Why not put the false bottom above the valve?

Why do most false bottoms have the dip tube going in the middle of it? Why can't you just take a false bottom and put some kind of riser (2" or so) on the bottom of it to raise it over the valve? Then you wouldn't have to worry about disconnecting the dip tube to get it out.

Am I missing something obvious?

It would require a very snug fit. I think some kettles are built like that but a one size fit all FB application such as that wouldn't work.

After fabricating one to fit perfect, the question would be "Why not just use a dip tube?"

You'd have a lot of space below where you'd have liquid mash. It's not exactly dead space but I'd call it slack space. This volume of liquid would be in addition to your desired strike water to grain ratio. It takes away from your sparge volume. In fly sparging, it wouldn't be all that detrimental though.

I was thinking Kettle as that is where I use a FB. Yeah, what Bobby said if in the MLT. You want your liquid to be with your grains.

In my case, I keep the false bottom below the port and use a dip tube because of how large the pots are. 1 gal in my pots is about 0.75" deep. My port is 2" off the bottom, which equates to about 2.75gal of water below the centerline of the port. My false bottom sits approx 1-1/8" off the bottom of the pot, leaving about 6-7qt below the false bottom. That 6-7qt impacts my mash ratios and doesn't contribute to my grain bed depth, so I'd like to keep that volume as low as possible. Make sense?

Some of us do this, and Bobby made a nice write-up of how to do it. Just use a false bottom with a drain in the bottom of your MLT.

Adventures in Homebrewing has one here -> http://www.homebrewing.org/Stainless-Steel-PICO-style-keg-kettle-false-bottom_p_1010.html

I've been using one for about 10 batches. As Bobby said, there is about 2 1/2 gal of slack space below the FB. I like not having to worry about hitting anything when I stir. It does reduce the overall capacity in the MLT however.

Beach

Because there would be less space in your mash tun for grain; therefore, less volume/gravity yield. I usually almost completely fill my mash tun.

I understand with the 2" of "dead space." Would one just add 2" of water? I'm using a keggle so it would be a gallon or two of water. Or would that screw anything up?

EDIT: forget it, I didn't read Bobby's post good enough.

I have a PICO style FB, with a dip tube going to the bottom.

I have one gallon of "dead space" below my full width false bottom and it poses no problems whatsoever. The drain port is below the false bottom, so there is no dip tube. I make no adjustments in the water/grain ratio to allow for the dead space. I circulate continuously while mashing with the hope that all of the wort will be equally exposed to the conversion enzymes. This method has been working very well for me for a very long time. IMO, a dip tube is used only because there is often no practical alternative for some kettles such as a converted keg with the concave bottom.