Par meter with and without water?

[splash]

I got the Par meter and was wondering if there would be any significant difference if I would measure without water?
I am finishing up the prototype of the lamp using CREE XM-Ls, while the tank will not be ready for some time. Any advice? Tank is 24x24x48"

 

[Gerryd]

I think the values out of water would be meaningless unless you are growing emersed...

There is a big difference in values in and out of water...

 

[pepetj]

Let me add that the difference in measurements with or without water may look negligible when filling a tank with new water for the first time, more so for experimental measurements when a bare bottom tank is used.

In my planted tanks I get noticeable difference in measurements taken just before a huge water change and once I complete the water change, so assuming one reading represents the actual amount of PPF is, at least in my tanks, an error since it's just a measure at that moment. The log function works well to estimate an average value here.

I use the leveling plate every time I take measurements; sometimes the air bubble is hard to center.

Pepetj
Santo Domingo