If this transfer standard could actually maintain even 10ppm/year stability I would have to consider that Calibratory has some very specific empirical knowledge regarding those commodity 1% tolerance metal film resistors. Let's call them CMFRs. Anyway, that's what they look like. The specs. for commodity metal films is +/-1000ppm/year. Thus, the D105 should become useless after 12 months. That said, I believe that the stability specs published are for CMFRs at rated current, and 80C temp. I haven't found data on CMFR stability at 23C over a year. I might conjecture that Calibratory has a secret temperature cycling procedure to improve stability of CMFRs. Perhaps CMFRs become more stable with time, just as the reference does. Merely having an ample inventory of 20 year old CMFRs may permit Calibratory to select the ones that have remained stable for that long. In order to maintain records on hundreds, or thousands of CMFRs for years on end would take a particular type of individual. Indeed. Has anyone on this forum ever built two identical reference circuits, one with CMFRs, and the same one with foil or ww to actually compare stability at, a small temperature range (+/-5C) above and below 23C for over a year or more? Then, of course, there's that 1K trimmer. That seems too much range, to me. As a disclaimer, I've purchased a D105, but I don't have a metrology lab setup that can track the progress or demise of my D105 over the course of a year. According to Calibratory, the D105 should remain unpowered, when not in use, for greatest stablility. That sentiment is not without support from some respected metrologists. In one scholarly paper LTZ1000 references were only powered on when needed. Aging was significantly less than other units left on continuously. IMHO, this is truly a Wait and See thread at this point.