Apparently my version of the user guide for the Amicon Ultra-4 was an older version, which did not say anything about this. I contacted Millipore and they sent me different document, which states, "When using a fixed-angle rotor, orient the device with the membrane panel facing up." This is odd because the membrane panel and the printed numbers are approximately 90° different, as I see things. I don't currently have Viva or Thermofisher concentrators, but I will try to obtain some.