Refractometer / Hydrometer alcohol calculation after supplementary sugar feed in fermenter

[DuncB]

I'm using a digital refractometer and it works really well.
Calculations for alcohol using the Brewfather tools converting from Brix to Gravity seem very accurate when compared to my hydrometer.

The calculator for alcohol needs the original gravity when it's input is fermented which normally isn't a problem, but for the imperial stout I'm making the recipe has called for 1.2 kg of dextrose to be added in increments at the end of the fermentation.
So my starting OG was 1.107 for 23 litres, do I just add the 386 points per kg per litre to bump up the OG figure for the calculation?

ie 386 x 1.2 divided by 23 and added to 1.107 for New OG?

So my new OG is 1.127 ?

 

[VikeMan]

That appoach will get you close. But the corn sugar will also add some volume, so the effective combined OG would actually be lower than your 1.127. About 1.122, assuming 42 PPG for the corn sugar and accounting for the additional volume.

 

[bracconiere]

i'd just take a hydro reading for the FG, and see what the refrac says...

should read about 14 BRIX? (and a hydro 1.015) i'd guesstimate...a calc would do it better....

 

[DuncB]

@VikeMan
I'm assuming 386 gravity points for a dry kg of dextrose added per litre. How can I calculate the volume that dry dextrose would add?

 

[VikeMan]

386 PKL would be almost 46.3 PPG, which would be a smidge high even for sucrose, which is ~ 46.21 PPG. And dextrose contains about 9% water, so it's only about 42 PPG or ~ 350 PKL.

For volume added, I used ~ 0.074 gallons per pound.

 

[DuncB]

Thank you @VikeMan
Brewfather has dextrose from gladfield as 100% fermentable and 1.046.
Briess dextrose is 99% fermentable but also 1.046

ppppg respectively.
I'm adding powdered dextrose as above, but given that we know the density change of a gallon or litre the volume change must be calculable.
It's more straightforward for additions to water but must be complicated for a mixture of water, unfermentables, alcohol and then dextrose.
I was hoping this was a bit simpler than it now appears having delved into it.

 

[VikeMan]

Thank you @VikeMan
Brewfather has dextrose from gladfield as 100% fermentable and 1.046.
Briess dextrose is 99% fermentable but also 1.046

Brewfather is wrong. Unless you buy anhydrous grade dextrose and take pains to keep it that way, it'll have/absorb water and be(come) dextrose monohydrate (corn sugar). And homebrew supplier corn sugar is the monohydrate form.

 

[DuncB]

Thank you so much, I'll work out the numbers and then use that in my gravity calculation to plug in as my OG.