When do you take your pre-boil gravity reading

[thunderchild]

Afternoon all, just a curiosity question. When do you take your pre-boil gravity reading?

I ask because I get varying readings from a calibrated refractometer which quite frankly Sh1ts me to tears.

If I take the reading immediately after the completed sparge (Batch) the reading tends to be several points lower than it should be. Of late I have taken a reading at 95 degrees just before the boil starts and that reading seems to be closer to the BeerSmith predictions.

At the end of the day it's not the end of the world but I am not a fan of inconsistency.....

 

[Dan Pratt]

The important thing to dial in is the points of gravity achieved from boil off. On std gravity beers 1040-1055 I get 6 gravity points per hour, basically 1 per 10mins and the same for 90min boils.

If you take a reading do it before you sparge and post sparge but be sure to cool the samples, I use a shot glass and store that in the freezer for a few minutes.

If you are +/- yor BS preboil target then you can calc before the boil starts where it will finish, you can assess a longer boil is you are lower.

 

[droid]

I have temperature compensation but also have discrepancies.

BG is taken at around 80% of total boil volume in the kettle, just to see how things are tracking.

Sometimes though when I have boiled off the right amount the OG is lower than expected, so something is wrong because that's happening after hitting BG. So either the calculation for increased gravity as evaporation occurs is wrong or the initial BG # was wrong.

Dunno

 

[Matplat]

I tend to ignore the refractometer... so bloody irregular, usually just causes me to panic thinking I've missed gravity by a huge margin.

Then I check my SG value with the hydro before pitching, and it's all good!

 

[tj2204]

I tend to ignore the refractometer... so bloody irregular, usually just causes me to panic thinking I've missed gravity by a huge margin.

Then I check my SG value with the hydro before pitching, and it's all good!

I'll second this, I'm still taking refractometer readings before and during the boil but not letting them stress me out anymore.

Hydro gives consistent results refractometer is a gamble.

 

[Rocker1986]

I BIAB with no sparge and don't use a refractometer either. I usually take the sample after the bag has been drained and squeezed, and the actual reading is taken when it cools down. I usually tend to either hit my target pre-boil SG and volume or go slightly over it. The volume may be overstated due to the temperature of the wort but at least it's consistently around the same level so that's good enough for me.

 

[MitchD]

I've witnessed the same thing but I discovered it was stratification, with the pre sparge runnings of higher gravity sitting towards the bottom and the later runnings on top. After running my pump for a few minutes it comes up to normal. Without the pump the convection currents will mix all the sugars giving you the correct reading, it just takes longer.

 

[trevgale]

I use a refractometer and get accurate readings throughout the brewing and fermentation. The key to getting an accurate pre-boil gravity is to take it after a vigorous boil has started. I had the same issues with seemingly inaccurate pre boil measurements, the problem is that the wort stratifies in the kettle during sparging, readings from the bottom of the kettle will be high and low from the top. Stirring did not seem to work very well. Anyway just take the reading after the boil has started an wait for the ATC to work, it can take a couple of minutes if boiling. Since doing this I get accurate repeatable results.

 

[trevgale]

I also batch sparge and if you take a reading as you transfer each batch to the kettle along with the volume you can calculate what you pre boil volume will be based on this. It also allows you to only transfer part of your final batch to get the exact gravity you are targeting if you want to. This gets around having to mix or wait for the boil to get around the stratification issue.

 

[thunderchild]

I use a refractometer and get accurate readings throughout the brewing and fermentation. The key to getting an accurate pre-boil gravity is to take it after a vigorous boil has started. I had the same issues with seemingly inaccurate pre boil measurements, the problem is that the wort stratifies in the kettle during sparging, readings from the bottom of the kettle will be high and low from the top. Stirring did not seem to work very well. Anyway just take the reading after the boil has started an wait for the ATC to work, it can take a couple of minutes if boiling. Since doing this I get accurate repeatable results.

That Was my thinking that led to my 95 deg policy.

I too found stirring useless

 

[Tex083]

@ the 60min mark. Just before the 1st hop addition. I never do a 90min boil. I found this to be most consistent and as long as I don't make an error in sparging I get close or above Beersmith prediction.
Hope it helps. I use a refract on brew day and take 1 OG via hydro. I don't usually test fermenting wort. Just check the FG. I will allow 2 weeks to ferment, I haven't had a stuck ferment yet but I'm avoiding WY3724

 

[trevgale]

If I am really trying to target a pre-boil gravity I will measure the gravity and volume of each batch sparge and calculate what the pre-boil gravity will be. This can be done pretty simply as follows:

Target Pre-boil: 60L of 1.040sg --> 60x40 = 2400 gravity points* required

First runnings - 15L of 1.080sg --> 15x80 = 1200
1st batch sparge - 15L of 1.045sg --> 15x45 = 675
2nd batch sparge - 15L of 1.030sg --> 15x30 = 450

Total before final batch sparge is 2325 meaning that we only need an 75 from the final sparge.

3rd batch sparge - 15L of 1.015sg --> 15x25 = 225

This would give a total of 2550 if all the final batch was used which would result in a pre-boil gravity of 2550/60 = 42.5 --> 1.0425sg

If we wanted to hit our pre-boil target then we only need 75 gravity points which would only require 75/15 = 5L of the final batch with the remaining 10L being topped up with water.

* gravity points it just wat I called it when I was setting up my brewing spreadsheet.

 

[Adr_0]

I've witnessed the same thing but I discovered it was stratification, with the pre sparge runnings of higher gravity sitting towards the bottom and the later runnings on top. After running my pump for a few minutes it comes up to normal. Without the pump the convection currents will mix all the sugars giving you the correct reading, it just takes longer.

This will likely cause hydrometer readings +/- a fair bit as well as refractometer. The pre-boil volume should be thoroughly mixed to prevent this, including running through any sample lines for a little while. If your gravity is short at this point, it SHOULD only be pointing to lower efficiency than expected.

Note that refractometers, including with automatic temperature correction, need the sample to be at room temperature. The ATC goes only as far as correction of the refractive index, and does nothing for different density at different temperatures.

 

[Lyrebird_Cycles]

Your "gravity points" divided by 385* will give you kg of extract. It's not exact but it's good enough for Government work.

In your example above the first runnings have 1200 / 385 = 3.12 kg of extract, the second have 1.75 kg and the third 1.17. The last runnings would have 0.58 kg of extract except you are going to throw away most of it.

This way you can at least calculate the efficiency hit: let's assume for the sake of example that the above figures are for 10 kg of pale malt at 80% FGDB and 4% moisture.

If you kept all your last runnings your efficiency would be 6.62 / 7.68 for 86%. With only the first 5 litres kept your efficiency is going to be 6.23 / 7.68 for 81% so you've lost 5%.

You may object that the cut has to made on wort quality but that's another issue.




*A quick way of doing this in your head is to divide by 400 and add 4%.

 

[Lyrebird_Cycles]

The ATC goes only as far as correction of the refractive index, and does nothing for different density at different temperatures.

The ATC on a Brix refractometer corrects for the effect of temperature on the refractive index of a sucrose solution as per the International Sugar tables. The correction is roughly -0.07 oBx per oC at the concentration levels in which we are interested. Since oBx is a mass fraction it is not affected by solution density as such. I have a copy of the correction tables somewhere if you are interested.

If the solution is pure sucrose and the sample temperature and machine temperature have equilibrated*, the reading would be correct even though it is not at 20 oC.

With solutions made up of many different sugars and other substances (eg wort) the rate of change of reading with temperature is different but the correction factor built into the digital refractometers seems to work reasonably well.



*This is especially important: you can't put a hot sample on a cold machine and expect it to work correctly. I think many of the errors reported are due to lack of equilibration.

 

[Adr_0]

The ATC on a Brix refractometer corrects for the effect of temperature on the refractive index of a sucrose solution as per the International Sugar tables. The correction is roughly -0.07 oBx per oC. at the concentration levels in which we are interested. Since oBx is a mass fraction it is not affected by solution density as such. If the solution was pure sucrose and the sample temperature was within the correction range of the machine* the reading would be correct even though it was not as 20 oC.

I have a copy of the correction tables somewhere if you are interested.

With solutions made up of many different sugars and other substances (eg wort) the rate of change of reading with temperature will be different but the correction factor built into the digital refractometers seems to work reasonably well.

* For the digital machines this range can be quite large eg my little Atago Pal-1 claims to have a range of 0 - 100oC.

The solution is the same - let it cool for a while, 10min say. I have had vastly different readings (1-1.5 plato) on the same wort sample from this.

Most people out there would have the optic refractometers rather than digital. These are meant to have ATC for +/- 10°C from 20°C. If you live anywhere north of Sydney you are probably sitting on the high end of this anyway, so a 45-80°C wort sample needs time to cool.

I daresay there are people who think ATC means any sample at any temperature is accurate.

Brix/Plato vs refractive index is not linear. My original comment was intending to highlight that a small compensation in refractive index does not equal an temperature compensated Brix/Plato - there is still a hell of a lot of error.

 

[Lyrebird_Cycles]

The solution is the same - let it cool for a while, 10min say. I have had vastly different readings (1-1.5 plato) on the same wort sample from this.

See comment above re equilibration, that's probably your problem there rather than the ATC.


Edit: I was editing my post above while you were replying, sorry about the confusion.

 

[Danscraftbeer]

Afternoon all, just a curiosity question. When do you take your pre-boil gravity reading?

I ask because I get varying readings from a calibrated refractometer which quite frankly Sh1ts me to tears.

Me too. I gave up from frustration stuffing around never being able to get a consistent reading trying all the usual advice and I cant see any advantage to get a pre boil reading. Mainly because all other estimated targets fall in line in the end anyhow.

 

[DUANNE]

i dont bother anymore. been running the same system for so long that unless i stuff something up along the way i know i will end up within a point or two of my predicted gravity at the end of the boil.

 

[Lyrebird_Cycles]

The solution is the same - let it cool for a while, 10min say. I have had vastly different readings (1-1.5 plato) on the same wort sample from this.

Brix/Plato vs refractive index is not linear. My original comment was intending to highlight that a small compensation in refractive index does not equal an temperature compensated Brix/Plato - there is still a hell of a lot of error.

OK I just ran a test on todays forerunnings which are at ~75 oC. All readings are in Brix.

I took a sample and cooled it to 20.0 oC (measured with a calibrated thermometer). Three sub samples were each allowed to equilibrate on the machine for approx one minute after which they measured 18.8, 18.8 and 18.8.

I took three samples from the runnings (hot) which were each allowed to equilibrate* on the machine for approx 1 minute after which they read 18.8, 18.7 and 18.8.

Equilibration was performed by taking successive readings on the same sample until it stabilised. The machine takes approximately 6 seconds to take a reading then it takes me another second or two to note the reading and press the go button for the next. A typical sequence was 18.0, 18.3, 18.5, 18.6, 18.7, 18.8, 18.8, 18.8, you can see the equilibration actually takes less than a minute.

The stated accuracy of the machine is +/- 0.2 oP but I find it's usually repeatable to +/- 0.1 on similar samples.