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Here are copies of the 4 emails that I received in response to my query on filament lifetimes; thanks to everyone who responded. I guess the summary is that filament lifetimes are considerably shorter (factor of 5?) running at 500 microamps than at 200 microamps. If anyone knows of some hard data on this subject, I'm still interested to see if there is a way to calculate a theoretical or empirical filament lifetime. Pete Simon Kelley: We run at 200 uA trap like all Ar people I guess and the filaments run at between 2.35 and 2.45 A. We use the standard MAP coiled filament which is on 24 hours a day (I know some people turn them of and claim it extends the lifetime). I guess we have had between 2-6 years per filament with the low end of that spectrum probably due to a badly fitted filament. David Graham We regulate at 500 uA emission. We typically have filament currents of 3.2 to 3.5 A. They last about 12 months. Sometimes less and sometimes more. One lasted as long as 3 or 4 years (way back when). Fin Stuart I replaced the MAP filament last week so I can tell you exactly how long it was in (since delivery early October 1999, plus some time in the factory). I run @ 500 uA, filament current was 2.65 A on delivery but over the last 6 months has decreased down to 2.1 A Bill McIntosh In the New Mexico Geochronology Research Lab we use use a standard MAP-supplied filament in our 215-50. We operate at a filament current of 2.5 A and a trap current of 200 um. We have only had to change the filament once, last summer, after seven years of always-on operation. We are hoping that our current filament will last as long. ______________________________________________________________________________ Dr Pete Burnard Division of Geological and Planetary Sciences, California Institute of Technology MS100-23 Pasadena CA91125 USA Tel: 626 396 3872 Fax: 626 683 0621 Website: http://www.gps.caltech.edu/~peteb