Hello Poppy
Your difficulty is not unusual for doctoral candidates. A neat way to handle it is to revisit your Chapter 1. Readers expect to be introduced to what your thesis contains and the rationale of the sequence of the chapters. THUS ~ use the chapter to explain the textual structure of your thesis followed by a brief account of each chapter. In that text, you can then justify the location of the chapters.
So ~ where to locate that chapter? If the measurements occurred after your main experiments then that chapter should follow the chapters that present your main experiments. The chapter will require an accompanying account of why further measurements were undertaken and an methodological/theoretical explanation for the apparently differnt outcomes. Be clear and explicit and ready to engage with your examiners in discussion the phenomena.
What did you decide? (up)

The conclusion chapter in doctoral theses is MORE than just a summary. Among other things, it needs to Remind readers what the research was all about, Tell readers what was discovered by the research, Sell the scholarly merit of the research ~ in an appropriate academic manner, and, Leave readers with a positive appreciation of the research that has been undertaken. It represents the final window of understanding into the thesis for readers.