We are first time pool owners and these guys and been helpful with the 100 and one questions we had for them. They have a system where our pool details (size, type, depth etc) are on record and we can take a sample down with our code number. They can then tell us if the pool chemical levels are correct and if not what we need to do eg add salt or whatever to get it to the right levels. Save us from swimming in a green pool!
Approximate cost: $Free plus the cost of any chemicals needed.