SG Measurement of hot wort

[mcblades]

I was making some cascade ale this afternoon and trying to guess what the final gravity of the boiling wort would end up at. Was hoping for 1048 at room temp. But the boiling wort was only 1030 ish. Googling a bit I came up with a temp conversion that went up to 50C and the look of the curve when plotted hinted at a rectangular hyperbola. I started curve fitting the data with no clear idea what happens to the right of 50C.... then smacked my forehead.

the WAH calc will convert brix to SG and the temp of 100ul of wort on a cold refractometer soon cools to 20C or so. result. 12.5% Brix = 1050 and no need to guess!

Cheers

Mark

ps. this will probably have been mentioned already in the forum but I am still chuffed.

[kampervan]

1.030 at 100 deg c is 1.070 http://hbd.org/cgi-bin/recipator/rec...ydrometer.html

[mcblades]

Hi Kampervan,

thanks for the link. Yes, I measured the wort about 10 min after it had gone in the boiler (it was at 65C). I appreciate this wasn't clear from the above post.

regards

MArk

[Cellar_Rat]

I must have had a refractometer for too long. I do everything on it. The maths is easier for adding sugar too.
The only thing I use a hydrometer for now is final SG.

[mcblades]

I am now appreciating the utility of this little device. I recently started a black cherry port recipe and was thinking how to calculate the ABV of the fermenting liquor. I thought that the presence of alcohol in the liquor would screw up the refractive index measurement, and this is indeed the case. BUT if you combine refractive index with a hydrometer reading (still your next best friend!) you can calculate the ABV of the ferment.

This is good for people like me that start with an arbitrary volume of liquor and adjust it to a target starting gravity, then add in sugar or juice or whatever later on. Most of the sites I looked at require the RI measurement in zeiss units (see here for example:

http://www.bellinghamandstanley.com/ltd/technical_centre/tc_calibrationuk.html

I haven't managed to track down a converter for zeiss to brix but I did get a dataset for brix vs Refractive index (RI) from 0 to 85 brix.

A bit of curve fitting yielded a not very pretty but quite good fit for a parabolic equation. So to convert from brix to RI you can use the following:

Screenshot.jpg

Now thankfully someone has already tackled replacing the zeiss unts in the above link with RI units that can be derived from the above formula. see below:

http://hbd.org/hbd/archive/2923.html#2923-13

To determine alcohol content, take a sample of the beer (100 mls or whatever amount you typically use for gravity determination). Degas the sample (shake it in a flask, use a blender on low, etc.). Warm/chill the sample to 20C, and determine its specific gravity at this temperature. Now put a drop of this degassed, 20C sample in your refractometer and note the measurement. Record your refractometer reading in terms of the refractive index ("RI") of the sample - --if your refractometer reads in degrees Brix, you'll need to use a conversion table such as found in the CRC Handbook (look for a table showing the "Index of Refraction of Aqueous Solutions of Sucrose") to convert degrees Brix (percent sugar) to RI. [At present, I have been unable to find a formula that provides this conversion. If anyone knows of one, lemme know and I'll revise the formula.] Using this data (SG and RI of the sample at 20C), calculate the alcohol by weight (A) of the sample as follows: A = 1017.5596 - (277.4 x SG) + RI ((937.8135 x RI) + 1805.1228) While you're at it, you can use this data to calculate your Real Extract (RE): RE = 194.5935 + (129.8 x SG) + RI ((410.8815 x RI) - 790.8732) (These formulas were derived from those contained in DeClerck and others from a 1980 ASBC Journal article by K.J. Siebert, and have been checked against examples given in those articles as well as in the ASBC Methods of Analysis.) Example: SG 1.0104, Refractometer reads 5.5 Brix. According the the CRC Table, 5.5 Brix = 1.3411 RILouis K. Bonham

so, if you are still awake, an example:

My black cherry port started out at about 1080ish and is now at a gravity of 1.006 (@ 29.4 deg C). Measuring the RI of the sample I get a brix of 10.1% (20C).
First convert the SG back to a 20C temperature reading = 1.008 (thanks kampervan).

to convert brix to RI we use the formula:
RI=0.00000706*(10.1*10.1)+(0.00141*10.1)+1.332
RI = 1.347

ABV is given by

ABV = 1017.5596 - (277.4 x SG) + RI ((937.8135 x RI) - 1805.1228)
= 1017.5596-(277.4 x 1.008) + 1.347x((937.8135 x 1.347)-1805.1228)
= 8.46 % alcohol (w/w)

to convert to abv we use the SG and the density of alcohol =0.79g/ml

so (8.46*SG)/0.79 = (8.46*1.018)/0.79 = 10.8% abv

phew..

Now I bet someones got a calculator that works that out automagically but it kept me occupied for a bit while my mash cooled.

ps. the equation in the original quote states +1805.1228 but I could only get sensible answers when I switched the sign (ie -1805.1228)

If anyone spots a howler please let me know.

regards

mark

[Cellar_Rat]

I tend to skip the maths this way.

Use the Refractometer to start and the hydrometer to finish. Easy peasey!

[mcblades]

I agree, especially for beers. But for wines where you add sugar and/or liquor after your initial SG measurement, eg when making high alcohol dessert style wines where you feed the must until the yeast croak, this can help to give an idea of the %ABV of the must so you can decide when enough sugar is enough.

[Cellar_Rat]

Agreed. But I can make that easy too - don't make dessert wines he he