Does LU matter on FLD detector baseline monitoring?

I am very new to FLD detectors and have been seeing some issues between some of our newer units and older ones.  We are thinking the optical units may have some etching on the older ones but because of our company's structure we cannot service them ourselves.  What I am trying to understand is how the baseline monitoring works.  When we start our systems, our newer systems seem to monitor closer to zero.  However our older models (1100 series) do not behave the same way.  They tend to drift upwards of 5LU, some even to 10 or 20 LU.  Is this something that should be of concern? We have cleaned with IPA and still see the same issues.  When we do test runs, we notice better reproducibility of injections the longer the system equilibrates, however it seems that 4/5+ hours is sometimes necessary to achieve more consistent reproducibility.  Again, total newbie here, please feel free to provide with any advice, feedback or even resources that would be available for better understanding of how these detectors work and what signs to look out for when assessing proper performance.  Thanks!

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