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  • OT: Displaying the output of a vacuum gauge

    I have a Pfeiffer cold cathode vacuum gauge just off the turbo pump on my RGA system. Simple to use, 24v in and it gives a signal out depending on pressure. I want to build a display to connect to it. Heres the thing, the output is linear, in a way. Anything over 8.5v is over pressure. 8.5means 1 Pa, 7.5 is 1e-1, 6.5 is 1e-2. Simple, every volt is a decade down to 1.8v where it peters out at 2x10^-7 Pa. If the output is below .5v that means there is something wrong with the gauge.

    Now the problem is that between each decade the voltage is logarithmic to the pressure. So at 6.5v the pressure is 1e-2, 6.8 is 2e-2, 6.97v is 3e-2, 7.2v is 4e-2, and so on.

    Any ideas how to deal with this? I am figuring using the analog input on something like an Arduino or Teensy++. On the arduino it will give me 10 bit resolution between 0-5v. Use a voltage divider to scale it and output the info on a LCD.

    But how do I process the data on something like this. I suppose I could do a lookup table where it subtracts the number that determines the decade and then does and inverse log of the resulting number to determine the pressure in that range. So if it saw a voltage of 5.92v it would set the range to e-3, subtract 5.5 to get .42v, calculate 10^.42 to get 2.63 and then display 2.63e-3Pa on the LCD. Also do an option to show mbar or torr.

    I really have no clue how to program something like this. Or even if I am doing it close to the right way. I do not want to do this on a PC!

    Any ideas?

    -Jerry

    Last edited by macona; 04-20-2010, 06:18 AM.

  • #2
    readout

    Macona,

    Have you checked to see if Pfeiffer offer a readout/controller. Installed similar units made by varian at a cyclotron years ago.

    Mark


    I have a Pfeiffer cold cathode vacuum gauge just off the turbo pump on my RGA system. Simple to use, 24v in and it gives a signal out depending on pressure. I want to build a display to connect to it. Heres the thing, the output is linear, in a way. Anything over 8.5v is over pressure. 8.5means 1 Pa, 7.5 is 1e-1, 6.5 is 1e-2. Simple, every volt is a decade down to 1.8v where it peters out at 2x10^-7 Pa. If the output is below .5v that means there is something wrong with the gauge.

    Now the problem is that between each decade the voltage is logarithmic to the pressure. So at 6.5v the pressure is 1e-2, 6.8 is 2e-2, 6.97v is 3e-2, 7.2v is 4e-2, and so on.

    Any ideas how to deal with this? I am figuring using the analog input on something like an Arduino or Teensy++. On the arduino it will give me 10 bit resolution between 0-5v. Use a voltage divider to scale it and output the info on a LCD.

    But how do I process the data on something like this. I suppose I could do a lookup table where it subtracts the number that determines the decade and then does and inverse log of the resulting number to determine the pressure in that range. So if it saw a voltage of 5.92v it would set the range to e-3, subtract 5.5 to get .42v, calculate 10^.42 to get 2.63 and then display 2.63e-3Pa on the LCD. Also do an option to show mbar or torr.

    I really have no clue how to program something like this. Or even if I am doing it close to the right way. I do not want to do this on a PC!

    Any ideas?

    -Jerry

    Comment


    • #3
      Am I missing something here? Isn't the voltage always logarithmic to the pressure? There's nothing special that happens at every decade.

      This might sound really stupid, but there are analog gauges/dials that already have a log scale printed on them. They're typically marked for pressure or volume.

      Comment


      • #4
        Following on

        I take bb's points here.

        If a standard pressure guage is accurate enough over the required range, the scale can be re-plotted/calibrated from linear to log.

        Perhaps the mechanical arm can be replaced with a rotary potentiometer/resistor that is wound for a log output and read off directly by a meter (volts? amps?). I guess the output could be used to illuminate an LCD display.

        Comment


        • #5
          Resolution???

          Jerry, the big question here is what resolution are you looking for?

          A Nat Semi LM3915N-1 will drive a 10 segment LED bar/dot display very simply.
          See http://docs-europe.origin.electrocom...6b806e8b6c.pdf for details

          If you need a higher resolution than this, then you need to look at a different solution.

          Stuart

          P.S. Data sheet also shows that you can cascade 2 units with a bit of work, giving you a 60dB range over 20 segments.
          Last edited by Stuart Br; 04-20-2010, 08:48 AM.

          Comment


          • #6
            Pfeiffer does sell a readout for it. Used they are at least $400.

            A meter won't do it. You can have a log scale on it but the meter is still linear in response.

            The output is logrithmic only between decades. I did find a math library for arduino so I might give that a try.

            This is a fully electronic cold cathode ion gauge. No moving parts.

            Comment


            • #7
              Jerry,

              I'm with beanbag here, the chart shows a "straight" logarithmic relationship between pressure and voltage across the whole range, I don't see anything strange going on between decades.

              8.5V = 1Pa Log10(1) = 0
              7.5V = 0.1Pa Log10(0.1) = -1
              6.5V = 0.01Pa Log10(0.01) = -2
              5.5V = 0.001Pa Log10(0.001) = -3
              4.5V = 0.0001Pa Log10(0.0001) = -4
              5.5V = 0.00001Pa Log10(0.00001) = -5
              6.5V = 0.000001Pa Log10(0.000001) = -6
              Last edited by Stuart Br; 04-20-2010, 09:42 AM.

              Comment


              • #8
                Yes, that shows the rough vacuum. To get the fine reading I have to take that to figure out the rest. So 4.8v would be 2x10^-4Pa (.0002Pa) 5.1v would be .0004Pa.

                Comment


                • #9
                  Its actually a lot easier than that.

                  if you take your reference voltage as 8.5 V giving 1 Pa
                  Pressure for any voltage is
                  pa= 10^(InputV -8.5)

                  So for 5.1V

                  5.1-8.5 =-3.4
                  10^-3.4= .0004 Pa

                  back to my original question, what sort of resolution are you looking for and what is the tolerance on the sensor?
                  Last edited by Stuart Br; 04-20-2010, 11:10 AM.

                  Comment


                  • #10
                    Hi Jerry

                    I would use a table if you intend to use a microcontroller...they give you speed at the expense of rom. You can achieve your results via calculation and math librarys...but they normally eat cpu cycles and are not that memory friendly either...Cordic routines may save on memory if thats an issue.

                    If your looking at that atmel based teensy..its 10 bit ad will give you 1024 elements (minus the error zones) in the table..so it shouldnt be that big if you keep to ints. depending on how you want to display the data.

                    Once you have the raw data in each table element its quite easy to convert from binary to decimal...then convert to ascii for the LCD..the web is full of C code for this...8052.com has plenty that I and other have posted over the years.

                    ST micro are currently selling the discovery package (http://www.st.com/mcu/contentid-130-...DISCOVERY.html) that includes a ARM based usb front end programming chip and a ST8 rear end that can be snapped in half..You also get a free IDE and 16k limit cosmic C compiler...all for $7 ..so you would have everything you'd need to get started.

                    The silabs 8051 kits are a great alternative aswell..

                    Rob
                    Last edited by MrSleepy; 04-20-2010, 01:19 PM.

                    Comment


                    • #11
                      Use an antilog amplifier.

                      Datasheet here:

                      http://www.datasheetarchive.com/data...A00501563.html
                      Free software for calculating bolt circles and similar: Click Here

                      Comment


                      • #12
                        Originally posted by macona
                        ...Now the problem is that between each decade the voltage is logarithmic to the pressure. So at 6.5v the pressure is 1e-2, 6.8 is 2e-2, 6.97v is 3e-2, 7.2v is 4e-2, and so on...

                        -Jerry

                        So, if you redrew the graph with linear spacing between the grid lines, the voltage traces would roughly resemble a staircase? Seems strange. Such an output would cause a lot of extra work for the circuitry it was feeding. Sounds like what might happen with a defective R2R ladder. Have you been able to actually look at the output with a scope whild sweeping the vacuum?
                        Weston Bye - Author, The Mechatronist column, Digital Machinist magazine
                        ~Practitioner of the Electromechanical Arts~

                        Comment


                        • #13
                          Stuart, wow, you hit that one. Didnt even see that. Geesh. the 10 bit of a microcontroller would be good. Accuracy of one of these gauges is not great (30% in the range) but its nice to have the resolution to see trends in pressure.

                          Rob, I am looking at using the teensy because I have one lying around that I never got around to using intended for another project. I only need it to update a few times a second. I think it ought to handle that. Nice thing about microcontrollers, if it does not work just reprogram it. The teensy will take up to 2.59v in so I will have to use a voltage divider. What would be the easiest way to make a 1024 entry lookup table?

                          Evan, that looks like a possibility as well. I would have to convert to a current output to get the range.

                          Weston, you have it exactly. My guess one of the reasons they do this is to make it more difficult to make a display. The only thing is my other ion gauge outputs in a similar fashion. As they put it in their manual, 1v to 9v out (1v per decade) and is proportional to the logarithm of the pressure. 1v at 1x10^-10 and 9v 1x10-2 torr. So its very similar, maybe a throwback from old time fully analog gauges.

                          The gauge is the white thing sticking up in this picture:

                          Comment


                          • #14
                            I have spent far too much time doing loss calcs on optical fibre for data transmission, so I get the whole logarithmic scale stuff.
                            You have opened my eyes onto these low cost micro-controllers, I never knew you could pick them up at such a low cost. Not much good to me though, I never got my head round "C". I can read some of the simple constructs, but I have never tried writing any code with it.

                            Comment


                            • #15
                              Originally posted by Stuart Br
                              I can read some of the simple constructs, but I have never tried writing any code with it.
                              You can do alot in the native assembly, the PicMicro site host alot of applications with code, also free assembler and debugger etc.
                              If you need an intro see Nigel Goodwins site
                              http://www.winpicprog.co.uk/pic_tutorial.htm

                              (My daughter is 'marooned' in Oxfordshire right now due to the closing of Heathrow A.P) !!
                              M.
                              Last edited by MaxHeadRoom; 04-20-2010, 06:34 PM.

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