Refractometer confusion

[fermenator]

Hello all,

I used a refracto for my last batch and have a question regarding the conversions. I use the MoreBeer spreadsheet (found here: http://morebeer.com/public/beer/refractbeer.xls)

to convert from Brix to SG and FG.

SG converts from 16.5 brix to 1.064
FG converts from 8 brix to 1.011

This is using the temp correction column as it should be for my ATC refracto.

However if you do the simple math converstion (16.5 x 4 = 64 which = 1.064) for the starting gravity, it works out the same. But if you so the same math for the FG it works out to be (8 x 4 = 32 which = 1.032) which is obviously too high of a gravity reading for a completed fermentation. I know fermentation is complete because I also took hydrometer readings which resulted in matching the spreadsheet values.

I am just confused on the differences between the "simple" math and the spreadsheet. The spreadsheet obviously counts for F.G. adjusted for Alc and Temp and I was wondering what the math was here in order to understand it more. I looked at the formula in excel but its a bit complex and I was hoping for a simpler explanation.

Thanks!

 

[bradsul]

You hit the nail on the head. The simple calculation only works pre-fermentation so doing the rule of 4 once fermentation has started won't work. I don't know the actual formula (as you say it's in the spreadsheet) but I'm sure it's more than you want to work out by hand.

From every report I've heard (including by a lot of members here) the refractometer shouldn't be used to try and get an accurate FG (the calculations are a guess at best). You can use it to see if the fermentation has completed but to get an accurate FG (for abv calculations for example) you need to use a hydrometer.

 

[fermenator]

You hit the nail on the head. The simple calculation only works pre-fermentation so doing the rule of 4 once fermentation has started won't work. I don't know the actual formula (as you say it's in the spreadsheet) but I'm sure it's more than you want to work out by hand.

From every report I've heard (including by a lot of members here) the refractometer shouldn't be used to try and get an accurate FG (the calculations are a guess at best). You can use it to see if the fermentation has completed but to get an accurate FG (for abv calculations for example) you need to use a hydrometer.

Thanks Bradsul, that is one thing that suprised me. My Hydrometer readings matched my refracto with spreadsheet conversion readings spot on. It's just the simple math that was giving me the off reading which is completely understandable. I think I will still use my hydrometer along with my refracto for the next few batches. If I don't see an arguable difference between the two I will ditch the hydrometer all together.

 

[ebeer]

Promash, Beersmith and others have calculators built in. I also have one here.

 

[fermenator]

Promash, Beersmith and others have calculators built in. I also have one here.

Thanks ebeer! My apologies as I forgot to metion that I plugged this into Promash and recieved the same result as the morebeer spreadsheet so I think im good to go! Nice website!

 

[WortMonger]

I always use my hydrometer post ferment, refractometer even with it's calculations is never on target. I will say I absolutely love the thing while brewing and as a reference for when to start my spund for carbonation.