The idea is to keep one meter always "brand new" - never in use except when testing the others. I did that for quite awhile, but then as the
older meters starting getting rattles with wear n' tear, I got overwhelmingly tempted to put the reserve unit into regular service. They are all
still perfectly fine and matched as far as reading goes. New Pentax digital spotmeters have gotten pretty damn pricey now that they're no longer made. Sooner or later I'll acquire another one, or perhaps a Minolta Spotmeter F (a little less convenient to use, but a bit more compact,
and when I've had em before, were identically calibrated to the Pentax).