Metric vs Imperial OG calculations

[Contrarian]

I've been building a spreadsheet to help predict OG for my brewdays. I know there are programs that can do the job but I like knowing the details behind the calculations. This brings me to a problem I've been having with calculating OG - When I use the ppg method my results align nicely to various brew calculators. However, when I use the metric system to generate plato my answer is very different and I'm unsure why.

Briefly. If I use 0.4536 kg (1 lb) pale malt in 3.785 L (1 g) with the specs of 80% extract and 37 ppg.

Via ppg this is 37 points * 1 /1 = 37 or 1.037 OG which is an accepted answer on online calculations.

Via metric we calculate
plato = 100*mass extract / (mass extract + mass water )
= 100*(0.4536 * 0.8) / ((0.4536 * 0.8) + 3.785)
= 8.75 plato or 1.0347 OG

If you double the mass of grain the variation is worse with Imperial 1.074 OG but Metric predicting 16.07 plato or 1.0657 OG

Am I getting the metric calculations hopelessly wrong?

 

[doug293cz]

I've been building a spreadsheet to help predict OG for my brewdays. I know there are programs that can do the job but I like knowing the details behind the calculations. This brings me to a problem I've been having with calculating OG - When I use the ppg method my results align nicely to various brew calculators. However, when I use the metric system to generate plato my answer is very different and I'm unsure why.

Briefly. If I use 0.4536 kg (1 lb) pale malt in 3.785 L (1 g) with the specs of 80% extract and 37 ppg.

Via ppg this is 37 points * 1 /1 = 37 or 1.037 OG which is an accepted answer on online calculations.

Via metric we calculate
plato = 100*mass extract / (mass extract + mass water )
= 100*(0.4536 * 0.8) / ((0.4536 * 0.8) + 3.785)
= 8.75 plato or 1.0347 OG

If you double the mass of grain the variation is worse with Imperial 1.074 OG but Metric predicting 16.07 plato or 1.0657 OG

Am I getting the metric calculations hopelessly wrong?

When using points, you are working with gallons of wort not gallons of water, so 1 lb of grain will produce 1 gal of 1.037 wort, which contains less than 1 gal of water, since the sugar makes up some of the volume. When you do your calculations with extract, you are using a whole gal of water, so your wort volume is higher, but with the same amount of sugar, so the SG is lower. Your wort volume in the second case is:

(0.4536 kg * 0.8 + 3.785 kg) / 1.0347 kg/L = 4.01 L = 1.06 gal​

You also need to remember that most grain potentials are reported based on dry weight, but that most grain you get has about 4% moisture, so you need to multiply the potentials by 0.96 to get the "as is" potential.

Brew on

 

[Contrarian]

Cheers mate! That really is helpful