Fly sparge efficiency vs. Total sparge water

When preparing to run my first very big beer on my 5Gallon all-grain setup (20lbs of grain), I realized that the sparge ratio would be very small in comparison to what is normally used. I usually collect around 7 or 7.5 Gallons in the boil kettle. Some quick estimates:

20lbs *1.25qts/lb = 6.25 Gallons
Mash loss of ~0.125 Gallons/lb = 2.5 Gallons
Total Mash collection = 6.25-2.5 = 3.75 Gallons
Sparge water to reach 7.5 Gallons = 7.5 - 3.75 = 3.75 Gallons
Sparge ratio = 4*3.75/20 = 0.75 qts/lb
This is half of the usual ~1.5qts/lb number

I was curious about the efficiency drop off as sparge water decreased, but I wasn't able to find any decisive documentation. So I decided to run a simple experiment.

Disclaimer: I know there are some gaps in measurement quality in this experiment. And I don't know how well it scales or how repeatable it is.

The experiment:
Start with 4lbs of 2-row. Assume 100% efficiency would yield 36 points/lb*Gallon.
Mash at 152F in a cooler
Collect runnings and fly sparge as normal
One difference: collect all the running in plastic cups that hold exactly 2 cups volume. The gravity of each of these 2cup samples can be measured to judge the increase in sugar received from each successive sparge sample.

Goal:
Build a table showing the efficiency change as sparge water increases

I mashed with 1.25 qts/lb ratio. 4*1.25/4 = 1.25 Gallons
Assuming 0.125Gallon/lb mash loss, I would collect 1.25-4*0.125=0.75Gallons
I sparged with 1.5Gallons of water. (1.5*4/4=1.5qts/lb)
In the end, I collected 18 2cup samples. (18*2/16=2.25 Gallons)

The first runnings from the mash tun can be used to find conversion efficiency. I measured a gravity of 1.1. 100 points. 36*4/1.25=115 ideal points. 100/115 = 87% mash conversion. So if I had 100% sparge efficiency, I would end up with 87% total efficiency.

Since I collected 2.25 Gallons and targeted this as 2 Gallon batch:
36*4/2*0.87=63 points or 1.063
(The batch size target is arbitrary but must remain a constant.)
So 1.063 is the 100% sparge efficiency or 87% total efficiency target.


Here are the results:





Some comments:

Moving from 0.75qts/lb to 1.5qts/lb sparge ratio moves my total efficiency from 70% to 80%. (Or moves from about 80% to around 90% sparge efficiency).

Ideally the first 0.75Gallons collected would be a constant gravity, but of course the sparge water will start to dilute the mash before all of the mash running are collected since it is a fly sparge. This is why the gravity drops from 100 points to 95 points between 0.5 and 0.75 Gallons collected.

Conclusions:
If the data holds up, I could lose around 10% efficiency when dropping the sparge ratio from 1.5 to 0.75.
I plan on using this data to budget the impact of big beer / low sparge ratio effects. I haven't tested it on a real beer yet.

Your conclusions appear to match my experiences. I'm sure the numbers and percentages vary per system, but I expect a similar result across all fly spargers.

I for instance, hit around 87% when brewing beers with around 7-10lb grain bills. Anything in the 10-14 range I average closer to 83%, so on. My biggest beer, a Yeti Clone with ~20lbs of grain netted me more like 72%. Same process across the board.

You can obviously adjust these numbers, instead of shorting your sparge ratio, plan a longer boil and have more sparge water. The trade off is obviously time, but keeping the ratio the same should keep your estimated efficiency more static.

Thanks for the feedback. Great point about the longer boil. I'm planning to do a 2 hour boil for the 20lbs of grain, 5 gallon batch. That should get the sparge ratio close to the normal range.

I brewed the 20lbs of grain, 5 Gallon batch over the weekend, and I did the 2 hour boil.

My kettle is only 10Gallons, so I initially collected 7.5 gallons of wort in the kettle. Then I collected another 2 gallons in a separate vessel. After boiling off about 2Gallons in the first hour, I added the extra 2 gallons and basically proceeded with a normal 1hour boil including hop additions and so forth.

Since I was separating the wort into 2 different vessels, I was able to get more insightful gravity readings than if I had not split them.

First 7.5 Gallons = 1.06. Corrected for 5 gallons = 60*7.5/5= 90 points
Last 2 Gallons = 1.02. Corrected for 5 gallons = 20*2/5= 8 points
Actual SG of the 5 gallons collected for fermentation = 1.1 or 100 points. This correlates reasonably well with the expected 90+8=98.


So I was able to increase my starting gravity from about 1.09 to about 1.1 by sparging with an extra 2 gallons. This increased the sparge ratio from 0.75 to 1.15