I'm having problems whilst sparging the grain bed. I vorlauter fine, and the wort begins to clear, so I then open the tap on the boiler and start to sparge the mash. This seems to go fine for about 10 minutes, with the level of hot liquor being about 1/2 - 1 inch above the grain bed. After about 10 minutes though, the liquor level on top of the grain bed was appearing to drop exposing the top of the grain bed, no matter how fast I let the liquor run into the mash tun. Looking at it more closely, it was not the liquor level that was falling, but the top of the grain bed rising. Watching this for the past 2 brews, the top of the grain bed rises by up to 1 and a half inches and seems stay at the surface of the liquor from about 10 minutes into the sparge until the end of the sparge. All the instructions that I've read seem to state that you should keep the surface level of the liquor about 1/2 to 1 inch above the top of the grain bed, but how do you do this if the grain bed expands to meet the surface level of the liquor? Presumably I'm losing out on the sugars at the top of the grain bed that can't be washed out.
I'm using a sparging arm so it's not that the grain bed is being disturbed. I use a insulated mash tun, so the grain bed is probably about 6-8 inches deep. Sparging normally takes about 40 minutes or so. Is this too slow and would a faster sparge compact the grain bed with the flow of water?
Any ideas what may be causing this?