Fly Sparge Questions After First AG Brew Day

I finally made the leap into AG brewing with NB's Smashing Pumpkin Ale. I went for extra credit and added pureed pumpkin that I roasted the day before. I almost can't believe the whole thing didn't blow up in my face but I guess I had put AG brewing on some kind of mental pedestal that only the demigods of homebrewing could reach. Needless to say I'm super pumped about the process now! So many possibilities!

Anyway, my question has to do with fly sparging: What is the proper level of water to maintain in the mash tun during sparging? On one hand I was told to maintain a constant "flux" of sparge water through the tun (sparge water in = wort out). But at the same time, I was also told to keep the water about 1" above the grist in the MT. If you start out with more than an 1" of water over the grist and keep flow in and out constant, you'd always have more than an 1" of water.

During this brew, I had 10.75lbs of grain, 3-4lbs of pumpkin, and a few handfuls of rice hulls (forgot to rinse them, oh well.). I heated 4Gal of water (~1.5 qts/ lb) but missed my strike temp (152F) by 4F (148F) so I added a few teapots of boiling water and hit 150-151F. After the rest, I mashed out with 1.5gal of boiling water (based on NB's DVD instructions) and missed the mash out temp aimed for 170, hit 160. The lesson: always calculate the volume to boil based on calculations, not guidelines. I probably missed it because of the additional thermal mass of pumpkin and extra water I added to hit the Sacch' rest temp. Based on some research of discussions here I don't think this will ruin anything.

I soldiered on and started to sparge but this is where my question stems from. I had about 6" of water over the top of my grain bed but decided to keep the in/out flow constant so I maintained more than 1" of water until I collected 6.5 gal of wort. In the end I don't think it mattered because I hit a pre-boil gravity of 1.046 and ended up with ~5 gal after the boil (1hr) with an OG of 1.056 (recipe OG was 1.054).

All in all I'm going to call it a success. I think the extra .002 gravity may be the result of the added pumpkin which I have no idea how to account for in calculating efficiency.

To recap:
- When fly sparging, is having >1" of water over the grain bed an issue?
- Extra credit: how would you factor in the pumpkin for the purpose of efficiency calculations?

The underlying reason for those suggestions is because you don't want to disrupt the grain bed and cause "channeling". Imagine you have a tub full of sand, and you dump a bucket of water into it... if you dump that bucket to quickly, the water will drill through the sand and go straight through to the bottom. If you pour that water slowly enough, it will spread out and slowly fall through the sand instead.

This is what you want to have happen when fly sparging. The "bed" of water on top of the grain bill allows gravity to do its thing while preventing the water from drilling down through the grist. You want the water to slowly and evenly sift through the entire grain bill so that the sugars are evenly pulled out. That is why it typically takes a while to complete.

It sounds like you did a great job!

It's better to have a few inches of water on top then to let the bed get dry. No worries.