Two measurements of the same object differed by a few millimeters. The difference was averaged out. 

It's neater to use words rather than numerals at the start of a sentence, especially if it's a small number.

A measurement is the datum obtained by measuring.

<em>http://english.stackexchange.com/questions/56387/measure-vs-measurement </em>

Remember to keep the verb tense consistant. :)

The second sentence is correct.