Strangely low SG reading part way through fermentation

[Jase]

Just took a sample from my FV and it is very low. Fermentation should be no where near complete. I pitched 1L starter of Wyeast 2308 (Munich Lager) just 36 hours ago and it is fermenting at 13C. My OG was 11.5B (1046). Just took a sample this morning and the refractometer is reading 5.8, which translates to a Specific Gravity of 1009. The airlock is showing considerable activity, so why is it that the reading is so low?

The one thing about this GF conical fermenter is that taking a sample is super easy, and I only needed a teaspoon worth, so I could use the refractometer. Could it be that this tiny sample was too small, and it has actually completely fermented? Or os this something to do with density of the wort, and sampling from the bottom, with a bottom fermenting yeast will give unpredictable results?

thanks

J

[Eric]

The calculator you used to convert your reading?

Try that embedded in Graham's Beer Engine downloadable from here.

The difference might surprise you.

[guypettigrew]

The calculator you used to convert your reading?

Try that embedded in Graham's Beer Engine downloadable from here.

The difference might surprise you.

I'm surprised, very surprised!!

The calculator I use is the one on Northern Brewer. This gives an OG of 1.050 for a Brix reading of 12.4, my last brew. Graham's BE calculator gives 1.048. Very different, and ruins my % efficiency!

Today the brew is showing a brix of 6.0. NB calculates this as a gravity of 1.007. Very low, as in the post from Jase. GW's calculator gives it as 1.017. The hydrometer shows about 1.013.

What's going on here, Eric?

Guy

[clarets7]

I use this calculator for my refractometer readings: - http://seanterrill.com/2012/01/06/refra ... alculator/

[guypettigrew]

Similar results to Graham's calculator.

Does this mean I've been overestimating my efficiency all this time?

Guy

[Eric]

Similar results to Graham's calculator.

Does this mean I've been overestimating my efficiency all this time?

Guy

No Guy, just the alcohol level.

A refractometer measures refraction of light through a liquid medium. Ours are calibrated for a specific type of sugar. We use them to measure the combination of various sugars in wort we create, often from a mix of different ingredients. For this purpose they do a fine job.

A problem comes when using them to measure a mix of alcohols with sugars, mostly non-fermentable, because of that refractive index being different, made worse by various methods to calculate the result.

[Eric]

Using Graham's calculator, it postulates a gravity reading of 1017 from those given figures. I find Graham's is usually a lot more accurate than the American ones and would suggest from my own findings the gravity would likely be 1016 if measured by hydrometer.

[Jase]

The brewersfriend calculator considers this with their calculations. I've been using it for a while and compared to my hydrometer readings which I took when first switching over to a refractometer, are pretty accurate.
Around 6 on my refractometer is normally what I read when fermentation is finished; obviously it varies a little depending on the batch and the yeast. Its only been going 36 hours, so shouldn't be anywhere near that (around 1016-1009).

[guypettigrew]

Thanks Eric.. Explanation much appreciated.

But it begs the question; why would the American calculators be so different to the UK ones?

Surely Brix is Brix, gravity is gravity? In the same way as a meter is a metre and a yard is a yard.

Guy

[IPA]

Thanks Eric.. Explanation much appreciated.

But it begs the question; why would the American calculators be so different to the UK ones?

Surely Brix is Brix, gravity is gravity? In the same way as a meter is a metre and a yard is a yard.

Guy

There is also a problem with American and Australian colour calculators.

[f00b4r]

Thanks Eric.. Explanation much appreciated.

But it begs the question; why would the American calculators be so different to the UK ones?

Surely Brix is Brix, gravity is gravity? In the same way as a meter is a metre and a yard is a yard.

Guy

The problem is that there alcohol from fermentation messes up the readings. The calculators attempt to compensate for this in different ways (from my experience relatively successfully - I use the Sean T cubic version). However, to complicate things even further, the grist will also make a difference to things and a “wort correction factor” that is used in these (all?) calculators should be reflective of the grist.

[Jase]

Took another sample this morning, a slightly larger one around 30-35 ml. This time the reading came in at 7.6B. So much more in line with what I would have expected. Perhaps it was because the previous sample was just too small, maybe only 1 or 2 ml, that it gave such a low reading.

The conversion calculator that I use puts the current SG around 1021. Given the earlier comments though, about calculator differences, I might just chuck my hydrometer in, when fermentation is complete and compare the end result with the calculated one. Haven't done that in ages, as I felt that the calculated values were pretty accurate, but now I have a little doubt in my mind now...

[guypettigrew]

Jase: 30-35 ml? What refractometer do you use which needs a sample this size?

Eric and f00b4r: Thanks. I've just had a read through Graham's notes on his refractometer calculation pop up on Beer Engine. He explains how to check the correction factor, which he's set at 1.04. I'll give it a go next brew..

Guy

[McMullan]

It's important when sampling anything that a representative sample (to whatever) is collected. A few drops from the inside of an FV tap, for instance, might not represent what's actually in the FV. Ideally, things are mixed before taking a sample, but, obviously, not always desirable in home brew. There's actually a good reason why I choose to use an 'old fashioned' hydrometer to monitor fermentation. It can tell you a lot about the beer, especially if it's hidden behind a stainless steel wall. In home brew, the most basic tools are often the best, in terms of cost and function.

[Jase]

Jase: 30-35 ml? What refractometer do you use which needs a sample this size?

No, I just took a larger sample into the glass, to ensure it was suitably mixed. I only needed a tiny tiny amount to go on the glass plate. Did it a few times to make sure it was accurate, tasted the sample (of course) and then chucked it (shame).

Eric and f00b4r: Thanks. I've just had a read through Graham's notes on his refractometer calculation pop up on Beer Engine. He explains how to check the correction factor, which he's set at 1.04. I'll give it a go next brew..

Can you tell me how to access this feature? I went all the way through BE several times (well, the python version) and couldn't see it anywhere. Brewers friend also talk about their "wort correction factor" which is often around 1.04, however they refer to a calibration process so as to ascertain exactly what that correction factor is, as it can vary starting from 1.00 (no correction factor at all).