I can do it easily on my portable and have to with the pulse dose, but I forget to on my home unit and will run it higher than it needs to be.
I did find one, but it cost only a few hundred dollars and the description and statistics were very sketchy. Beware of big promises with a low price tag, there are some awful concentrators out there, as @A MyCOPDTeam Member described.
I wish then I could save a lot of money by being able to turn it off and on, as when sitting or sleeping I don't need the O2, only when I'm up and active.