Pre boil vs post boil gravity figures are different in BS (even though boil off is accurate)

[Truman48]

So I have my Grainfather G40 profile setup with a 5 litre boil off per hour, (which is about right depending on time of year ambient temps etc)
But I've noticed that my gravity point increase is always much less than what BS predicts.

For example, last brew BS said that my SOB = 1.044 and EOB = 1.056 (12-point increase for a 60 min boil with 5 litres lost)

My actuals were SOB=1.046 and EOB = 1.053 and 5.5 litres lost. So, I lost an extra .5 litres in the boil but still only gained 7 points. (Not 12 poiunts that BS said I should have gained.)

Ive noticed this with all of my brews. The actual gravity point increase is always 5-6 points below what BS says even though i boil the same amount of water off and sometimes more.

I know that boil vigour is related to this, but shouldn't that be related to volume lost anyway?

What settings if any can I change for this?

 

[GigaFemto]

I don't know if this has anything to do with your problem, but one issue is that BeerSmith's gravity predictions are for the density at wort temperature, while you most likely make your measurements on samples at room temperature. That means that the start-of-boil prediction by BeerSmith will be ~4% lower than expected, because the volume expands by 4% from room temperature to boiling. If you set the expansion to zero in BeerSmith it solves that problem but causes volume predictions (volume recovered from mash, pre-boil volume) to be low.

--GF

 

[Truman48]

I don't know if this has anything to do with your problem, but one issue is that BeerSmith's gravity predictions are for the density at wort temperature, while you most likely make your measurements on samples at room temperature. That means that the start-of-boil prediction by BeerSmith will be ~4% lower than expected, because the volume expands by 4% from room temperature to boiling. If you set the expansion to zero in BeerSmith it solves that problem but causes volume predictions (volume recovered from mash, pre-boil volume) to be low.

--GF

I changed the cooling shrinkage % from 3% to 0% but all it did was reduce my sparge volume from 5.85 litres to 5.22 litres. Nothing else.

 

[BOB357]

User inputs are a major factor in BeerSmith calculations. This includes inputs in your equipment profile, so don't rely on an existing profile. Create one or edit an existing profile to match your individual system and techniques. Here's a link to a video on creating equipment profiles: https://brulosophy.com/2014/08/04/beersmith-tutorial-equipment-profile-setup/ As with other software, BeerSmith predictions are only as accurate as your inputs.

If you're relying on the volume marks in your Grainfather, be aware that they're often inaccurate so don't trust them without verifying first. Also, keep in mind that volumes will change with temperature, so be sure to either take measurements at the same temperature or apply a correction factor.

Because volume and gravity are relative to each other, one thing you can rely on is that the total number of points in your wort will remain constant so, as the volume is lowered by evaporation, the total number of points will remain the same. Water evaporates, sugars don't.

Example: 6 gallons @1.065 (6X65=390), boiled down to 5 gallons will have a gravity of 1.078 (390/5=78)

If you apply the above to your wort and the SOB and EOB total points don't agree, you've verified that measurement error is the cause. (Be sure to correct hydrometer readings for temperature.) Note that the error(s) may be either in actual readings, readings entered into the software or a combination of both and may be in gravity and/or volume reading(s).

 

[Truman48]

User inputs are a major factor in BeerSmith calculations. This includes inputs in your equipment profile, so don't rely on an existing profile. Create one or edit an existing profile to match your individual system and techniques. Here's a link to a video on creating equipment profiles: https://brulosophy.com/2014/08/04/beersmith-tutorial-equipment-profile-setup/ As with other software, BeerSmith predictions are only as accurate as your inputs.

If you're relying on the volume marks in your Grainfather, be aware that they're often inaccurate so don't trust them without verifying first. Also, keep in mind that volumes will change with temperature, so be sure to either take measurements at the same temperature or apply a correction factor.

Because volume and gravity are relative to each other, one thing you can rely on is that the total number of points in your wort will remain constant so, as the volume is lowered by evaporation, the total number of points will remain the same. Water evaporates, sugars don't.

Example: 6 gallons @1.065 (6X65=390), boiled down to 5 gallons will have a gravity of 1.078 (390/5=78)

If you apply the above to your wort and the SOB and EOB total points don't agree, you've verified that measurement error is the cause. (Be sure to correct hydrometer readings for temperature.) Note that the error(s) may be either in actual readings, readings entered into the software or a combination of both and may be in gravity and/or volume reading(s).

Hi Bob, So if I use your equation for my actuals I get this (Done in litres)

My actuals were SOB=1.046 and EOB = 1.053 and 5.5 litres lost.

26.6 litres SOB x 1.046 = 1223.6 EOB= 21.1 @ 1.053 = 1223.6/21.1 = 1.057 so slightly out, but your equation will come in handy to use.

Ive checked the GF measurements before and made sure they were right. Ill double check everything again.

I do always ;et samples cool down or allow for shrinkage etc when taking a measurement. For example if my EOB is 22.4 litres I round down to 22 litres.


Thanks for your response.

 

[BOB357]

The equation actually shows your EOB gravity to be 1.057.99, which is, for all intents and purposes, 1.058. This is a 5 point (over 9%) difference between your reading of 1.053 and what the gravity should be if your inputs are correct. That's a significant disparity and the same as you cited in your original post.

Time to double check all of your measurements and inputs and make corrections as needed. Once you've determined everything is accurate, make incremental adjustments in Brewhouse Efficiency as needed. Be sure to check the calibration and accuracy of your thermometer(s) and hydrometer and use the proper temperature correction for all gravity readings.