calculating expected first wort gravity

[DryStout]

Hi All,
Im having some trouble calculating my expected first runnings gravity. I'm going by the calculation in the MBAA basic brewing calculations handbook and getting some pretty out there numbers. Im mashing with 7.25BBL of water (1865.425lbs)

the equation given is
Plato of 1st runnings= % extract cg, db X (1- % moisture) / water weight + ((% extract cg, db X (1- % moisture))

Plugging in my values from a briess I have.

plato= .795 X (1-.042)/ 1865.425 + ((.795 X (1-.042))
This leads me to an expected first runnings of .000404

Using the Kunze calculation my numbers make much more sense.
(3hl/100kg)(20plato)/(8.5hl/272.2kg)= .6/.03123= 19.2 Plato

I would like to avoid having to convert all my numbers from hectoliters and kilograms every time I want to calculate first runnings. Is something missing from the top MBAA calculation

Thanks for the help

[a10t2]

I would like to avoid having to convert all my numbers from hectoliters and kilograms every time I want to calculate first runnings. Is something missing from the top MBAA calculation

Yes, you need to multiply what you have there by the grist mass and projected conversion efficiency if less than 100%. If you were doughing in with 600 lb (for a 1.5 qt/lb mash ratio), the projected first runnings would be about 600*.0004 = 0.24, or 24°P.

You need to knock about 7-10% off of that to account for the wort expansion, so you'd expect to see something like 22°P.

[jwalts]

a10t2 is correct about multiplying by the grainbill, but it needs to happen both on the top and bottom. In a more simplified form, the MBAA equation would show sugar weight over total wort weight expressed as a percentage (i.e. the definition of degrees Plato). DryStout: you also misplaced one of your parentheses, but it doesn't look like the error made it into your calculation. Here's how the equation should look:

Plato of 1st runnings = 100 x CGDB X (1 - moist) x grain weight / (water weight + CGDB x (1 - moist) x grain weight)

For 600 lbs of grain, it becomes this:

Plato = 100 x 0.795 x (1 - 0.042) x 600 / (1865.425 + 0.795 x (1 - 0.042) x 600) = 19.7 Plato

You can compensate for wort expansion by calculating water mass with the density of water at the temperature where you measure its volume. However, your foundation water may not be evenly distributed after vorlauf and could cause your initial runnings to have a lower gravity than the average of all your first runnings wort. I hope this helps!

Joe
Narrows Brewing
Tacoma, WA

[a10t2]

a10t2 is correct about multiplying by the grainbill, but it needs to happen both on the top and bottom.

Oops. Good catch.