Pre-boil Gravity Accuracy Problem

[Heaviside]

Hi All,

This is my first post on AHB and as a new AG brewer I'm pretty impressed with the level of expertise and knowledge of all things brewing shown here.

I've been missing my target OG values by a couple of points so to track down whats happening I started taking pre-boil gravity measurements for my last two brews so that I could adjust my kettle volume. The results were surprising!

The first time my pre-boil gravity was high by about 10%, so I adjusted my volume to correct then started the boil. After the boil I measured my gravity again and found it was about 10% too low! I was so surprised I remeasured with the same results. I know my evaporation rate accurately, and I'm using a refractometer so I assumed my initial measurement was just plain wrong.

The next brew I made sure I'd stirred the kettle before taking any measurement. I took two refractometer readings and a hydro sample for good measure and all were consistantly about 10% too high. So I adjusted the pre-boil volume again. After the boil I found my gravity was again too low by about 10%.

If I'd not made any adjustments, both OG values would have been correct - so I'm puzzled.What could cause my pre-boil measurements to be wrong? Would break material in the wort affect my gravity measurements before it is precipitated out during the boil? The post boil readings all showed the expected amount of fermentable material in the wort, whereas the preboil readings seemed to show that I had more than expected.

Has anyone experienced anything similar, or have any explanation for why my pre-boil readings seem to be wrong?

Thanks for reading and looking forward to any ideas/suggestions.

 

[cubbie]

I have always had issues with gravity readings in the kettle. 1-2 points out or 500-1000ml out can give significant difference in a 24L brew. I still take the readings but am less concerned if I am a fraction out from what was expected.

Will be interested in the responses you get.

have you made adjustments for temperature?

 

[mika]

Always had that problem as well. Gave up taking pre-boil and just take post-boil values, which is ultimately what matters.

 

[matr]

Always had that problem as well. Gave up taking pre-boil and just take post-boil values, which is ultimately what matters.

I thought pre boil would matter as it would affect your hop utilisation. If you have poor mash efficiency then you will end up with higher IBU.

If you measure it you can make adjustments.

Cheers, Mat.

 

[cubbie]

I thought pre boil would matter as it would affect your hop utilisation. If you have poor mash efficiency then you will end up with higher IBU.

If you measure it you can make adjustments.

Cheers, Mat.

You are correct, but you are probably only taking about a couple points of IBU. On a larger scale this becomes more important.

 

[beerdrinkingbob]

i do preboil just with a hydro and use the hydro temp adjust on Beersmith, my post boil usually ends up between 2 to 6 above, this would be from the 4 ltrs of top up sparge water but ends up where i expected.

It really doesn't make sense for it to lower unless you are topping up with water through the boil.

 

[Maxt]

It sounds like you are doing all the right things in taking measurements, (stirring, adjusting for temp etc). The next culprit is your brewing software...I imagine this is where you are getting your projected figures from?
All you are after is consistency, so next brew, if you are hitting the same numbers, just use this a 'set point' and ignore what the software tells you. For instance, Beersmith never gets my pre and post boil gravities correct, even adjusting for cooling, water loss and evaporation. I just know how much I will boil off, and how many gravity points this will give me. I can also rely on my mashing to be consistent, so normally I knw that x amount of grain with x amount of water will give me x gravity. It's all about getting to know your gear.
I have even been not checking the starting gravity on some beers lately, because I have been using the same gear doing the same beers for so long...I know what I'll get.

 

[MHB]

If we assume that the refractometer is working well and that you have zeroed it then it's most likely to be your volume reading. How sure are you that it's accurate?
As Cubbie said it doesn't take a very large error in either reading to give the sort of error you are reporting.

Handy little equation known by a few names but mostly it's called "The Standard Dilution" equation.
C₁V₁=C₂V₂ where C is the Concentration and V is the Volume
Let's just say you started with 28 L of wort at a gravity of 1.044 and you boiled away 3 L of water (now this is important leave out the 1 part it's not there OK).
0.044*28=25* C₂ or (0.044*28)/25= C₂ = 0.04928 or 1.050 near as dam it.
Try the same thing with a 1L error
0.44*29=25* C₂ or (0.044*29)/25 = C₂ = 0.05104or 1.051
(0.04928/0.05104)*100= 96.55%

OH and remember that your wort contracts by 4% when it cools, if you are taking volume measurements at boiling and ambient you have to watch for that, it amounts to about a 4% error in 25 L.
Errors always accumulate so it probably isn't just one thing you need to find. Make sure your refractometer is calibrated, but look at your volumes and just check that wort contraction isn't compounding the error.

MHB

 

[Heaviside]

i do preboil just with a hydro and use the hydro temp adjust on Beersmith, my post boil usually ends up between 2 to 6 above, this would be from the 4 ltrs of top up sparge water but ends up where i expected.

It really doesn't make sense for it to lower unless you are topping up with water through the boil.

I agree this sounds strange. I'm not topping up the volume during the boil, so I suspect that the adjustment I made before the boil made the pre-boil gravity too low. I know what the evaporation loss for my system is, so the post boil gravity would also be too low. Maybe I should do as others have suggested and not measure it.

 

[Heaviside]

It sounds like you are doing all the right things in taking measurements, (stirring, adjusting for temp etc). The next culprit is your brewing software...I imagine this is where you are getting your projected figures from?
All you are after is consistency, so next brew, if you are hitting the same numbers, just use this a 'set point' and ignore what the software tells you. For instance, Beersmith never gets my pre and post boil gravities correct, even adjusting for cooling, water loss and evaporation. I just know how much I will boil off, and how many gravity points this will give me. I can also rely on my mashing to be consistent, so normally I knw that x amount of grain with x amount of water will give me x gravity. It's all about getting to know your gear.
I have even been not checking the starting gravity on some beers lately, because I have been using the same gear doing the same beers for so long...I know what I'll get.

You have some good points there Maxt. The couple of points I'm normally off by could be just my calculations (I'll take a closer look).

 

[Heaviside]

If we assume that the refractometer is working well and that you have zeroed it then it's most likely to be your volume reading. How sure are you that it's accurate?
As Cubbie said it doesn't take a very large error in either reading to give the sort of error you are reporting.

Handy little equation known by a few names but mostly it's called "The Standard Dilution" equation.
C₁V₁=C₂V₂ where C is the Concentration and V is the Volume
Let's just say you started with 28 L of wort at a gravity of 1.044 and you boiled away 3 L of water (now this is important leave out the 1 part it's not there OK).
0.044*28=25* C₂ or (0.044*28)/25= C₂ = 0.04928 or 1.050 near as dam it.
Try the same thing with a 1L error
0.44*29=25* C₂ or (0.044*29)/25 = C₂ = 0.05104or 1.051
(0.04928/0.05104)*100= 96.55%

OH and remember that your wort contracts by 4% when it cools, if you are taking volume measurements at boiling and ambient you have to watch for that, it amounts to about a 4% error in 25 L.
Errors always accumulate so it probably isn't just one thing you need to find. Make sure your refractometer is calibrated, but look at your volumes and just check that wort contraction isn't compounding the error.

MHB

MHB These are also good points. I have checked my refractometer with water and it seems OK. The readings check out with my hydrometer also.

My pre-boil and post boil volumes were taken at different temperatures so there could very well be some error accumulating there so I 'll have a look at this too! Thanks!

 

[Hatchy]

I've had similar issues where my pre boil gravity is lower than my last runnings which can only mean an inaccurate reading somewhere from my understanding. I always get (close to) my target post boil gravity so I just pretend that my pre boil gravity doesn't exist. On Monday my 1st runnings were 24, 2nd runnings were 19 (with a question mark on the brewsheet) but my pre boil gravity was 10. I'm guessing that the 2nd runnings was where I got the inaccurate measurement my post boil gravity was 13 which was what beersmith told me I'd get.

I know that none of this helps at all but at least yr not the only one getting strange readings.

 

[peted27]

I've had similar issues where my pre boil gravity is lower than my last runnings which can only mean an inaccurate reading somewhere from my understanding. I always get (close to) my target post boil gravity so I just pretend that my pre boil gravity doesn't exist. On Monday my 1st runnings were 24, 2nd runnings were 19 (with a question mark on the brewsheet) but my pre boil gravity was 10. I'm guessing that the 2nd runnings was where I got the inaccurate measurement my post boil gravity was 13 which was what beersmith told me I'd get.

I know that none of this helps at all but at least yr not the only one getting strange readings.

10 and 13 sound ok to me... my last 4 brews are

Preboil and OG:

11 and 13
12 and 14.5
9.5 and 12
10 and 12

so depending on boil off its going to vary of course, but seems in the right range (unless mine is buggered too)

as i BIAB i cant take measurements of first and second runnings.... i find it extremely hard to get gravities and volumes to match software.... there is just too many, trub loss, dead space, boil off. and this is across multiple vessels, kettle, cube, fermenter..... its just not worth stressing.... if your hitting OG and getting the correct volume in to bottles or keg.... then forget the rest

 

[Hatchy]

It wasn't the pre & post boil gravity that had me scratching my head, it's the 2nd runnings being higher than the pre boil that confuddled me.

At the end of the day I'm making beer, that's the important thing & when mates come round & I offer them the choice of homebrew or coopers they invariably go the homebrew over the coopers so I'm not too concerned about it.

My volumes are what I'm (slightly) concerned about. I'm pretty consistent with boilovers so my batch size into fermenter is pretty miss & miss (that wasn't a typo). Fortunately for me they've banned mobile phones at work so I'm taking designing great beers to work tomorrow as well as the latest copies of BYO & the other magazine I have whose name escapes me. Apparently reading AHB when it's quiet will distract me if someone rings up but a book/magazine won't. I thought about taking in my laptop & spending the day on beersmith but I reckon my boss probably wouldn't appreciate that.

 

[np1962]

Irony- A phone company banning phones at work. :blink:

 

[cubbie]

If we assume that the refractometer is working well and that you have zeroed it then it's most likely to be your volume reading. How sure are you that it's accurate?
As Cubbie said it doesn't take a very large error in either reading to give the sort of error you are reporting.

Handy little equation known by a few names but mostly it's called "The Standard Dilution" equation.
C₁V₁=C₂V₂ where C is the Concentration and V is the Volume
Let's just say you started with 28 L of wort at a gravity of 1.044 and you boiled away 3 L of water (now this is important leave out the 1 part it's not there OK).
0.044*28=25* C₂ or (0.044*28)/25= C₂ = 0.04928 or 1.050 near as dam it.
Try the same thing with a 1L error
0.44*29=25* C₂ or (0.044*29)/25 = C₂ = 0.05104or 1.051
(0.04928/0.05104)*100= 96.55%

OH and remember that your wort contracts by 4% when it cools, if you are taking volume measurements at boiling and ambient you have to watch for that, it amounts to about a 4% error in 25 L.
Errors always accumulate so it probably isn't just one thing you need to find. Make sure your refractometer is calibrated, but look at your volumes and just check that wort contraction isn't compounding the error.

MHB

I assume this is applicable for IBU also. For example brew to a certain IBU and then top up your fermenter.

 

[silvana]

Heaviside,

I assume you added water to bring your pre boil gravity down the 10% to hit your traget?

Did you then take out that extra water you put in so your pre boil volume was what you usually have (and what I assume you evaporation rate is calculated on)?

This makes a deffinite difference on my system.

 

[katzke]

I tossed the software and made adjustments to a spreadsheet called Suds. I am always close enough in my pre boil gravity and my post boil gravity and volume to keep me happy. I have found that the other software I tried was never quite right on water or gravity. Before the spreadsheet I was doing calculations by hand.

Why do I say close enough? Any home brewer that can collect 23 liters of 1.056 wort every time they brew is charmed. If I am off 2 liters or points I am happy, as long as I get a full corney keg.

The second running problem is most likely the way it is tested. For true results the test needs to be taken from a collected full run off, that has been well mixed.

 

[Heaviside]

Heaviside,

I assume you added water to bring your pre boil gravity down the 10% to hit your traget?

Did you then take out that extra water you put in so your pre boil volume was what you usually have (and what I assume you evaporation rate is calculated on)?

This makes a deffinite difference on my system.

Hi Yogi,

I did add water to adjust the pre-boil gravity to my target. The pre-biol gravity measurement showed me that my total gravity point value was too high by about 10%. So I adjusted the volume to get my intended pre-boil gravity. After the boil and using the post-boil volume in the kettle, I found that my post-boil gravity was too low by about 10%.