It could have problems. Check the manual if you have different LED indications.

Looking at the alternative configurations (set to emulate a different sensor) some could have an output around 9.6 V at atmospheric pressure.

I think you should figure out how to talk to it with the serial port. In addition to reading the output in a text response you can request the status of the 901P and see if it is in some strange mode or has some kind of problem.

I think you said you had a Mac. I just looked on You can search there just for Mac software. Here's a link that should take you there, set-up for Mac:

On that page put 'Serial terminal' in the search term. That gave a number of results. Maybe one will work for you on the Mac. I don't have any experience to suggest one, but I'm thinking I may try one or two of the PC programs to see how they compare. I have used RealTerm on a PC but I don't think they have a Mac version. That one isn't the easiest to figure out either.

--- In response to your latest ---
If you can talk to it via serial now, check and make sure it is configured in normal mode, not some emulation.

To avoid as many plumbing issues as possible, connect the sensor to only the pump first with the most direct connection you can make. You first want to check only your pump and sensor, so exclude everything else for now.
Rex Allers

Bruce Meagher
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Re: MKS 901p

Post by Bruce Meagher » Fri Jul 28, 2017 10:16 pm


Good to see you were able to get your gauge working over the serial link. I just purchased one from eBay, and thought I’d share my results for comparison.

At atmosphere (in San Diego today) the analog out on the 901P measures 8.87V. Over the serial link the Pirani is reads 900 Torr, the Piezo reads 14.5 Torr (relative to ATM), and the combined reading (PR3) is 739 Torr.

The gauge was set to the default address (253), default baud rate (9600), and default emulation (AO1 = 30 - combine pressure value, MKS standard). Below are the command/response values from my gauge for comparison (I used the broadcast address in case the previous user had changed it from the default value).




Seems like it'll be a nice little gauge. A small battery powered Arduino with display would make a nice combo.

Last edited by Bruce Meagher on Wed May 20, 2020 10:58 pm, edited 1 time in total.

Pablo Llaguno
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Re: MKS 901p

Post by Pablo Llaguno » Sun Jul 30, 2017 1:25 am

Haven't measured analog with a repeatable volt meter, I think the one I used isn't working properly (because it measured 9.6volts). Today the serial (PR4) measured 711.9 Torr, but I live at about 500m altitude, that could be why. I will get that TTL serial adaptor and learn some code so I can communicate the transducer with the Arduino directly an have it displayed in a LCD. Also, I had some problems with the backlight pot of the LCD, because it interfered with the pressure when I tried to use the Arduino to read the transducer. I checked every connection and it doesn't make sense, how can the pot interfere with the analog output of the transducer.

I will upload some pics later in Images du Jour of the Star Mode I was able to get, the pressures I was receiving with the transducer seem to match the plasma I had, according to the Demo Furor Pictorial under the FAQs.

Rex my gauge is working fine, the led turns red for about 2 seconds then green and stays that way. I also sent @253T?;FF and received @253ACKO;FF which means both sensors are working properly.

Thanks for the tips on connecting the sensor directly to the pump, also I will look into vacuum conductance.

I am using the app serial at the moment to do communication, but it will expire soon, what kind of drivers will Zterm need?

John Myers
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Re: MKS 901p

Post by John Myers » Sun Jul 30, 2017 5:05 am

How much current are you supplying to the LCD? strange things can happen when the current draw of the system is a little more then the power supply can handle. The LCD backlight can use up a good amount of current compared to the rest of the circuit.

Rex Allers
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Re: MKS 901p

Post by Rex Allers » Sun Jul 30, 2017 6:52 am


On serial terminal software. I mentioned earlier that you might try looking on SourceForge. While I was browsing there one of the programs I found was ScriptCommunicator. You might want to check it out. It looks amazingly powerful. The only tricky part might be learning to use it just as a simple terminal, it has so many features.

Here's a direct link:

It has versions for Windows, Mac OS X, and Linux. Click on the files tab to see directories for downloading each version.

The description there:
ScriptCommunicator is a scriptable cross-platform data terminal which supports serial port (RS232, USB to serial), UDP, TCP client/server, SPI, I2C and CAN.

All sent and received data can be shown in a console and can be logged in an html, a text and a custom log.

In addition to the simple sending and receiving of data ScriptCommunicator has a QtScript (similar to JavaScript) interface.

It looks like it should be easy to make a list of strings to send so you can click from a list of commands you build.

Looks like it has a very powerful scripting capability and I think you can even build a GUI interface to use it from.

I haven't tried it yet but I downloaded it to use later.

There's a big PDF User Guide in the source zipfile. I think it may be in the installed versions too but I haven't installed the prog yet so I checked and found the PDF was available in the Source zip.
Rex Allers

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