Partial least squares calibrations last forever if done with modern stable instruments!

In the “old days” of NIR, some instrument drift and seasonal variation was acceptable (as frankly there was a notable lack of options) but it was also understood that “every now and then” calibrations, of course, had to be updated or adjusted. Whether the specific reason was, for example, the amount of sunshine on the
grassland changing the chemical composition of the feedstock analysed by NIR, or the analytical instrument was subjected to a replacement (a big change), or just suffered a little change in optical response (drift), was
not so important since one had to update the calibrations anyway.