David, The long storage is not the cause. Extended operation of the ion pump causes the "whiskers" to grow. They can give rise to field emission leakage currents. This manifests as a raised ion pump current but does not inhibit proper operation unless the current gets too high. I don't remember at what current a 5071A will shut down at but I can take a look in the manual. Corby ____________________________________________________________ Click here for free information on how to reduce your debt by filing for bankruptcy. http://thirdpartyoffers.juno.com/TGL2141/fc/PnY6rw22U6KSiqbzGnzo8ijTIrFXhPWyQU8EVhPuk3kNAOgi96JqR/