Digital BW, The Print

Does the reflection plaque (X-Rite #302-12) degrade over time?  I read
where you should replace it every 18 month but at $71 it's bit steep.
 It's a ceramic coating and I would think it would last quite a while.

I get readings of 0.110 - 0.140 in the 0% patch for HPR and Dourian
Art with Eboni and 0.140 on Epson Premium Luster with PKN ink.  I'm
questioning my readings and have calibrated the 810 using a new
transmission film (X-Rite #810-68) but the plaque is about 10 years
old.  Should I pop for a new one?  Are there cheaper ones around?

Could the lamp be suspect too?

Thanks,

Tom