Strange Hydrometer And Refractometer Readings

[Brad_G]

Hi guys,

Ive just had some strange and inconsistant readings from my hydrometer compared to my refractometer. Ill tell the full story:

I mashed 2 different brews, at 2 different temps, one at 65/64, and the other at 67/66. Both brews had an original gravity of 1052 as read from my refractometer which i tested at 0 with distilled water (suggested in another thread). Now the strange thing... The beer that I mashed at the lower temperatures reads 6.1brix, which is fine, but my hydrometer reads 1008 (at 20deg which is also tested ok). The beer that I mashed at higher temps reads 5.9brix, but strangely enough, the hydrometer reads 1011???? Im confused somewhere. Im using beersmith to convert the readings.

Any answers to this, I have checked three times now. :unsure:

Brad

 

[AndrewQLD]

Brad,
I have had this problem too when comparing between one instrument and the other, I have also taken 2 refrac readings from the same sample but minutes apart and had up to 1 brix difference. Temperature of the wort and also wort clarity play a big part in the accuracy of your refrac readings. Cloudy worts or worts with trub in them are not the best samples to take and can slew the readings.

My only advice is:
1, stop comparing between your refrac and hydro, accept one as accurate and use that only, otherwise you will go mad comparing them.
2, always use clear, cool wort for your samples. an ATC refrac is temp corrected for the ambient air temp not boiling wort so your sample still needs to be cooled to air temp to read accurately.

Frustrating I know, but a refrac still has similar limits as a hydrometer does and can also give incorrect readings if not used properly.
Also darker beers will not measure as accurately with a refrac as lighter ones are.
Cheers
Andrew

 

[Darren]

Hydrometer is cracked?

 

[Brad_G]

Darren,

Hydrometer is almost new and in perfect condition.

Andrew,

Thanks for your advice. The two brews have different clarity which could be the answer. I think that I will use the refrac only when measuring during the mash/boil process, the hydrometer seems ok at 20deg.

Cheers

 

[Darren]

Vjval,
I think I understand now. You are talking about the final gravities?

Brix and gravity are determined from a refractometer via a complex equation. There are many confounding possibilities to the actual read-out.
Hydrometer is probably a more accurate measure of final gravity as it doesn't rely on a mathematical formulae for conversion.


cheers

Darren

 

[AndrewQLD]

Ahh, sorry I didn't read the original thread properly, Darren is right, post fermentation gravities are extremely inaccurate if read using a refrac because the alcohol creates false readings and the formula to conteract that is very complicated I believe.
I only use the refrac for pre fermentation readings and that is what my original post was refering to.
Remember that a refractometer measures sugar density and not alcohol.

Cheers
Andrew

 

[Brad_G]

Darren,

Yeah, I used the refrac before and after the boil. got my results as its easier to cool a couple mils of wort than say 50ml. After it was cool, just pitched and closed the lid. The latest readings are 6 and 4 days into fermentation. Used beersmith to work out the readings using its algorithm. It asks for tho OG which in this case, both beers came out the same at 1052. I would have thought the readings would have been consistant. Obviously there is something different, and as andrew suggested, it could be the clarity as the younger is quite cloudy.

Ta

 

[tangent]

also don't forget that stratification happens quicker than you think
rolling boiling wort or a freshly racked brew seems to be the most accurate readings for me
you're getting a bigger sample with a hydro, so maybe that could be a variable

 

[sosman]

The formula to convert to FG when you have the original brix reading and the final brix reading I believe is:

def brixToSg2(OB, B):

	return 1.001843 - 0.002318474*OB - 7.775e-6*OB*OB - 3.4e-8*OB*OB*OB + \

		0.00574*B + 3.344e-5*B*B + 8.6e-8*B*B*B

In fact that is a snippet of working python code. I got it from a reference somewhere and I am not sure if I have tried it so beware. It might look complicated but it is just a 3rd order approximation.

 

[Brad_G]

Ok guys, got it. thanks heaps. I will only use the refrac on pre-fermentation

 

[pint of lager]

Alcohol also screws up the density readings of your hydrometer. Alcohol is less dense than water. So the hydrometer is trying to read a solution of three things from a fermented beer, water, remaining dissolved sachaarides and ethanol. The water wants it to float at 1.00, the dissolved sachaarides want it to float at greater than 1.00 and the ethanol want it to float at less than 1.00.

Both the hydrometer and the refractometer have their limitations.

Refractometers are great for tracking brews during fermentation. You do not use much to do a sample every day and the reading will be stable when it has finished fermenting.