Over the past couple of weeks I've done a bit of work with refractometers and how to best utilize them. Here is a collection of tables and formulas that have been useful to me, and hopefully will be useful to you.

### Spread Sheet Friendly Equations

OGR = original gravity in Brix (eg 9)CGR = current gravity reading in Brix (eg 9)

OGH = Original gravity as measured with a hydrometer (eq 1.036)

CGH = Final gravity as measured with a hydrometer (eq 1.004)

S%W = Sugar percentage by weight (eg 4 would indicate 4%)

ABV = Alcohol by volume as a percentage (eg 4 would indicate 4%)

**Alcohol**by volume based on

**current refractometer**measurement and

**original refractometer**measurement

ABV =0.6967*(OGR-CGR)

*(extrapolated from physical model)*

**Sugar**by weight based on

**current refractometer**measurement and

**original refractometer**measurement

S%W =1.128*CGR-0.128*OGR

*(extrapolated from physical model)*

**Alcohol**by volume based on

**current refractometer**measurement and

**current hydrometer**measurement

ABV =1.421*FGR-357.4*FGH+357.4

*(based on physical model)*

**Sugar**by weight based on

**current refractometer**measurement and

**current hydrometer**measurement

S%W =0.7488CGR+68.55*CGH-68.60

*(extrapolated from physical model)*

**Original Gravity**based on

**current refractometer**measurement and

**current hydrometer**measurement

OGH =-1.728*FGH+0.01085*FGR + 2.728

*(extrapolated from physical model)*

**Specific gravity**based on

**current refractometer**measurement and

**original hydrometer**measurement

SGH =0.00628*CGR-0.0025*OGR+1.0013

*(based on physical model)*

Equations that are "based on a physical model" are algebraically derived from other equations. These include the Balling Observation that 2.0665g of sucrose are required to created 1g of Ethanol. They also use some basic analytical chemistry to convert from various units.

Equations labeled as "extrapolated from a physical model" are derived from the physical models as describe above. Instead of listing complicated equations that are algebraically correct these equations are simplified linear fits to the physical equations. In most brewing applications these should be just as accurate as their algebraic equivalents. When compared to the physical model these linear fit equations were accurate to two decimal places, or 0.01%.

In my experience it seems that final gravity measured by refractometer is always few points up/down from what calculation predicts.

ReplyDeleteI measured a lot of SG/FG samples with refractometer and checked them with hydrometer. I used seanterrill's calculator to calculate FG (Brix) and I had to use different correction factor each time to correct reading to what hydrometer measured.

I decided not to use refractometer for FG reading anymore (at least not as only measurement).

It is great tool for checking OG during the boil or for measuring starters OG, but for final FG hydrometer is still indispensable tool.. I am speaking about my experience with cheap e-bay refractometer, it could be that quality models are accurate.

Thanks for providing your opinion on this. Every tool has it's weakness. Sean's calculator attempts to correlate the refractometer to the hydrometer. These equations look at it a different way. They are based on physical models treating each tool as equal. The true is both tools have inaccuracy.

DeleteHistorically hydrometers have been used for final gravity so that is what people know. For evaluating fermentation from a scientific perspective I find percent sugar by weight more valuable. Both tools are equally good at providing that information.