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weird diffusion pump behavior

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  • Guy Brandenburg
    Our old no-name, government-surplus diffusion pump has a problem. After pumping the diff pump for a while (minutes or hours, doesn t seem to make a difference)
    Message 1 of 16 , Dec 26, 2010
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      Our old no-name, government-surplus diffusion pump has a problem. 
      After pumping the diff pump for a while (minutes or hours, doesn't seem to make a difference) and after loading the mirror to be coated into the bell jar, this is what happens:
      when I close the connection between the mechanical pump and the diffusion pump, and begin pumping down the bell jar directly, the pressure in the diffusion pump all of a sudden rises dramatically, going up to ~200 millibars if I don't immediately switch back to pumping down the diffusion pump directly again. It becomes quite a dance, closing one valve then opening the other, then reversing again, and again, and again.
      Eventually it always settles down and I get decent vacuum levels, but it's annoying for a while.
       
      Guy Brandenburg, Washington, DC 

      My blog, mostly on Education in DC:
      http://gfbrandenburg.wordpress.com/


      My home page on astronomy, mathematics, education:
      http://home.earthlink.net/~gfbranden/GFB_Home_Page.html
      or else   
      http://tinyurl.com/r6fh2
      =====================================



    • James Blackett
      Hi, this seems very weird, I dont understand!! 200 mb is very high. Can you pump the bell jar down without bypassing the diff pump? what happens then? Because
      Message 2 of 16 , Dec 26, 2010
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        Hi,
         
        this seems very weird, I dont understand!! 200 mb is very high. Can you pump the bell jar down without bypassing the diff pump? what happens then? Because you can achieve a good final vacuum (how much?), there cant be too much wrong with your system, I wonder if there is someting wrong with the gauge(s), I've seen some pirani gauges do stupid things when they have been contaminated,
         
        sorry this is not much use, but just my ramblings,
         
         
        James
        ----- Original Message -----
        Sent: Sunday, December 26, 2010 8:48 AM
        Subject: [VacuumX] weird diffusion pump behavior

         

        Our old no-name, government-surplus diffusion pump has a problem. 
        After pumping the diff pump for a while (minutes or hours, doesn't seem to make a difference) and after loading the mirror to be coated into the bell jar, this is what happens:
        when I close the connection between the mechanical pump and the diffusion pump, and begin pumping down the bell jar directly, the pressure in the diffusion pump all of a sudden rises dramatically, going up to ~200 millibars if I don't immediately switch back to pumping down the diffusion pump directly again. It becomes quite a dance, closing one valve then opening the other, then reversing again, and again, and again.
        Eventually it always settles down and I get decent vacuum levels, but it's annoying for a while.
         
        Guy Brandenburg, Washington, DC 

        My blog, mostly on Education in DC:
        http://gfbrandenburg.wordpress.com/


        My home page on astronomy, mathematics, education:
        http://home.earthlink.net/~gfbranden/GFB_Home_Page.html
        or else   
        http://tinyurl.com/r6fh2
        =====================================



      • Guy Brandenburg
        BTW I haven t been able to photograph the haze that sometimes occurs because it hasn t occurred. Guy ________________________________ From: Guy Brandenburg
        Message 3 of 16 , Dec 26, 2010
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          BTW I haven't been able to photograph the haze that sometimes occurs because it hasn't occurred. 

           
          Guy 


          From: Guy Brandenburg <gfbrandenburg@...>
          To: VacuumX@yahoogroups.com
          Sent: Sun, December 26, 2010 3:48:42 AM
          Subject: [VacuumX] weird diffusion pump behavior

           

          Our old no-name, government-surplus diffusion pump has a problem. 
          After pumping the diff pump for a while (minutes or hours, doesn't seem to make a difference) and after loading the mirror to be coated into the bell jar, this is what happens:
          when I close the connection between the mechanical pump and the diffusion pump, and begin pumping down the bell jar directly, the pressure in the diffusion pump all of a sudden rises dramatically, going up to ~200 millibars if I don't immediately switch back to pumping down the diffusion pump directly again. It becomes quite a dance, closing one valve then opening the other, then reversing again, and again, and again.
          Eventually it always settles down and I get decent vacuum levels, but it's annoying for a while.
           
          Guy Brandenburg, Washington, DC 

          My blog, mostly on Education in DC:
          http://gfbrandenburg.wordpress.com/


          My home page on astronomy, mathematics, education:
          http://home.earthlink.net/~gfbranden/GFB_Home_Page.html
          or else   
          http://tinyurl.com/r6fh2
          =====================================




        • Daryl P. Dacko
          Just a guess, but that sort of behavior has been linked to a weak heater on the diff pump. The pump oscillates between backstreaming and proper behavior. It s
          Message 4 of 16 , Dec 26, 2010
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            Just a guess, but that sort of behavior has been linked to a weak
            heater on the diff pump.
            The pump oscillates between backstreaming and proper behavior.

            It's a long shot, but it's been known to happen...

            Daryl
          • Guy Brandenburg
            A weak heater, and not a hyperactive, too-hot one? Hmm. Sent from my iPhone
            Message 5 of 16 , Dec 26, 2010
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              A weak heater, and not a hyperactive, too-hot one? Hmm.

              Sent from my iPhone

              On Dec 26, 2010, at 9:12 AM, "Daryl P. Dacko" <mycrump@...> wrote:

               

              Just a guess, but that sort of behavior has been linked to a weak
              heater on the diff pump.
              The pump oscillates between backstreaming and proper behavior.

              It's a long shot, but it's been known to happen...

              Daryl

            • Guy Brandenburg
              I suppose I could, theoretically, pump both the bell jar and the diffusion pump at the same time, but I think it would be a bad idea since the dp would rise to
              Message 6 of 16 , Dec 26, 2010
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                I suppose I could, theoretically, pump both the bell jar and the diffusion pump at the same time, but I think it would be a bad idea since the dp would rise to 1atm for a while.
                We get down to about 
                ~9 to 7*10^-5 torr. Can't seem to go any lower. The coatings look good and seem to last a reasonable length of time (years).

                Sent from my iPhone

                On Dec 26, 2010, at 8:16 AM, "James Blackett" <jamesrblackett@...> wrote:

                 

                Hi,
                 
                this seems very weird, I dont understand!! 200 mb is very high. Can you pump the bell jar down without bypassing the diff pump? what happens then? Because you can achieve a good final vacuum (how much?), there cant be too much wrong with your system, I wonder if there is someting wrong with the gauge(s), I've seen some pirani gauges do stupid things when they have been contaminated,
                 
                sorry this is not much use, but just my ramblings,
                 
                 
                James
                ----- Original Message -----
                Sent: Sunday, December 26, 2010 8:48 AM
                Subject: [VacuumX] weird diffusion pump behavior

                 

                Our old no-name, government-surplus diffusion pump has a problem. 
                After pumping the diff pump for a while (minutes or hours, doesn't seem to make a difference) and after loading the mirror to be coated into the bell jar, this is what happens:
                when I close the connection between the mechanical pump and the diffusion pump, and begin pumping down the bell jar directly, the pressure in the diffusion pump all of a sudden rises dramatically, going up to ~200 millibars if I don't immediately switch back to pumping down the diffusion pump directly again. It becomes quite a dance, closing one valve then opening the other, then reversing again, and again, and again.
                Eventually it always settles down and I get decent vacuum levels, but it's annoying for a while.
                 
                Guy Brandenburg, Washington, DC 

                My blog, mostly on Education in DC:
                http://gfbrandenburg.wordpress.com/


                My home page on astronomy, mathematics, education:
                http://home.earthlink.net/~gfbranden/GFB_Home_Page.html
                or else   
                http://tinyurl.com/r6fh2
                =====================================



              • Vladimir Chutko
                Guy, What do you mean: when I close connection between the mechanical pump and the diffusion pump ? If you close backing valve and open gate valve, system
                Message 7 of 16 , Dec 26, 2010
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                  Guy,

                  What do you mean: "when I close connection between the mechanical pump and the diffusion pump"? If you close backing valve and open gate valve, system won't work.  Only cryo and other sorption pumps work with closed backing valve. Dif and turbo pumps must be backed always.

                  Do you have gauge on the dif pump input flange under the gate valve and on its output? What are the pressures?

                  Sometimes valves may leak in some position, for instance, there is no leak when it closed and a great leak when it opened. So everything looks fine when a gate valve is closed, and a big leak appears when you open it for high vacuum pumping.

                  Best regards,

                  Vladimir

                  On 12/26/2010 12:48 AM, Guy Brandenburg wrote:
                  Our old no-name, government-surplus diffusion pump has a problem. 
                  After pumping the diff pump for a while (minutes or hours, doesn't seem to make a difference) and after loading the mirror to be coated into the bell jar, this is what happens:
                  when I close the connection between the mechanical pump and the diffusion pump, and begin pumping down the bell jar directly, the pressure in the diffusion pump all of a sudden rises dramatically, going up to ~200 millibars if I don't immediately switch back to pumping down the diffusion pump directly again. It becomes quite a dance, closing one valve then opening the other, then reversing again, and again, and again.
                  Eventually it always settles down and I get decent vacuum levels, but it's annoying for a while.
                   
                  Guy Brandenburg, Washington, DC 

                  My blog, mostly on Education in DC:
                  http://gfbrandenburg.wordpress.com/


                  My home page on astronomy, mathematics, education:
                  http://home.earthlink.net/~gfbranden/GFB_Home_Page.html
                  or else   
                  http://tinyurl.com/r6fh2
                  =====================================



                • Guy Brandenburg
                  In our setup, the mechanical pump has tubes and valves that connect either to the bell jar or to the rear end of the diffusion pump. I ll have to see exactly
                  Message 8 of 16 , Dec 26, 2010
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                    In our setup, the mechanical pump has tubes and valves that connect either to the bell jar or to the rear end of the diffusion pump.
                    I'll have to see exactly where the gauge for the dp is located.

                    Sent from my iPhone

                    On Dec 26, 2010, at 3:30 PM, Vladimir Chutko <chutko@...> wrote:

                     

                    Guy,

                    What do you mean: "when I close connection between the mechanical pump and the diffusion pump"? If you close backing valve and open gate valve, system won't work.  Only cryo and other sorption pumps work with closed backing valve. Dif and turbo pumps must be backed always.

                    Do you have gauge on the dif pump input flange under the gate valve and on its output? What are the pressures?

                    Sometimes valves may leak in some position, for instance, there is no leak when it closed and a great leak when it opened. So everything looks fine when a gate valve is closed, and a big leak appears when you open it for high vacuum pumping.

                    Best regards,

                    Vladimir

                    On 12/26/2010 12:48 AM, Guy Brandenburg wrote:

                    Our old no-name, government-surplus diffusion pump has a problem. 
                    After pumping the diff pump for a while (minutes or hours, doesn't seem to make a difference) and after loading the mirror to be coated into the bell jar, this is what happens:
                    when I close the connection between the mechanical pump and the diffusion pump, and begin pumping down the bell jar directly, the pressure in the diffusion pump all of a sudden rises dramatically, going up to ~200 millibars if I don't immediately switch back to pumping down the diffusion pump directly again. It becomes quite a dance, closing one valve then opening the other, then reversing again, and again, and again.
                    Eventually it always settles down and I get decent vacuum levels, but it's annoying for a while.
                     
                    Guy Brandenburg, Washington, DC 

                    My blog, mostly on Education in DC:
                    http://gfbrandenburg.wordpress.com/


                    My home page on astronomy, mathematics, education:
                    http://home.earthlink.net/~gfbranden/GFB_Home_Page.html
                    or else   
                    http://tinyurl.com/r6fh2
                    =====================================



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