Re: [Cider Workshop] Different readings between hygrometer and refractometer

[Andrew Lea]

I think you mean a hydrometer. A hygrometer is something quite different ;-)

Anyway the short answer is that you cannot use a refractometer to measure SG in a finished cider or wine.. That’s because the presence of alcohol affects the reading as you have found.

In this situation you can only trust the hydrometer. 

Andrew 

Wittenham Hill Cider Portal
www.cider.org.uk

On 21 Jan 2020, at 21:40, Max Marshall <maxhasvi...@gmail.com> wrote:

So I’m bottling today, and plan on bottle conditioning 12 750ml champagne bottles as a little experiment.  I”m figuring out how much yeast/dextrose/SO2 to put in, I have a sample with a hygrometer in it reading 1.000 SG at 60F and VOILA my refractometer is giving me in between 1.015 and 1.020 SG.  I’m playing who do I trust now with both these devices. Both were purchased from my local brew equipment shop.

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[Claude Jolicoeur]

Refractometer will NOT give you a correct reading when there is alcohol present. It can only be used before the fermentation starts.

As of your hydrometer, I suggest you test it with water in order to have an idea if it is well calibrated - some low cost units bought from brew shops may be quite a bit off.

[Max Marshall]

It’s such an honour to have two of my favourite authors respond so quickly but it’s such a shame to have it be for a silly question with a silly spelling error. With no clear option how to edit the thread name. I digress! I have calculated my dextrose and yeast needed for my bottle conditioning but it’s calling for sulfites as well. Should I do like I am sulfiting a must and wait a day before I bottle condition with my yeast/sugar?

[Claude Jolicoeur]

Le mardi 21 janvier 2020 18:29:45 UTC-5, Max Marshall a écrit :

It’s such an honour to have two of my favourite authors respond so quickly

... and fortunately we both said the same thing! (which is not always the case, but most of the time)

calculated my dextrose and yeast needed for my bottle conditioning but it’s calling for sulfites as well.  Should I do like I am sulfiting a must and wait a day before I bottle condition with my yeast/sugar?

 

Adding sulfite for bottle conditioning is not really standard procedure. It won't give you much as all the SO2 will get bound very quickly. The only thing adding sulfite at that stage will do is to prevent lactic acid bacteria (LAB) to perform some malolactic fermentation in the bottle. If this is something you wish to do, you may add some 30 to 50 ppm of SO2. I have made some tests with up to 70 ppm of SO2 at bottling, and this dosage did not prevent bottle conditioning from happening.

Claude

[Max Marshall]

Perfect. I’ll leave SO2 out of the equation!

[Eric Tyira]

Claude/Andrew

Does alcohol in a refractometer give consistent, but incorrect values?  Can true levels of sugar be obtained through some mathematical model?

Or is the refraction random due to a combination of alcohol and sugars in a way that makes it completely useless?

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[Wes Cherry]

It should be possible to adjust for alcohol.  Here’s a method to use a refractometer to monitor sg during fermentation. I haven’t used this method  personally.

http://valleyvintner.com/Refrac_Hydro/Refract_Hydro.htm

-'//es Cherry

Dragon's Head Cider

Vashon Island, Wa US

www.dragonsheadcider.com

On Jan 22, 2020, at 4:47 AM, Eric Tyira <ety...@gmail.com> wrote:



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[Andrew Lea]

I haven’t used it either, but I’m kind of doubtful how well it would hold for all situations. It would need considerable validation IMO. 

It assumes that all the sugar goes to ethanol and CO2 and nothing else, but in real life that’s not actually what happens.  Yeasts produce variable amounts of glycerol, polysaccharides and organic acids (both synthesis and breakdown), all of which are likely to affect the refractive index in a significant way. Because of the way a refractometer works, a small amount of such solutes will likely have a much greater distorting effect on refractive index than on SG when it is measured by a hydrometer (which is more proportional to simple mass of solutes in solution). 

There are existing protocols for allegedly estimating ethanol levels in wines by using a combination of refractive index and hydrometer readings, and these have been around for many years. They are empirically accurate to around 1% ABV but not much more, simply because they do not and cannot take into account the variable amounts of non-alcohol non-sugar stuff that yeasts produce and indeed also metabolise. They are also affected by residual sugar in solution (a factor if wines or ciders are sweetened). Many years ago (maybe on the list which was the predecessor to this one?) there was a long discussion about use of these protocols for determining alcohol levels in cider and it turned out they were a good deal less accurate than had been hoped for. I know some people in the UK do use them as a “rough and ready” guide for ciders, but generally they are not regarded as very reliable. 

Andrew

Wittenham Hill Cider Portal
www.cider.org.uk

On 22 Jan 2020, at 15:37, Wes Cherry <w...@dragonsheadcider.com> wrote:

It should be possible to adjust for alcohol.  Here’s a method to use a refractometer to monitor sg during fermentation. I haven’t used this method  personally.

To view this discussion on the web, visit https://groups.google.com/d/msgid/cider-workshop/B4592693-62B3-43BD-A0C7-B4D1C4D828BB%40dragonsheadcider.com.

[Bartek Knapek]

It is possible to translate a refractometer reading to SG or Brix during fermentation, however it is not super accurate (yet).

I was fermenting 20+ batches of the same juice once, different yeast, taking both hydrometer and refractometer readings along the way.
The relation between the hydrometer and refractometer readings is pretty much linear:

I was able to derive a formula that could translate the refractometer readings into SG with 0.002 accuracy. I believe I could get an even better accuracy, but for the most of the experiment I was using a cheap chinese refractometer, and only for the last part I upgraded to a much better japanese model, and the readings became far less scattered. The formula worked fine only for this particular juice of course, as it was trained against only one Original Gravity. Basically the angle between these two trend lines above will be different for different OGs.

 

 

There are several formulas for correcting the refractometer reading, that account for OG, available in the Internet. The most modern I think is the one by Novotny: https://www.diversity.beer/2017/01/pocitame-nova-korekce-refraktometru.html (google translate is your friend!). It is used in some internet calculators afaik. The thing is that it has been trained with beer, which behaves slightly differently as maltose has other refractive properties than fructose - and because of this, when used for cider it suffers a slight 0.001-0.002 offset; hence overall I guestimate its accuracy for cider to be ~SG 0.004.

It is actually also worth to keep in mind, that even for a pure juice the refractometer reading will not be equal to hydrometer reading! It is only for a pure sucrose solution they will match. In the presence of maltose (hence in beer) the refractometer reading will be greater than hydrometer, hence the beer folks will talk about "wort correction factor" and will divide the refractometer reading by 1.04, if I remember right. In the apple juice it is the other way around - fructose makes the refractometer reading smaller:

Perhaps adding a "cider correction factor" to the formula would futher improve the accuracy.
I need to finally kick-off a little project for collecting data to derive the translation formula for cider.

Anyone would like to contribute with samples?
I only need two reading per sample: initial + after fermentation complete.

//Bartek

[Bartek Knapek]

I have done additional measurements. My observations are:

1. Refractometer reading during fermentation follows decrease of SG/density in a linear way

2. It is possible to estimate the end reading of a refractometer, after fermentation complete

Hence:

3. Refractometer can be used to estimate how far the fermentation progressed so far

4. It is possible to "translate" the refractometer reading during fermentation to a hydrometer reading (although with an error of ~ SG 0,002 / Brix 0,5)

The linear behavior was shown in my previous post. That was done for multiple samples of the same juice, hence same OG and same final alcohol content. Now I have measured aditional 7 juices, with OG ranging from SG 1,040 to SG 1,060. I used digital refractometer Atago PAL-1 and high-precision hydrometer from Alla France with accuracy of 0,0002. The hydrometer is scaled in density, which I converted to Brix using Circular 440 table. Refractometer automatically accounts for temperature, while for hydrometer I applied the corrections manually.

Readings for raw juice show difference between Brix readings from refactometer and hydrometer. This is expected and explained in the previous post.

The x-axis is in hydrometer Brix.

The measurements taken after the fermentation has completed show major difference in the readings of both instruments. Hydrometers and refractometer respond to the presence of alcohol completely differently, which is well known.

There is plenty of calculators in the Internet that "correct" the refractometer readings, to match hydrometer. They have been primarily made for beer. There are several different formulas in use, but I think the most recent is by Novotny, linked in previous post. So I applied this correction to my mesurements, and found that it underestimates the result with ~0,5Brix. I made exactly the same observation when taking samples for my previous post, which makes it >60 samples in total. Good thing is that the results are "consistently" under-estimated! So in principle the beer formula works. If I compensate with adding 0,5 Brix, all the results come into SG 0.002 / Brix 0,5 accuracy window, comparing to the hydrometer heading.

It leads to a final finding, which I think is the most interesting. If I have a formula, I can predict what will be the final reading for a refractometer, for any original SG/Brix:

x-axis is the original Brix of raw juice, y-axis is the expected value after fermentation.

Knowing the expected end value, I can use the refractometer to monitor the fermentation progress same way as I can do with hydrometer. The end value is estimated, so it cannot be expected to be spot-on - but that is exactly the same for hydrometer, where the end value is only expected somewhere around SG 1.000 / Brix 0. In my case, the actual results were always within 0,25 Brix range from the estimate, so quite accurate for usual use.

I realize this is only few samples, so not super trustworthy. But since the patters are quite clear and matching expectations, I thought it would be beneficial if I shared anyway.

By the way, if you want to implement the "Brix correction", this is the formula I have derived so far:

<corrected Brix> = ( <current Brix reading> - 0,366 * <original Brix> + 0,165 ) / ( 0,634 + (0,165 / <original Brix> ) )

This is basically Novotny formula, compensated with +0,5, and updated to give results in Brix.

Curious your thoughts.

//Bartek

W dniu 22.01.2020 o 23:41, Bartek Knapek pisze:

[Erik Nilsson]

Hello Bartek,

This is an old post but I thought I would add some observations on your formula for translating refractometer readings to corresponding hydrometer values in fermented cider.

If I'm understanding things right it seems a simple rule of thumb might give very similar results to your equation: Simply subtracting 150% of the refractometer-measured drop from the initial reading gives close enough "hydrometer" values. This might be useful for giving an estimate without needing a calculator.

To get results even closer to the estimation from your formula, one can subtract 155% instead, and if measured in Oechsle 153% seems spot on.

Attached is a figure comparing the results of the 155%-drop formula (red lines) to the one you presented (black). Separate lines for different starting Brix values (10, 15 & 20). The fit looks very close, indicating the simpler formula might be similarly useful.

If you still have the datapoints I would be happy to test how this aligns with them.

Kind regards, Erik

t

[Bartek Knapek]

Well.. your simple formula works perfectly on my datapoints :)

Good job!

mvh // Bartek

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