Refractometer usage

[Matplat]

Evening all,

I did a bit of a search on this, but could only find about people who hadn't used a calculator to correct their FG reading.

So i just got a refractometer, and immediately calibrated it. Being most of the way through my current fermentation, i thought I would give it a go.

First thing i noticed was that whereas when calibrating with water the line was perfectly defined, it was now much more blurry. I assume this is due to cloudiness from suspended yeast?

I converted my original gravity reading to brix (10.5) so I could use a calculator, using my approximate blurry FG reading of 5 brix, I get an FG of 1.006 however this does not match my hydrometer reading of 1.010?

Where am I going wrong?

Cheers, Matt

 

[Ducatiboy stu]

Honestly........

Refractometer suck. Sorry, but they do. I made the mistake of buying one in the big refractometer rush of 2000 & something. They are a bit of a wank

First thing i noticed was that whereas when calibrating with water the line was perfectly defined, it was now much more blurry. I assume this is due to cloudiness from suspended yeast?

Stick with a Hydro. B)

My kids ended up using it to amuse themselves with the flappy bit at the end....and using it as a bit of a thing to look at small objects with.

 

[Denobrew]

Hey Matplat, have you tried turning the eyeglass to focus it? You probably have, not trying to be a smarty pants. Cheers Tim

 

[mckenry]

Yeah, theyre a pain in the arse. I have one thats retired. But the answer to your question is that they are not for measuring samples that contain alcohol, unless you have the correction factor.

 

[gaijin]

I've plugged your numbers into the formula I use and 1.006 seems to be correct. Not sure why the hydrometer reading is high - was there perhaps a few tiny bubbles in the lens of the refractometer making the fluid seem appear thinner/give a lower reading? Maybe try taking another reading as this has happened to me before.

 

[Danscraftbeer]

Refractometer used only for pre ferment readings.
I use my calibrated refractometer as a quick easy gravity reading any time through the brew process. Then the final confirmation measurement against my Hydrometer for the opening gravity reading.

 

[mikec]

Clear sample and good light will help with readability. You do need to use a calculator to compensate for the alcohol factor but it sounds like you are doing that.

 

[Ducatiboy stu]

Clear sample and good light will help with readability. You do need to use a calculator to compensate for the alcohol factor but it sounds like you are doing that.

And that bit as well that makes them more of a PITFA

 

[MichaelM]

There is a correction factor that needs to be determined for your refractometer. This is done by taking numerous readings for unfermented wort with the refractometer and hydrometer and plugging them into a spreadsheet. This may be why you got a reading of 1.006 compared to a hydrometer reading of 1.010. Brewersfriend website has a spreadsheet for doing this.

 

[Ducatiboy stu]

There is a correction factor that needs to be determined for your refractometer. This is done by taking numerous readings for unfermented wort with the refractometer and hydrometer and plugging them into a spreadsheet. This may be why you got a reading of 1.006 compared to a hydrometer reading of 1.010. Brewersfriend website has a spreadsheet for doing this.

Can we have that in English please

At least the old Hydro has easy to read markings that are easy to read

 

[MichaelM]

Can we have that in English please

At least the old Hydro has easy to read markings that are easy to read

not saying that refractometer is better. Just giving a reason for the discrepancy. At least a hydrometer is more straight forward to use.

 

[stewy]

Refractomoters are great to use after the mash, during boil etc. Once the beer begins fermenting use your hydro

 

[Bribie G]

I have no problem at all with my refrac, used during the boil or before pitching. After that let hydro be your friend.

ed: when I lived on Bribie Island a pineapple farmer (such people are the main users of refracs to measure sugar content, you don't seriously think in a million years that they actually manufacture such devices for the sole use of home brewers, ffs?) had a look at mine and said it's good. The ones with OG scale are a very very small subset of the refracs out there, and they are aimed at home brewers but are just bog standard fruit juice ones.

 

[btrots87]

I only ever use my hydrometer at two points these days, getting an OG just before pitching and confirming the FG once I'm sure the ferment is finished. All other readings I just use the refractometer and plug the numbers into Brewer's Friend which converts it to a gravity reading.

I agree the hydrometer can be a bit easier to read and is potentially more accurate, which is why I use it for the OG and FG readings, but the refractometer has other advantages. It uses a lot less sample which means less beer wasted, and you don't have to worry about cooling a couple hundred mL of sample to get a reading during the mash/boil. I find it really helpful to use to keep an eye on my ferment gravity and getting mash/boil gravities to track efficiency.

For what it's worth I always find that my hydrometer readings and corrected refractometer values are within 1-2 points of each other anyway, even after the ferment. The Brewer's Friend calculator seems to be pretty reliable.

 

[Lyrebird_Cycles]

The refractometer measures the refractive index of the solution.

Refractive index is a colligative property: it depends on the number of solute molecules present*, not on their properties such as molecular weight.

Density is not a colligative property: it is highly dependent on the weight of the solute molecules present.

Therefore the equivalence between refractive index and density changes when the type of molecules in solution changes.

A standard brix refractometer converts the refractive index to the equivalent sucrose solution in weight %.

Since wort solids are not sucrose, a correction factor must be applied which is dependent on the average molecular weight of the wort solids as compared to sucrose.

The average molecular weight of your wort solids depends on the mash profile and the types of malts and adjuncts used so it will vary for each brew.

A good average factor is about 4% eg on an average wort if a densitometer reads 12.5 Brix (or Plato) a refractometer will read about 13.0.

During ferment two things happen that affect the reading: one is the production of alcohol which breaks the rules for solutes and increases the refractive index.

The other is that the yeast uses up the low molecular weight sugars first so the average molecular weight of the remaining wort solids keeps increasing, thus changing the relationship between density and refractive index further.

The calculators commonly used account for the first of these effects but not the second, so they are to be taken with a grain of salt.

Despite this, with a bit of practice you can use a refrac during ferment and get quite good results. You'd just better like maths is all.



* This is on the assumption that the solute molecules themselves do not affect refractive index. This is the case for most solids but it is not the case for dissolved liquids such as ethanol.

 

[Blind Dog]

^^^^

Or what DBS said in English...

 

[Lyrebird_Cycles]

I converted my original gravity reading to brix (10.5) so I could use a calculator, using my approximate blurry FG reading of 5 brix, I get an FG of 1.006 however this does not match my hydrometer reading of 1.010?

Where am I going wrong?

Further to post #15 above, from the best information I can find, the numbers you gave are possible if the average MW of the remaining wort solids corresponds to that of saccharides with a mean degree of polymerisation of about 3.5.

I'm guessing from that figure that we are talking about a wort produced from a highish temperature mash (>66 oC)

 

[Matplat]

So many responses! I pretty much went to bed after posting this... guess I'm turning into a lightweight. I bought the refrac so I could monitor gravity during the mash, monitor gravity of the runnings and so that smaller FG samples needed to be taken. Seems like I will just drop the last one, which isn't such a bad thing because I like tasting the samples anyway.
Lyrebird, you continue to impress me with your understanding and knowledge of the process. This wort was mashed between 63-64, however I'm wondering if something isnt right though because I was expecting FG below 1.010 for that mash temp.

 

[droid]

Come on Stu - refractometers are awesome and much easier to use than hydro for taking BG and OG Gravities. Then like others have said - it's over to the hydro once the yeast is involved.

Take a reading when your 80% full during filling your kettle - you can see how your gravity and of course efficiency is doing. Then at 90%. Much better to be getting the OG exactly where you want it (at least you've made a calculation) than to wait till your filling a fermenter. Measuring your BG then at full boil volume based on your (80% - 90% kettle volume ~ whenever you measure) findings. If you get a better eff you can run some more wort in and alter your hops quantities accordingly.

I got 100ltrs more beer on a commercial system because I ran up to the kettle and did a BG - it was 3 or 4 points higher so we put in an extra 100ltrs and hit the OG exactly, there is no reason to not be doing the same at home.

 

[Mardoo]

They'll never get my refractometer until they pry it from my cold, dead hands. Unless of course I'm doing sugar additions during ferment. Well that's a completely different story.

But, yes, you are relying on relative mathematical extrapolation when you use a refractometer. And on the little piece of paper inside the tube staying in one place when using a hydro.

There's a thread with a massive debate about whether to use the top or bottom of the blurry line to take your refractometer reading. I myself am not seeking absolute precision.

Edit: But you need to get a good one if you get a refractometer. Not worth going cheap, IMO.