Hi all, so I tried another experiment to verify the accuracy of the gradations on my sight glasses. If you saw my very long previous post last year in this group, you know I did this experiment previous using a BSG bucket where I filled it up to the 5 gallon mark of the bucket (assuming the bucket volume markings were accurate) and poured it into the kettle to see if it matched up with the sight glass 5 gallon mark...did this 3 times to get to 15 gallons...anyway long story short since you can read it all in the previous post, the sight glass volumes were off and I posted an equation that corrected for this (it lets you know what to expect the sight glass to read for any volume added). As I mentioned - all of this will only hold true IF the BSG bucket volume markings are accurate....
So, I bought a 65LB capacity Anvil scale (shown here:
https://www.morebeer.com/products/anvil-large-scale.html) and it's fantastic!!! To verify its accuracy I put a butterfly valve on my smaller precision scale, noted the weight, and put the same valve on my new Anvil scale, and am happy to say it read the same weight (I was unsurprised since they mention the scale has been calibrated ahead of time, but wanted to verify). Had the idea to check my sight glass gradation accuracy BY WEIGHT using this scale instead of the previous volume method I used. Here is my process:
I placed an empty 6.5 gal capacity BSG bucket on the scale and filled it with 5 gallons of water by weight. Water weighs 8.34LB per gallon at 62F, so I tared the scale with the empty bucket and started filling it up until it reached a weight of 41.7LB (5*8.34LB) to know that I had 5 gallons of water I was working with. This volume of water only came up to the ~4.5 gallon marker on the bucket so I was intrigued. I poured the 5 gallons of water into my boil kettle and turned on my control panel to see the temp (it read 66F...close enough to the required 62F for 8.34LB/gallon to be true). I also put a level across the top of my kettle to make sure it wasn't sloping or anything. I then noted the volume shown on the site glass was 4.6 gallons! The site glass sheathing that has the gradations on it can move up and down slightly, so please note that the sheathing in this test is all the way DOWN (which I assume is normal). I then did this 2 more times at 10 and 15 gallons...
5 gallons by weight = 4.6 gallons on site glass
10 gallons by weight = 9.6 gallons on site glass
15 gallons by weight = 14.5 gallons on site glass
Putting this data into an excel graph with 5,10,15 on the X-axis and 4.6, 9.6, and 14.5 on the Y-axis yielded the following linear equation with an R^2 = 1:
y = 0.99x - 0.3333
This means that if you have a pre-boil volume of 15 gallons, the site glass will read 14.52 gallons. My previous equation based on volume actually went the opposite way... site glass would have read around 15.5 gallons. If this is all true and I should only be adding a site-glass-displayed volume of 14.52 gallons (for an actual pre-boil volume of 15 gallons), it will yield a slightly higher pre-boil gravity and higher mash efficiency since that last 0.5 gallons (or 1 gallon if you were using my previous equation) will be super diluted last runnings and, although there is some sugar being added still, it's most likely so diluted that it it's knocking down my pre-boil gravity instead of bring it up.
I did some math for a beer I recently brewed to see what the pre-boil gravity would have been if I had stopped filling my kettle at 14.5g (per new equation) instead of going to 15.5g (per the old equation). I measured a pre-boil gravity of 1.055 on that brew day (let's say 55 pts to make it easier) at 15.5 gallons in my kettle. If I had stopped filling at 14.5g, I would have had something like...
(55)(15.5) = (X)(14.5) -------> X = 58.8 pts (round up to 1.059)
This is a pre-boil gravity error of nearly 4 points!
Anyway, this was a very interesting exercise and it may vary slightly between these 20 gallon spike systems if they're not all EXACTLY the same. If any of you have some time, let me know if you see an error in my ways and please try this on your own and post your results here! I would be great and validating to see if others are getting the same results for true consistency.
- Mike (Fictional Brewing Company)