L8-M "Tach" error

It went down to 350 microns in about 5 minutes. Then suddenly lost vacuum, but quickly recovered back to 350. I'm monitoring this closely now.  After 9 minutes it just dropped to 300.

Could the oil I mentioned earlier be a sign of a leak/broken tube?

Ok, it got down to 20 microns in 13 minutes.

Every time I try to get it spinning the Speed reading stays at zero and the TACH code appears.  I have cleared the code by pressing Enter then Start.  

I greatly appreciate any help you can give.  

CTL,The "window" you are looking at is for temperature readout and control.  It has nothing to do with the TACH warning that you are getting.  The TACH and OVERSPEED pickoffs are small black plastic devices found attached to the drive spindle.  What you need to do first is wet a paper towel with 95% ethanol and puch it up underneath the spindle and then rotate the soindle against the towel.  Underneath the spindle is where the segments that the pickoff looks at are found and if you are seeing a lot of oil underneath the black cover plate then there is a high probability that you will also have oil accumulation underneath the spindle.  Once you have cleaned it off try to perform a run.  Your vacuum will NEVER pull down properly until the rotor speed exceeds 3,000 RPM which is when the diffusion pump actually turns on to heat the diffusion pump oil.  Typically you should have a vacuum of less than 20 microns within 20 minutes.  I will also add that if you are seeing lots of oil under the chamber plate them that needs to be cleaned up and you also should be changing out both the vacuum pump and diffusion pump oils because they may be contributing to the oil accumulation.  The other possible source of that oil accumulation is the drive motor itself as it too can burp oil up into the vacuum chamber.  Let me know how things go after you have cleaned everything up.

Don

Thanks a bunch, Don.  Wow, was I wrong about the tach sensor!  I will have to go back and clean it as you described.  That said, the centrifuge was able to function properly after I made my posts here.  Somehow.  I should clean the tach sensor and change the oil ASAP just in case.  I have changed the oil in an LE-80K before, and this L8-M looks to use the same sort of pumps.  Can you remind me of how much diffusion pump oil I should add after draining the diffusion pump?  And is it correct that I should use a 704 oil?

Thank you so much for your knowledge and advice. 

CTL, You are welcome and you are correct as to the similarities between the models.  When it comes to cleaning up the oil sitting under the black plastic plate, the best thing to do is remove the radiometer in order to be able to mop underneath the refrigeration chamber.  You will probably find that oil just keeps coming out from underneath the refer can.  Mop up what you can, let it sit for awhile and then mop some more.  You can get underneath the can with paper towels folded together but be careful to not get any of the towel stuck underneath the chamber.  Remember to clean the old thermal paste off and then reapply a film of thermal paste underneath the mounting tab of the radiometer before reinstalling it.  The correct diffusion pump oil volume is 100ml.  I honestly do not know of the correct type of oil to use in the diffusion pump if it did not come from Beckman.  If 704 is a silicon based oil then that will probably work just fine.  Remember to check the large vacuum hose for cracks especially at the top of the diffusion pump because of the heat generated by the pump.

One other important thing to do that I always did when I first worked on any L8/M/LE ultracentrifuge was to check EVERY phillips head screw on the big capacitors to make sure that they are tight.  In fact, anywhere you see a phillips head screw on the slide out chassis try to tighten it.  Periodically check out the slide out chassis for hot spots as traces burning off were not an uncommon occurence.  If I can think of anything else I will let you know but it has been almost 10 years since I last saw one of these let alone work on them.  Fortunately, I still have some of my old paper service manuals and the L8/M is one of those.

Don

Hi Don,

One more follow-up about this centrifuge.  It took about 1.5 hours to cool down to 4 °C recently.  Do you have an idea of why it's taking so much longer than our other centrifuges.   Or is this typical for this model?

Thank you very much for your help.

CTL, I'm not sure the criteria that you are using to come up with the 1.5 hours.  Are you trying to perform an actual run or just trying to cool the chamber down.  Remember, temperature in the chamber is directly affected by how well the vacuum is pulling and the diffusion pump does not turn on unless there is a run going on and the vacuum gets down to at least 100u which during a run should happen in less than 20 t0 30 minutes depending on the status of the oils, the vacuum pump itself as to wear and tear, and even the condition of the vacuum hoses too.  If you are performing a run and you have not precooled the rotor then around 1.5 hours would not surprise me at all.  Remember, centrifuge refrigeration systems are design to maintain temperatures of a rotor more than actually cooling a rotor down to temperature.  Hope that helps.

Don

Oh, I mean the temperature as seen on the controller/display.  It seemed to take a very long time to go from ambient to 4 °C.  It seems all signs are pointing to a vacuum-related issue.  I hope that after we change the oil in both pumps we'll see improved performance all around.  I'll take a look at the vacuum hoses as well.  Thank you.

I understood that you were talking about the display but that doesn't change the direction of my question.  Were you trying to actually perform a run?  Again, the diffusion pump will not activate until the the speed gets past 3,000RPM and the drive motor will not accelerate past that point unless at least 100u's of vaccum is attained.  That's just the way that it is designed and there is no easy way around that.

Don

This was just a test of the centrifuge after being able to stop the "Tach" error from appearing.  I hope I'm able to answer your question here.  Even though I cleaned the temperature sensor (thinking it was the speed sensor), the centrifuge was finally able to pull a vacuum and not give the Tach error.  So we had the centrifuge operating at a hard vacuum, the rotor was spinning at the setting we input.  We then set the temperature control from ambient to 4 °C.  From that point it took 1.5 hours for the display to reach 4 °C.  The end user told me his other centrifuges cool down more quickly. 

I am not the end user, so I don't know if that is a typical amount of time for the chamber to cool down.