[kcjaz]

I use a SABCO converted keg for my mash tun and I brew primarily 11 gallon batches.  The way the SABCO mash tun is built there is about 1 gallon of volume below the false bottom.  This means that when I shoot for 1 qt total water per lb of grain the mash ends up thicker because of the liquid below the false bottom.  For example, with 20 lbs of grain, I need 5 gallons (20 qt) of water for 1:1 but what is really happening above the false bottom is 16 qt of water mixed with 20 lbs of grain for a 0.8 qt/lb ratio.  This has never really seemed to cause any problem and sometime I do two infusions with the first one a little thicker than 1:1 and the second one ending up thinner at 1.2 or so.  I use a recirc pump and I've got my process up to about 85% efficiency.  It all seems to work fine at 11 gallons.

I want to brew some half batches (5.5 gallons) but I am concerned about the one gallon volume below the false bottom as it will become a greater porportion of the total volume.  If I use a 1:1 ratio, for 10 lbs of grain, I'll be at about a 0.6 ratio above the false bottom which seems too thick and I would think would hurt my efficiency.  Is all I have to do is bump up to 1.4 to keep at 1.0 above the false bottom?  In other words, not count the gallon of water below the false bottom in my water to grain ratio?  Is the only affect here extraction efficiency? 

[euge]

Since you do a recirc it seems to me it should work pretty much the same for 5.5 as for 11 even though the space below the FB is in greater proportion.

[lonnie mac]

Typically, the dead space is just that. Dead space. You wouldn't count this space. Your water added will be what your recipe calls for above this dead space, so like you say, your mash is sitting in exactly what you were shooting for in your recipe you worked hard on. You also worked hard on hitting your strike temps too, and not accounting for the losses in your mash by not adding up water above the dead space will effect that too. But if you are making great beer, then by all means keep doing it that way!

Another way I like to explain this, and of course this is a very extreme example, lets imagine that you had five gallons of dead space, in a five gallon batch...

[tschmidlin]

I have about a 1.5 gallons or so below my false bottom, and that does not get included when I calculate my mash thickness.  If I want 1:1 for 10 lbs of grain then that's 10 qts of water, plus what is below the false bottom.  That number, 4 gallons for the sake of argument, gets used to calculate my strike temperature.  The valve is set low enough though, that the amount of wort left behind in the mash tun is much less than 1.5 gallons.

[jeffy]

I have about a 1.5 gallons or so below my false bottom, and that does not get included when I calculate my mash thickness.  If I want 1:1 for 10 lbs of grain then that's 10 qts of water, plus what is below the false bottom.  That number, 4 gallons for the sake of argument, gets used to calculate my strike temperature.  The valve is set low enough though, that the amount of wort left behind in the mash tun is much less than 1.5 gallons.

Yes, there are actually two things you're calculating: the ratio of malt to water and the strike water volume's temperature.  The latter would include all the thermal mass in the tun to get an accurate strike temp when you add the grain.
I have a Sabco, too and this is usually what I do.  Once the mash has rested for a while I recirculate, which brings the water from under the false bottom into the mash.  But this is mostly to maintain the temp and to clarify the run-off.

[Steve]

If I'm following...  if you have .5, 1 or 1.5 gal. of dead space you still go with your infusion ratio because the liquor in the dead space will still get recirculated through the mash.

[tschmidlin]

No, I ignore it for the ratio because the water under the false bottom does not affect the mash thickness, which is what the ratio is about.  You include the water under the false bottom for temperature calculations.

I don't like referring to the space under the false bottom as dead space, to me the dead space is what will not drain during runoff.  It doesn't really matter though, as long as it's clear what we mean.

[kcjaz]

Regarding the strike temp calculation, I had a theory that predicted (before I added the recirc pump) I would miss my target temp on the low side due to the "dead space" under the false bottom.  I reasoned that this would happen, because I would effectively only be mixing a lower amount of hot water with the grain above the false bottom and I would have a layer of hot water below the false bottom.  Think of it as a stratified mixture with a "hot" layer under the false bottom.  Given enough time, the heat might rise through the mixture but this would be very slow.  

Well, this never really seemed to happen, in fact, when I first started doing all grain batches, I would overshoot my target by 4 to 5 degrees.  I use Promash to calculate the strike temp.  Now I just subtract 5 from what Promash tells me and I hit the target (more or less).  I just figure that the specific heat vs. temperature assumption for the grain is off a little bit in Promash.  I have to add about 20F to the grain temp entered into Promash to make it match what happens in my mash tun.

[tom]

Since you do a recirc it seems to me it should work pretty much the same for 5.5 as for 11 even though the space below the FB is in greater proportion.

I agree with euge. Not sure why you would ignore it or separate it.

[Pi]

I'm in tschmidlin's camp. My RIMS mash tun is an inverted keg. I cut the bottom out and fitted the top (now the bottom) with a 1/2" ball valve. this way i get every drop of wort.
There's about 1.5 gal. of "dead space" below the false bottom/plumbing/heater etc. but since i am recirculating the wort, I dont think mash thickness has much effect on overall efficiency. So i usually figure a 1:1 water to grist ratio and add another 1.5g, and use that total quantity for determining strike. any thoughts?