A pachymeter is any of a group of instruments which are used to measure the thickness of something.
I don't know what a pachymeter is. But here's how to get the average of any five numbers, no matter where they came from: -- Add up all five numbers. -- Divide the sum by 5 . The quotient is the average of the five numbers.
A pachometer is another term for a pachymeter - any of several instruments which are used to measure the thickness of something.
There is a nice article on this optical device in wikipedia. It is better you don't suffer my transcription errors.