Refractometer vs hydrometer reading

[malt81]

Hi. I have used hydrometer so far. Now I bought refractometer also.
After mashing refractometer showed SG 10.6. I converted this value using different tables and calculators and it is the same like 1.043. BeerSmith says even 1.041. Refractometer has also SG scale and it showed appr. 1.043. But hydrometer showed 1.046, so quite big difference. Est OG by recipe is 1.046 also. Both, refractometer and hydrometer show 1.000 in plain table water.

So, why is there difference between refractometer and hydrometer readings? I couldn't find any answer from internet.

Another question - stable measurement values with refractometer should also show that fermentation is done? I know I cant trust the values because of alcohol. FG should be measured with hydrometer.

Thanks

 

[GrogNerd]

.003 difference? .3% (approx)?

not quite so big, at least well within my level of tolerance

calibrate in DISTILLED, not drinking water. at whatever temps are indicated for the instrument.

& yes, don't trust the value of the refractometer. I have limited experience (just bought one), so I'm not sure about the reliability of the stable measurements

 

[bernardsmith]

Not a scientist, malt81 but I can think of two good reasons to account for the difference between the readings a refractometer gives and the reading an hydrometer provides. First both instruments measure two very different phenomena but it is those phenomena when measured that we can use to ESTIMATE the alcohol content of a liquid. But neither instrument is DIRECTLY measuring the alcohol content: A refractometer measures the refraction (bending) of light through the liquid. An hydrometer measures the density of a liquid. Presumably there can be all manner of things in the liquid that will cause the light to bend and so give you a reading but there is no really good basis to assume that how the light bends is always equivalent to the density of that same liquid. That there is a calculator to convert one to the other does not mean that the conversion is exact.
The other reason is that the real difference between 1.043 and 1.046 may be less significant than you assume. An hydrometer's reading depends on the angle at which you view the miniscus, depends on the temperature of the liquid, depends on the viscosity of the liquid, on the accuracy of the instrument itself... we are talking about a device that costs about $10... and we are making all kinds of assumptions about the surface flatness etc of the table on which the hydrometer stands. Alcohol that is 5.6% ABV or 5.3 ABV is for all intents and purposes about the same... (using a rule of thumb calculation to estimate the ABV).

 

[kh54s10]

The refractometer should be just as accurate as a hydrometer before fermentation. It is skewed with the presence of alcohol and you need to use a correction calculator.

Either instrument could be off if the sample is not near the temperature it is calibrated to.

Distilled water should read 1.000, tap water could be the same or different depending on it's chemistry.

If you are not concerned with the actual FG number. Using a refractometer and just looking for the same reading to confirm it is stable is perfectly acceptable.

 

[seminaryfleming]

To add to what those above said, each refractometer has a different "wort conversion factor." One online calculator (I can't remember which) has a default of 1.08. Playing with the numbers I come up with .93 as a conversion factor for your readings. That seems to me to be a reasonable conversion factor.

I use a refractometer in part because it uses significantly less wort making 1 gallon batches feasible. However I have learned to take my OG and FG readings with a hydrometer to double check. (I sanitize the hydrometer, turkey baster I use as a wine thief, and jar along with my other equipment so I can return those two samples to my batch.)

 

[Johow]

It doesn't take much of a temperature variance to be off by 3 points.

 

[malt81]

Thank you. All makes sense
I just keep using both instruments for FG to get some statistic, for SG I use refractometer and save beer

 

[PADave]

.003 is not much. I bought a refractometer and tried using it when I was doing 1 gallon brews. Couldn't get a consistent reading or anything accurate. I'd even take a reading, then take another within minutes and couldn't get anything to repeat. Once I moved up to 5 gallon batches I would take both, a hydrometer and refractometer reading and could never get anything that was close. So I gave up on the refractometer and only use the hydrometer.