German Brewing

Bryan, that looks like a very neat solution. I like it! Thanks so much for posting the pictures. What did you use to cut the lid? Nice circle!
Re: the oil; never used it, so can't make any claims. But mineral oil density is about 800g/L so it should float easily on plain water and sweet wort. I haven't used a mash cap yet, but last brew, (my first attempt at LODO), I used a little stainless steel weight to hold my mash return hose down onto the top of the grain bed, instead of letting it float near the top like I used to do. The wort at the surface barely moves, so I think that oil mixing in due to turbulence would not be a problem. It's odourless and flavourless, so it shouldn't affect the mash in any harmful way. (I was thinking about cooking oils initially until I remembered the polsinelli site mentioning their oil). If it doesn't flavour white wines, fruit juices or olive oils, it's probably ok.

I ordered an extra lid, and used my angle grinder with a cut off wheel. I followed the raised lip with the edge of the wheel, which left a perfect 3/16" gap. I then used a gasket from a 7.3 gallon stout tanks conical, and here we are.

The other way around. Grain is already in the tun, add water from bottom. Then put on mash cap tight to the liquid.

Caps are a big issue for folks with keggles, as the diameter of the keg is wider than the diameter of the opening. Even if you cut one all the way to edge, the handles preclude you from getting a lid in there. You need to build a folding or two-piece cap. Press-fitting will be difficult, and floating may be the best option.

So I am thinking about making a mash cap out of food safe acrylic or polycarbonate and have a few question you may be able to help with. Would oxygen permeability of the material be a problem? Also, I've seen plenty of food safe acrylic, they use it to make cake circles and such, but how do you know if it can handle constant contact and the mash temperature? I've seen the temp rating as high as 190ºF, but I'm not sure if this would work with a liquid.

Any other reason making a mash cap out of one of these materials is not wise? I'd go the stainless route, but I want to be able to still see the recirculation, especially during spare.

I would drill a ⅞" hole in the top to hook my supply water up to. Under the lid would be the circular flow locline. I think I saw something similar in one of the threads here. It was made from a stainless lid however. My worry is that if I don't match the flow rates exactly, I won't be able to tell if the liquid level drops without removing the lid if I use stainless.