IBU claculation vs equipment selction

Hi All,

I'm trying to understand why the IBU calculation result is changing when I change the volume of the mashing equipment I use.

My mashtun is a 50liter keg.  There is a 10 to 12 liter 'dead' space below the false bottom.  In fact it is a RIMS.  Ther is a welded ring on which I put the flase bottom.  Underneath the false bottom there is a 1500W electric element.  I heat the wort and pump it on top of the mash.  When I get the right temperature, I stop the heater.

The question is why, if I select one or another equipment that seems to match my rig, the IBU calculation result differs from one to another?

I always get good beer but I'd like to get a better idea of what I brew exactly.

Thanks,
Fil.

tough to say without seeing the Equipment profile output, but it might be that the mash tun volume changes (and loss changes) are carrying over into the boiler and affecting the gravity of the wort, which would affect utilization of the hops. 

Would you consider the volume underneath the flase bottom as a deadspace since that volume will be transfered in the boil kettle?

I personally would not, if that volume moves forward in the process and is not lost.  BSmith treats that amount as lost, and makes up for it elsewhere to make the target batch volume.   

I suspect a lot of users enter the volumes under a valve, and then in practice, they tilt the vessel to more completely drain it, which misleads BSmith on the true losses. 




o Lauter Tun Deadspace - Represents the deadspace in the lauter tun - how much wort will likely be lost to the lauter tun screen and piping
o Losses to Trub/Chiller - The amount of wort lost in the trub or chiller during the transfer from the boiling vessel to the fermenter