Lab reports are usually written in third-person past tense (so no I/We/Us etc)
I don't know why it is used apart from making the report impersonal and objective. Appears to be a standard of sorts, I believe.
From my understanding, it's about being impersonal and objective. The more objective it is, the less likely it is to be influenced by bias - from both the writer and the reader. If you absolutely must refer to yourself for the sake of clarity use "the experimenter" (again, to keep that objectivity) but you shouldn't have to do this.
Past tense is the most clear and accurate, because when you're writing this up, it did occur in the past
There's actually some interesting debate about how scientific writing is generally passive "eg. the beaker was washed rather than ____ washed the beaker" because passive tense aids the scientific ideal of being objective /but/ can hinder the goal of having clarity. To be on the safe side, stay passive