MKS 925 MicroPirani Interfacing

[SteveHansen]

There's been a lot of discussion here regarding the various active transducer vacuum gauges that are now on the market. In a nutshell, these gauges provide scaled analog and digital outputs and are pretty much self contained units.

I am most familiar with the MKS 925 "MicroPirani" as 1) I used to work for MKS and 2) I have been using this gauge (and a couple of others in the 900 series family) for quite a few years.

I believe that the 925 is somewhat unique in that it uses a proprietary MEMS sensor which permits honest to goodness readings down to 0.01 microns (10^-5 Torr) whereas most conventional Pirani gauges give up between 6 and 1 microns. For many applications you won't need an ion gauge / Pirani gauge pair.

While you can purchase a really nice controller for the 925 (it also handles all of the other transducers in the series), you can also download a reasonable piece of software that will perform set ups (zero, span, gas type), give a real time read out and output a text file for data collection with Excel or other spreadsheet program.

The cable I made for wiring my 925 is shown in the figure. I used a common 12 volt regulated DC wall wart but any voltage from 9 to 30 will work. There are many options for the 925 and they can come with 9 pin or 15 pin high density connectors. Mine have the 9 pin and the illustration reflects that. Be sure to check the transducer manual before wiring.

There are two outputs: 1-9 volts analog (1 volt per decade) and RS232 (RS485 is also available). The analog signal can go right to a DVM or to the differential input of a A/D. (Single ended inputs can cause errors.) For the analog output the scaling is P(Torr) = 10^(V-6).

Now, if you want a direct readout, you can download the free software from http://www.mksinst.com/vtsw . Select the MKS 900 Series software, current version is 1.6.2. The software isn't visually elegant but it works fine.

The transducer index page is at http://www.mksinst.com/product/category ... egoryid=35

[Jerry Biehler]

I use a pfeiffer CC ion gauge that has a similar analog output. I used a teensy++ to decode the volt reading and display the pressure on a 2x24 line VFD display. It also handle switching on/off the turbo and has set limits for the rga sampler valves.

[Brian_Gage]

Hi Steve,
After reading your post and comment about the software, I just had to test it. Simple software may not be visually attractive, but it is often best for people using adaptive technology. I downloaded, installed and ran the software using my JAWS screen reader and it worked great. The options displayed and I could radial through them even without a transducer connected. Now that I know the software works in my situation, I'll be looking to get the hardware for my upcoming vacuum system.

Carl suggested a 1 Torr and 10 Torr Pirrani gauge, but your post seems to suggest that would be unnecessary for most applications?
(I'll be starting with a demo fusor and the KISS principle-fusion isn't my immediate goal)

Regards, Brian

[SteveHansen]

Hi Brian,

Not sure about the 1 and 10 Torr Pirani comment - that sounds more like capacitance manometer full scale ranging. Pirani gauges are generally offered in two flavors - standard for a few milliTorr through about 100 Torr and convection enhanced for expanding the upper end to about 1000 Torr. The MKS, as I pointed out, has the advantage of an expanded low end - down to 0.01 milliTorr. The upper end (over 100 Torr) of this gauge is a bit wobbly as the MEMS sensor doesn't really work quite the same way as the wire sensors. It works but you have to check the span more frequently. However, we don't generally play in the rough vacuum regime anyway.

Interesting that the software works with your adaptive application.

Steve

[Brian_Gage]

Hi Steve,
Yes, I realized later that I confused vacuum gauges when writing. Main thing was that the MKS software was neat, simple and worked with my screen reader. Many programs are not accessible, or cludgy with screen readers because they are written fancy with screen graphics for sighted mousers.
Eventually, I will get vacuum terminology, acronyms and brands sorted out (a little) and not make similar dense comments. Appreciate the offer for a vacuum gauge, but isn't that a bit pricy for a beginner? How about durability in the hands of a newby? That's a lot of dollars if it breaks easily.
Brian

[SteveHansen]

Pretty durable if you don't incorrectly bias it or allow contaminants to get into the sensor area. I designed this product into teaching systems for tech college level students and one has yet to fail.

I think that pretty much goes for any of these gauges, no matter who the manufacturer.

Steve