As the LoudCreatureWhatSharesMySpace says, we have the Apogee MQ-200. It is nice, easy to use and I have had a chance to compare it to a couple of more expensive meters, though it is a bit clunkier than the higher end meters the readings are comparable.
I guess I just do not fully trust the readings. Perhaps I have not had it long enough but the readings vary in ways that make me uncomfortable. Some of it has to do with the preciseness with which you place the sensor. Sometimes the placing of the sensor changes the environment I am trying to measure. I suppose this is a problem with measuring anything the act of measurement itself changes the measurement.
Some has to do with the variables of water; I suspect that what I may be seeing is in part an explanation of why people with apparently the same setups have such different results.
It also may be that I am just having a hard time coming to grips with photons as opposed to the light my eye perceives. :gw