Refractometer software, WARNING!

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gregorach
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Re: Refractometer software, WARNING!

Post by gregorach » Thu Jan 12, 2012 10:20 am

andybiochem wrote:The quality of the refractometers on the market (ebay, brew shops, etc) will vary wildly in quality. How do you know that the brix reading given is exactly what it should be? Even the most expensive unit I can find on ebay says nothing about the traceability of the calibration process, let alone give details on regression analysis in comparison to gold standards. There is just no interest in true scientific standardisation at this level of the market. If we were all to measure exactly the same beer with all our different refractometers we'd probably have a whole spectrum of results. As we each plug our brix values into a generic equation, some will have good outcomes, some not.
Very good point there Andy. Lord knows I bang on enough about proper calibration for expensive thermometers and pH meters, so expecting perfect calibration from a sub-20-quid refractometer is clearly being overly optimistic.

Mind you, I'm not exactly convinced about the accuracy of my hydrometer either... Not much to be done about it, short of buying a set of proper excise-certified ones.
Cheers

Dunc

andybiochem

Re: Refractometer software, WARNING!

Post by andybiochem » Thu Jan 12, 2012 11:01 am

A bit more on this...

Last night I got thinking about an experiment to see if my refractometer gives expected specific gravity results when using the onebeer/primetab equation for brix-to-FG in the presence of alcohol (via my BrixCalc app). I did the experiment this morning, however, I have a sneaky feeling that the might be some circular reasoning flaw to it, although I have so far convinced myself there isn't...please shout out if you spot logic issues here [-o< :lol:

Here goes,
experiment 1 :
- I mixed some hot & cold water together to get 20°C.
- I then added sugar to the water until it was about 1.016 with my glass hydrometer.
- Then I took 90 mls of this water and added 10mls of 37.5% ABV Gin (tesco value - used for air locks!!), giving a final alcohol content of 3.75%.
- I then measured the gravity again with the glass hydrometer: now 1.009.
- I also quickly measured the Brix with my digital refractometer (3.9%).
- In order to end up with 3.75% alcohol at 1.009, one would need to start with a sugar solution of 1.0375 (back-calculated).
- Calculating the original brix from the 1.0375 gives 8.4% (all calculators seem to agree on OB-to-OG results).
- Putting all this data (starting brix of 8.4, final brix of 3.9) back into BrixCalc gives me:
A predicted Final Gravity of 1.009 (exactly as measured with the glass hydro 1.009)
A calculated ABV of 3.7% (slightly below actual...but this is a different equation again)

experiment 2 (same as above, but more careful) :
- I mixed some hot & cold water together to get exactly 200mls at 20°C.
- I added 8g of sugar to the water to get a gravity of 1.020 (measured with the glass hydro to check)
- I took 84mls of the water and added 16mls of the 37.5% Gin, to give a final alcohol content of 6%.
- I re-read the gravity of the solution to be 1.018 with the glass hydro.
- Quickly measured the brix with the refractometer (7.5%)
- To end up with 6% alcohol at 1.018, one would need to start with a sugar solution of 1.062
- Calculating an original brix from 1.062 gives 14.3%
- Putting all this data (OB = 14.3, FB = 7.5%) into BrixCalc gives:
A predicted Final Gravity of 1.016 (0.002 below the glass hydro of 1.018)
A calculated ABV of 6.0% (as prepared)

In short, this is an experiment to see what effect alcohol has on the refractometry measurement of brix, and the conversion of that to specific gravity using the equation at primetab. For me, the equation works acceptably (spot on in the 1st experiment, 0.002 out in the 2nd) with the correction I set in my BrixCalc app of +0.004 on the calculated SGs.

In simpler, more practical terms, imagine I brew a beer with a starting Brix of 14.3%. All calculators seem to agree that this converts to 1.062.
I let it ferment for a couple of weeks, until a good 4 or 5 days of no activity, and get ready to bottle it. I take my refractometer, and it tells me that the Final Brix is now 7.5%. Putting this into BrixCalc tells me that the Final Gravity is 1.016. The important point now is that if I were to dust off my glass hydrometer and measure the actual Final Gravity, it would say 1.018 ish... roughly the same as predicted. I'm now happy to bottle without priming (I don't prime anymore anyway).

Putting this data in BeerEngine's calculator leaves me wondering why my beer is stuck at 1.025.

I think BeerEngine is great, I used it exclusively for my first 20 or so beers, and still use it in conjunction with another brewing program. However, for my gear, the refractometry tool doesn't match up to what my glass hydro says....even after setting the calibration factor. This is simply a mis-match of equipment and calculations, so I use a different combination of the two in order to get useable results.

jonnyt

Re: Refractometer software, WARNING!

Post by jonnyt » Thu Jan 12, 2012 11:20 am

But you still have the sugar in solution, which you don't when brewing.

andybiochem

Re: Refractometer software, WARNING!

Post by andybiochem » Thu Jan 12, 2012 11:29 am

jonnyt wrote:But you still have the sugar in solution, which you don't when brewing.
Thanks for the reply, although I'm not quite sure I get what you mean...

There is sugar in solution in beer after fermentation, that's why the hydrometer floats above 1.000.
These are sugars unfermentable by yeast, but sugar nonetheless.
There's an assumption when using a refractometer that all sugars act in the same way when bending light, as you'll never know what concoction of sugars you'll actually end up with in your beer.
In fact, using simple sugars in solution to test the effect will give a more accurate result than using an actual beer with unknown sugars.

Manx Guy

Re: Refractometer software, WARNING!

Post by Manx Guy » Thu Jan 12, 2012 12:29 pm

Very interesting thread...

I do use my refractometer when brewing but dont rely on it during or post fermentation...
Any results I get from a small sample I verify with my hydrometer before bottling or racking etc.

8)
Guy
Last edited by Manx Guy on Thu Jan 12, 2012 8:20 pm, edited 1 time in total.

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Re: Refractometer software, WARNING!

Post by Aleman » Thu Jan 12, 2012 12:50 pm

As long as the Brix value is constant over three days at the end of fermentation it is safe to bottle.

Graham

Re: Refractometer software, WARNING!

Post by Graham » Thu Jan 12, 2012 1:53 pm

andybiochem wrote: Here goes,
experiment 1 :
- I mixed some hot & cold water together to get 20°C.
- I then added sugar to the water until it was about 1.016 with my glass hydrometer.
- Then I took 90 mls of this water and added 10mls of 37.5% ABV Gin (tesco value - used for air locks!!), giving a final alcohol content of 3.75%.
- I then measured the gravity again with the glass hydrometer: now 1.009.
- I also quickly measured the Brix with my digital refractometer (3.9%).
- In order to end up with 3.75% alcohol at 1.009, one would need to start with a sugar solution of 1.0375 (back-calculated).
- Calculating the original brix from the 1.0375 gives 8.4% (all calculators seem to agree on OB-to-OG results).
- Putting all this data (starting brix of 8.4, final brix of 3.9) back into BrixCalc gives me:
A predicted Final Gravity of 1.009 (exactly as measured with the glass hydro 1.009)
A calculated ABV of 3.7% (slightly below actual...but this is a different equation again)

experiment 2 (same as above, but more careful) :
- I mixed some hot & cold water together to get exactly 200mls at 20°C.
- I added 8g of sugar to the water to get a gravity of 1.020 (measured with the glass hydro to check)
- I took 84mls of the water and added 16mls of the 37.5% Gin, to give a final alcohol content of 6%.
- I re-read the gravity of the solution to be 1.018 with the glass hydro.
- Quickly measured the brix with the refractometer (7.5%)
- To end up with 6% alcohol at 1.018, one would need to start with a sugar solution of 1.062
- Calculating an original brix from 1.062 gives 14.3%
- Putting all this data (OB = 14.3, FB = 7.5%) into BrixCalc gives:
A predicted Final Gravity of 1.016 (0.002 below the glass hydro of 1.018)
A calculated ABV of 6.0% (as prepared)
There is something wrong with either your methodology or instrumentation.
Take the second experiment, cos that has the largest idiosyncrasy. There is no way that a 1.020 solution with 6% alcohol added is going to read 1.018 by hydrometer.

(94*1.020 + 6*0.789)/100 = 1.0061

Your figure is approximately 12 points out, which is is the sort of error that many people find the aforementioned formula to be out by.

andybiochem

Re: Refractometer software, WARNING!

Post by andybiochem » Thu Jan 12, 2012 3:15 pm

Graham wrote: There is something wrong with either your methodology or instrumentation.
Take the second experiment, cos that has the largest idiosyncrasy. There is no way that a 1.020 solution with 6% alcohol added is going to read 1.018 by hydrometer.

(94*1.020 + 6*0.789)/100 = 1.0061

Your figure is approximately 12 points out, which is is the sort of error that many people find the aforementioned formula to be out by.
Thanks Graham!

I'll re-run the experiment again, I did have a lot of alcohol solutions flying around at this point from the first experiment, but I'm sure it was correct... I'll also use a different liquor - that Gin was quite old.

post back soon!

andybiochem

Re: Refractometer software, WARNING!

Post by andybiochem » Thu Jan 12, 2012 3:55 pm

Ok just redone the second experiment with a fresh bottle 37.5% Smirnoff, and indeed the SG went from 1.016 to 1.006 with the addition of vodka.

Looks like I have a bottle of 5% Tesco Value Gin :roll: :evil:

So, here's the second experiment AGAIN, but with the Vodka...

- Added 16g sugar (used 2x as much) to 200mls 20 degree water.
- Hydro says this is 1.030, Brix reading 7.0% (expected 1.036...perhaps some sugar didn't dissolve?)
- Added 16mls Vodka to 84mls sugar soln, to give 6% ABV
- Hydro now says 1.018 (hurrah!), Brix reading 7.8%
- Again assuming a pre-ferment value of 14.3% brix (1.062)
- Putting into BrixCalc (OB=14.3, FB=7.8 ) gives:
A predicted Final Gravity of 1.018 (same as the glass hydro of 1.018)
A calculated ABV of 5.8%


Despite the Gin debacle, the figures still stand up.

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Re: Refractometer software, WARNING!

Post by Andy » Thu Jan 12, 2012 4:42 pm

andybiochem wrote:Ok just redone the second experiment with a fresh bottle 37.5% Smirnoff, and indeed the SG went from 1.016 to 1.006 with the addition of vodka.

Looks like I have a bottle of 5% Tesco Value Gin :roll: :evil:

So, here's the second experiment AGAIN, but with the Vodka...

- Added 16g sugar (used 2x as much) to 200mls 20 degree water.
- Hydro says this is 1.030, Brix reading 7.0% (expected 1.036...perhaps some sugar didn't dissolve?)
- Added 16mls Vodka to 84mls sugar soln, to give 6% ABV
- Hydro now says 1.018 (hurrah!), Brix reading 7.8%
- Again assuming a pre-ferment value of 14.3% brix (1.062)
- Putting into BrixCalc (OB=14.3, FB=7.8 ) gives:
A predicted Final Gravity of 1.018 (same as the glass hydro of 1.018)
A calculated ABV of 5.8%


Despite the Gin debacle, the figures still stand up.

But the hydro still reads 1.018 after adding the alcohol ? I though Graham's point was that such a value was incorrect ?
Dan!

andybiochem

Re: Refractometer software, WARNING!

Post by andybiochem » Thu Jan 12, 2012 4:52 pm

Andy wrote: But the hydro still reads 1.018 after adding the alcohol ? I though Graham's point was that such a value was incorrect ?
I doubled the amount of sugar in the solution to end up with something closer to an end point than 1.006.

So in this case:

(94*1.030 + 6*0.789)/100 = 1.0155

1.016 ≈ 1.018

I put the difference down to measurement error of the equipment (hydro, accurate liquor ABV?, etc).

boingy

Re: Refractometer software, WARNING!

Post by boingy » Thu Jan 12, 2012 4:58 pm

I have a much more pragmatic solution to this particular problem - I don't use the refractometer once fermentation has started.
Once you are familiar with your process you can get pretty good at estimating when fermentation is done. At that point the 100ml hydrometer sample gives you a perfect excuse for a crafty first snifter of the beer.

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Re: Refractometer software, WARNING!

Post by GrowlingDogBeer » Thu Jan 12, 2012 5:20 pm

I often can't be bothered to get the refractometer or hydrometer out, so I just have a little taste.

Does it taste sweet and yeasty?
Yes, then leave a bit longer.
No, get it in the keg then.

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Re: Refractometer software, WARNING!

Post by gregorach » Thu Jan 12, 2012 5:59 pm

But what about us data junkies? I like to know that my fermentation is following the expected gravity / pH curve established by previous runs. There's at least one time that I had set my fermentation temperature wrong, but only noticed it because I didn't see the gravity and pH dropping at the expected rate over the first 24 hours...
Cheers

Dunc

Martin G

Re: Refractometer software, WARNING!

Post by Martin G » Thu Jan 12, 2012 6:08 pm

Just a thought and I stand to be corrected but isn't there a dilution of the sugar solution when the gin is added? It looks to me like 84ml of solution is diluted to 100ml. So instead of;

(94*1.030 + 6*0.789)/100 = 1.0155

it should be more like;

(94*1.025 + 6*0.789)/100 = 1.011

There you go data junkies, have a think about that and rip it appart!

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