Which FG measurement to believe?

[MrDave]

Much like the man with two watches who never knows what time it actually is, I'm struggling to decide where my beer is at.

Haven't brewed for a while and the first couple of weeks of COVID-19 related WFH had me thinking I better slap something together fast, so time for an extract batch.
Darling Wife keeps saying "Make that Honey Porter again", so I look at what's in the cupboard and decide to start with Coopers Stout.
Ingredients were Coopers Stout tin (1.7kg), 1.25kg of LHBS' "Stout Supercharger" and 1kg of Honey. I pitched a single rehydrated packet of Danstar Nottingham yeast.
That stuff, in a 22L batch should get me an OG of about 1055.

I didn't measure SG with a hydrometer or refractometer, but I did remember to sanitise and throw my Tilt into the fermenter.
It tells me 1.057, which was within coo-ee, so I thought I was golden.
Today is day 10, and it's reading 1.010.
Sounds about right, but I have this nagging doubt that the Tilt isn't calibrated, so I grab the refractometer.

Refractometer says 1.030. If we assume 1.055 to start, that's only 44% attenuation and the sample should taste like hopped cola.
Take a sip - it tastes like a stout that is pretty much finished, has a hint of sweetness and appropriate roastiness on the finish.

Plop a glass hydrometer into the graduated cylinder with the sample in it.
Hydrometer says 1.020. I've been fermenting at about 20C, which is the appropriate temperature for the hydrometer's scale.
Grab another hydrometer. 1.020 again.

I just splashed the fermenter around a bit in case the Tilt was having a lay down. It soon settled back to 1.010.
I guess the thing to do is keg it, keep a sample in a jar, re-calibrate the Tilt and re-test.

Anyone want to posit a theory?
If I sent a sample to a lab, I'd be putting my money on the glass hydrometer.

 

[philrob]

You need to adjust your refractometer reading for the alcohol content.
Here's your chart to do just that.
I brew AG and use my refractometer only during the brewing process. Once my batch is done, I use an hydrometer to measure my FG, so I don't have to faff about with this chart.

 

[MrDave]

You need to adjust your refractometer reading for the alcohol content.
Here's your chart to do just that.
I brew AG and use my refractometer only during the brewing process. Once my batch is done, I use an hydrometer to measure my FG, so I don't have to faff about with this chart.

Nice work @philrob.
That's an interesting result. I too would typically only use the refractometer when brewing AG and hadn't considered the change in refractive index that the alcohol would create. Thank you for the chart.
Adjusting per the chart, it seems that the Tilt and the refractometer concur

I guess I could take another sample for the hydrometer and check its temperature, in case I need to adjust for that.

 

[BrewLizard]

If two hydrometers are agreeing on 1.020, then that's probably your most accurate bet.

Can you make a sugar solution of 1.020 (5% sugar) and check your hydrometers vs. your tilt?

 

[MHB]

I get closer to 1.0586 as an OG, given 1.020 as an FG you are looking at 5.2% alcohol but<66% Apparent Attenuation.
I would expect a bit more out of Nottingham, mind you the stout supercharge might have a wack of Lactose in it that wont ferment.
With 1 packet you have under pitched which wont help at all, if you were thinking of bottling I would be a bit careful, it probably isn't all the way there yet.
Mark

 

[MrDave]

If two hydrometers are agreeing on 1.020, then that's probably your most accurate bet.

Can you make a sugar solution of 1.020 (5% sugar) and check your hydrometers vs. your tilt?

Hydrometers were within one point of each another.
Tilt was out by about 12.

 

[MrDave]

I get closer to 1.0586 as an OG, given 1.020 as an FG you are looking at 5.2% alcohol but<66% Apparent Attenuation.
I would expect a bit more out of Nottingham, mind you the stout supercharge might have a wack of Lactose in it that wont ferment.
With 1 packet you have under pitched which wont help at all, if you were thinking of bottling I would be a bit careful, it probably isn't all the way there yet.
Mark

Fair call.
Kegged it today and the hydrometers were indicating more like 1.014 so perhaps it hadn't finished after all.
I've calibrated the Tilt for both RD and temp.
I've got another Coopers "Ruby Porter" tin to make. I've grabbed a magnetic stirrer (need distractions during social distancing time) and will build a decent starter for that brew.

Will be sure to take a hydrometer reading and ensure the Tilt is not talking jibberish before next ferment.

 

[davemac]

According to this calculator, assuming a 1.04 correction factor for your hydrometer, Refractometer Calculator - Brewer's Friend your FG is at 1.020