A conversion can fail for one record while the rest of the file is usable. try expression otherwise fallback handles that error where it occurs. For example, try Number.FromText([Reading]) otherwise null keeps an unparseable reading as missing. It doesn't turn missing data into a measured zero.
Place try near the operation that can fail. Wrapping a whole table expression may miss errors stored inside individual cells until another step evaluates them. A conditional is the better tool for a known zero denominator: division by zero has numeric behavior in M and should not be treated as a guaranteed conversion error. These fictional inspection records include numeric text, invalid text, null and zero units. Your result must preserve the records and make the missing-value policy explicit.