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Homebrew Cold cathode vacuum gauge circuit

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  • chrisoe1
    I am building a small vacuum system with a diffusion pump to obtain about 10E-5 Torr. To monitor the vacuum I have built a Pirani gage which works allright on
    Message 1 of 10 , Dec 6, 2005
      I am building a small vacuum system with a diffusion pump to obtain
      about 10E-5 Torr. To monitor the vacuum I have built a Pirani gage
      which works allright on the rough vacuum but below 1 millitorr [1E-
      3Torr] it becomes unreliable. For the actual vacuum chamber I have
      built a Cold cathode gauge operating at 2500 Volt, but cannot detect
      any current with my 'electrometer', which is a microampere meter with
      1.5 uA [1.5E-6 A]full scale. I have searched the internet for a
      reference to what current a CC vacuum gage should measure but found
      very little and somewhat conflicting info. One reference according to
      my interpretation should measure 10mA at 1E-3 Torr which seems very
      high and would indicate gas discharge rather than electron
      generation. Another literature indicates a gage current below 10nA
      [1E-9 A]. The questions I have now are:
      What is the pressure/current function for a CC vacuum gage? I
      understand it is a logarithmic function but a one point value
      midscale as a design starting point for circuit development would be
      helpful too.
      Does anyone have a schematic for a homebrew CC vacuum gage.

      Any help is greatly appreciated.

      Chrisoe1
    • jrrymiller
      I think what you are proposing is a form of Penning gauge. My reference, Vacuum Technology and Space Simulation by Donald T. Santeler et. al., describes such
      Message 2 of 10 , Dec 6, 2005
        I think what you are proposing is a form of Penning gauge. My
        reference, Vacuum Technology and Space Simulation by Donald T.
        Santeler et. al., describes such a device. There are two issues that
        I think will lead you to success. First the discharge path should be
        through a strong magnetic field. To quote the book:
        "...After the discharge starts, electrons are supplied from the
        ionization of the gas molecules and by secondary emission as ions
        impact on the cathodes. These electrons are constrained to take
        helical paths along magnetic field lines, and cannot readily reach the
        anode without undergoing collisions with gas molecules or other
        particles, further ionizing the gas and releasing additional
        electrons...."
        Second some source of ionization is required to initiate the current.
        I put a small piece of thoriated tungsten in mine.(welding supply
        houses have this). Of course natural radiation may be enough to start
        it.
        I found on the web a discription of the type of gauge that you propose.

        "Modified, PDX-type Penning gauges [3,4] were installed on the upper
        and lower
        divertors of NSTX. These gauges have a cylindrical anode measuring
        1.27cm in diameter
        and 1.26cm long. 2.6cm2 steel plates separated from the cylinder ends
        by 2.6mm, act as
        the cathode. Voltages in the range of 2 to 5kV were used for
        experimentation. A voltage
        of 3.12kV was selected in the final configuration. This increases the
        maximum operating
        range for the pressure while ensuring that the Penning discharge
        initiates at the lower
        pressures. Since installing them more than two years ago, these gauges
        have required no
        maintenance. The calibration data, as shown in Figure 1, at a toroidal
        field of 0.45T at the
        machine axis shows the calibration to have remained essentially
        unchanged over the past
        two years [5] even though NSTX has gone through several machine bake-
        out cycles at
        3000 Celsius and numerous boronization and He glow discharge cycles.
        These gauges are
        not isolated from the machine during these machine conditioning
        events. These factors as
        well as their capability to operate at high values of the toroidal
        field, and retain their
        calibration over long periods may make them better suited for harsh
        environments such
        as that which may exist in ITER. On NSTX, these gauges have a maximum
        operating
        pressure of up to 1mTorr although by reducing the bias voltage this
        could be extended up
        to about 4mTorr."

        http://www.pppl.gov/pub_report/2004/PPPL-3944rev.pdf

        My own experience was to find first a penning gauge controller on
        ebay, then persist in finding a penning gauge head. They were not a
        matched pair but they did work although the calibration was off.

        Jerry Miller
      • Guy Brandenburg
        The book on laboratory procedures by C.L.Strong (I think) that first detailed many of the modern techniques for high-vacuum aluminization says that as soon
        Message 3 of 10 , Dec 7, 2005
          The book on laboratory procedures by C.L.Strong (I think) that first detailed many of the modern techniques for high-vacuum aluminization says that as soon as the pressure is under 10*-4 Torr, and you've done the ion bombardment, then it's fine to aluminize. We try (but don't always succeed) to get to 10*-6 Torr. What pressure do others typically throw the switch at?

          Guy


          Guy Brandenburg
          Washington, DC
          My home page:
          http://home.earthlink.net/~gfbranden/GFB_Home_Page.html
        • James Lerch
          ... From: Guy Brandenburg We try (but don t always succeed) to get to 10*-6 Torr. What pressure do others typically throw the switch at? Guy 2.2x10 -5 torr
          Message 4 of 10 , Dec 7, 2005
            ----- Original Message -----
             
            We try (but don't always succeed) to get to 10*-6 Torr. What pressure do others typically throw the switch at?

            Guy
            2.2x10 -5 torr here.  My old smaller chamber, if left long enough could get to 9x10 -6 torr, but it took about an hour to get there from the 2.2x10-5 mark.
             
            Of course, this is measured with a Penning Cold Cathode gauge and controller off E-Bay, so who knows if it is calibrated or not. 
             
          • jrrymiller
            ... first detailed many of the modern techniques for high-vacuum aluminization says that as soon as the pressure is under 10*-4 Torr, and you ve done the
            Message 5 of 10 , Dec 7, 2005
              --- In VacuumX@yahoogroups.com, Guy Brandenburg <gfbrandenburg@y...>
              wrote:
              >
              > The book on laboratory procedures by C.L.Strong (I think) that
              first detailed many of the modern techniques for high-vacuum
              aluminization says that as soon as the pressure is under 10*-4
              Torr, and you've done the ion bombardment, then it's fine to
              aluminize. We try (but don't always succeed) to get to 10*-6 Torr.
              What pressure do others typically throw the switch at?
              >
              > Guy

              My reference, Vacuum Technology and Space Simulation by Donald T.
              Santeler et. al.,develops the concept of "mean free path". The table
              of values goes:

              PRESSURE MEAN FREE PATH
              760 torr 6.69E-6 cm
              1 torr 5.09e-3 cm
              10E-3 torr 5.09E0 cm
              10e-6 torr 5.09E3 cm

              This is roughly speaking of course since the value is gas
              dependent. At 10^-3 helium's mean free path is 14+ cm
              and nitrogen is 5+ cm. This is predicated on the actual size of the
              molecules.
              Once I have pumped the chamber below the scale of the thermocouple
              gauge I figure that I am below 10^-3 torr and another 10 minutes of
              diffusion pump and I am ready to melt aluminum.
              All this makes sense to me but of course you must realize that I am
              an electrical engineer.

              Jerry Miller

              > Guy Brandenburg
              > Washington, DC
              > My home page:
              > http://home.earthlink.net/~gfbranden/GFB_Home_Page.html
              >
            • Matt
              Can anyone give me quotes on aluminizing a 16 F/8 mirror for my telescope? I live near Dallas, Texas.... Thanks, Matt ... ... think)
              Message 6 of 10 , Dec 7, 2005
                Can anyone give me quotes on aluminizing a 16" F/8
                mirror for my telescope? I live near Dallas,
                Texas....

                Thanks,

                Matt

                --- jrrymiller <gandamiller@...> wrote:


                ---------------------------------
                --- In VacuumX@yahoogroups.com, Guy Brandenburg
                <gfbrandenburg@y...>
                wrote:
                >
                > The book on laboratory procedures by C.L.Strong (I
                think) that
                first detailed many of the modern techniques for
                high-vacuum
                aluminization says that as soon as the pressure is
                under 10*-4
                Torr, and you've done the ion bombardment, then it's
                fine to
                aluminize. We try (but don't always succeed) to get
                to 10*-6 Torr.
                What pressure do others typically throw the switch
                at?
                >
                > Guy

                My reference, Vacuum Technology and Space Simulation
                by Donald T.
                Santeler et. al.,develops the concept of "mean free
                path". The table
                of values goes:

                PRESSURE MEAN FREE PATH
                760 torr 6.69E-6 cm
                1 torr 5.09e-3 cm
                10E-3 torr 5.09E0 cm
                10e-6 torr 5.09E3 cm

                This is roughly speaking of course since the value is
                gas
                dependent. At 10^-3 helium's mean free path is 14+ cm
                and nitrogen is 5+ cm. This is predicated on the
                actual size of the
                molecules.
                Once I have pumped the chamber below the scale of the
                thermocouple
                gauge I figure that I am below 10^-3 torr and another
                10 minutes of
                diffusion pump and I am ready to melt aluminum.
                All this makes sense to me but of course you must
                realize that I am
                an electrical engineer.

                Jerry Miller

                > Guy Brandenburg
                > Washington, DC
                > My home page:
                >
                http://home.earthlink.net/~gfbranden/GFB_Home_Page.html
                >






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              • Matt
                Can anyone give me quotes on aluminizing a 16 F/8 mirror for my telescope? I live near Dallas, Texas.... Thanks, Matt
                Message 7 of 10 , Dec 7, 2005
                  Can anyone give me quotes on aluminizing a 16" F/8
                  mirror for my telescope? I live near Dallas,
                  Texas....

                  Thanks,

                  Matt
                • Guy Brandenburg
                  Thanks for your feedback. (I understand the mean free path concept. I teach HS/JHS math and have worked with astrophysicists at NRL a couple of summers. The
                  Message 8 of 10 , Dec 7, 2005
                    Thanks for your feedback.

                    (I understand the mean free path concept. I teach HS/JHS math and have worked with astrophysicists at NRL a couple of summers. The author of my reference, John Strong (my bad, sorry) works it out for nitrogen, and he worked it out that the MFP at
                    760 mm of Hg (i.e., Torr) is 8.5 E-6 cm; at
                    1 Torr is 6.5E-3 cm; at
                    1E-3 Torr is 6.5 cm; at
                    1E-4 Torr is 65 cm; at
                    1E-5 Torr is 6.5 meters; and at
                    1E-6 Torr is 65 meters.
                    This is on page 95. I tried working out the numbers myself in an oxygen-nitrogen mix, and got numbers that were close to his, but not identical. Same ballpark, though.I never really believe our gauges, however, not knowing how to really calibrate one with another. And I guess my question is, since 65 cm is just about 2 feet, and it's a bit more than 2 feet from our coil to the mirror, am I just being paranoid in trying to get down to 1E-5 Torr?)

                    Guy

                    jrrymiller <gandamiller@...> wrote:
                    --- In VacuumX@yahoogroups.com, Guy Brandenburg <gfbrandenburg@y...>
                    wrote:
                    >
                    > The book on laboratory procedures by C.L.Strong (I think) that
                    first  detailed many of the modern techniques for high-vacuum
                    aluminization  says that as soon as the pressure is under 10*-4
                    Torr, and you've done  the ion bombardment, then it's fine to
                    aluminize. We try (but don't  always succeed) to get to 10*-6 Torr.
                    What pressure do others typically  throw the switch at?
                    >  
                    >   Guy

                    My reference, Vacuum Technology and Space Simulation by Donald T.
                    Santeler et. al.,develops the concept of "mean free path". The table
                    of values goes:

                    PRESSURE    MEAN FREE PATH
                    760   torr  6.69E-6  cm
                    1     torr  5.09e-3  cm
                    10E-3 torr  5.09E0   cm
                    10e-6 torr  5.09E3   cm

                    This is roughly speaking of course since the value is gas
                    dependent.  At 10^-3 helium's mean free path is 14+ cm
                    and nitrogen is 5+ cm.  This is predicated on the actual size of the
                    molecules. 
                    Once I have pumped the chamber below the scale of the thermocouple
                    gauge I figure that I am below 10^-3 torr and another 10 minutes of
                    diffusion pump and I am ready to melt aluminum.
                    All this makes sense to me but of course you must realize that I am
                    an electrical engineer.

                    Jerry Miller



                    Guy Brandenburg
                    Washington, DC
                    My home page:
                    http://home.earthlink.net/~gfbranden/GFB_Home_Page.html
                  • Guy Brandenburg
                    Thanks for the feedback. We have some cold cathode gauge that a physics prof, now retired, from UMd/CP gave us and installed. I don t trust its numbers any
                    Message 9 of 10 , Dec 7, 2005
                      Thanks for the feedback. We have some cold cathode gauge that a physics prof, now retired, from UMd/CP gave us and installed. I don't trust its numbers any more than you trust yours. We have similar problems getting down to 9 E-6 torr; it takes forever, and usually depends on the diffusuion pump having been on all night from the day before. Otherwise, foreget it. I am now beginning to think that 2, 3, 4, or even 5 E-5 is just fine. It will save us a lot fo time, too!
                      Guy

                      James Lerch <jlerch1@...> wrote:
                      ----- Original Message -----
                       
                      We try (but don't always succeed) to get to 10*-6 Torr. What pressure do others typically throw the switch at?

                      Guy
                      2.2x10 -5 torr here.  My old smaller chamber, if left long enough could get to 9x10 -6 torr, but it took about an hour to get there from the 2.2x10-5 mark.
                       
                      Of course, this is measured with a Penning Cold Cathode gauge and controller off E-Bay, so who knows if it is calibrated or not. 
                       



                      Guy Brandenburg
                      Washington, DC
                      My home page:
                      http://home.earthlink.net/~gfbranden/GFB_Home_Page.html
                    • chrisoe1
                      Jerry, Thanks for your prompt reply. I hope my question is not too far OT, if so please let me know: It is indeed called a Penning gauge. The dimensions and
                      Message 10 of 10 , Dec 7, 2005
                        Jerry,
                        Thanks for your prompt reply. I hope my question is not too far OT,
                        if so please let me know:
                        It is indeed called a Penning gauge. The dimensions and construction
                        design was taken from various operating manuals for commercial
                        gauges. Mine has a separate cathode where the electrometer operates
                        at ground potential and not in the HV circuit directly, this way the
                        leaking current in the HV supply and the whole Anode circuitry does
                        not interfere with the pressure related current measurement. It is a
                        one inch diameter Al tube 3 inch long with a 2 inch SST pin of 1/8
                        inch on its axis. Electrical insulation is with acrylic and the whole
                        thing is glued together with epoxy. Its not the best vacuum material
                        choice but for prototyping I thought it is ok.
                        Your suggestion to use eBay as a source is good but for educational
                        (and $) reasons I prefer to construct my own instrumentation or
                        modify discarded equipment. I have experimented with electrometers
                        from FID detectors and pH meters but without much success. At over
                        2000 Volt it is easy to blow the MOSFET input of such devices. Then I
                        built a VTVM with 100 Megohm input resistance (A clone of a 1960's
                        HewlettPackard Model HP410) The input has been modified to permit
                        current readings to 5nA [5E-9A] full scale at a 0.5 Volt load, but
                        still no current on the CC gauge. Experimentation with magnets from
                        discarded computer HDs occasionally caused gas discharge with glow
                        and high currents (Geissler Tube) that have little relation to the
                        pressure but still no Penning current.
                        I believe the problem is either with my current measurement equipment
                        and or with the shape and strength of the magnetic field. I have not
                        yet built a yoke that would confine the magnetic field appropriately
                        perpendicular to the electric field.
                        If the current is in the pA range [1E-12A] then my equipment is not
                        capable at this time and I need to improve the electronics. The only
                        schematic for a Penning gauge I have is from the Internet for a Fermi
                        National Accelerator experiment in 1981. It uses a AD759P log current-
                        to-voltage circuit but I have not found a data sheet for this
                        obsolete IC yet. The operating instruction tests the circit with 10
                        nA in to produce 9V out while the pressure range 3E-4 to 1E-7 Torr
                        causes 1.2 to 7.2 Volt out. At about 2V/decade I estimate 9Vout to
                        represent about 1E-2Torr. Assuming a linear relationship between
                        cathode current and pressure, the gage current for the pressure range
                        would be around 10 pA to 1 nA. - Of course this is going way out on a
                        limb with reverse engineering of existing design info and in stark
                        contradiction with a statement of 0.5 A/Torr I vaguely remember from
                        elsewhere.
                        Any info about actual electrode current during normal operation or
                        pertinent design info or literature reference is appreciated!
                        Chris Oertli


                        --- In VacuumX@yahoogroups.com, "jrrymiller" <gandamiller@v...> wrote:
                        >
                        > I think what you are proposing is a form of Penning gauge. My
                        > reference, Vacuum Technology and Space Simulation by Donald T.
                        > Santeler et. al., describes such a device. There are two issues
                        that
                        > I think will lead you to success. First the discharge path should
                        be
                        > through a strong magnetic field. To quote the book:
                        > "...After the discharge starts, electrons are supplied from the
                        > ionization of the gas molecules and by secondary emission as ions
                        > impact on the cathodes. These electrons are constrained to take
                        > helical paths along magnetic field lines, and cannot readily reach
                        the
                        > anode without undergoing collisions with gas molecules or other
                        > particles, further ionizing the gas and releasing additional
                        > electrons...."
                        > Second some source of ionization is required to initiate the
                        current.
                        > I put a small piece of thoriated tungsten in mine.(welding supply
                        > houses have this). Of course natural radiation may be enough to
                        start
                        > it.
                        > I found on the web a discription of the type of gauge that you
                        propose.
                        >
                        > "Modified, PDX-type Penning gauges [3,4] were installed on the
                        upper
                        > and lower
                        > divertors of NSTX. These gauges have a cylindrical anode measuring
                        > 1.27cm in diameter
                        > and 1.26cm long. 2.6cm2 steel plates separated from the cylinder
                        ends
                        > by 2.6mm, act as
                        > the cathode. Voltages in the range of 2 to 5kV were used for
                        > experimentation. A voltage
                        > of 3.12kV was selected in the final configuration. This increases
                        the
                        > maximum operating
                        > range for the pressure while ensuring that the Penning discharge
                        > initiates at the lower
                        > pressures. Since installing them more than two years ago, these
                        gauges
                        > have required no
                        > maintenance. The calibration data, as shown in Figure 1, at a
                        toroidal
                        > field of 0.45T at the
                        > machine axis shows the calibration to have remained essentially
                        > unchanged over the past
                        > two years [5] even though NSTX has gone through several machine
                        bake-
                        > out cycles at
                        > 3000 Celsius and numerous boronization and He glow discharge
                        cycles.
                        > These gauges are
                        > not isolated from the machine during these machine conditioning
                        > events. These factors as
                        > well as their capability to operate at high values of the toroidal
                        > field, and retain their
                        > calibration over long periods may make them better suited for harsh
                        > environments such
                        > as that which may exist in ITER. On NSTX, these gauges have a
                        maximum
                        > operating
                        > pressure of up to 1mTorr although by reducing the bias voltage this
                        > could be extended up
                        > to about 4mTorr."
                        >
                        > http://www.pppl.gov/pub_report/2004/PPPL-3944rev.pdf
                        >
                        > My own experience was to find first a penning gauge controller on
                        > ebay, then persist in finding a penning gauge head. They were not a
                        > matched pair but they did work although the calibration was off.
                        >
                        > Jerry Miller
                        >
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