how to calculate boil evaporation volume needed

[jascarver]

Hello,

I want to calculate the volume to evaporate while boiling with the gravity pre-boil and the gravity post-boil. Will this equation work (it should but it has the hypothesis that the evaporated water is pure)?

Initial mass= final mass
Initial mass/density water= final mass/density water

Gravity preboil * preboil volume = Gravity post boil (OG) * postboil volume + water gravity (1)* evaporated volume

Gp*Vpre=OG*(Vpre-x) + x
Gp*Vpre=OG*Vpre+(1-OG)*x

x=Vpre*(Gp-OG)/(1-OG)



thanks

 

[Gavin C]

Usually people will work the other way. Know the boil-off-volume and that is factored in when calculating the volumes needed.

The boil-off-volume is easy to measure.

Gravity preboil* preboil volume= Post boil gravity * Post boil volume. Edit: + boil off volume

One goes up as the other goes down. The same amount of sugars are present in each.

Just seems a needlessly complex way to solve a problem that doesn't exist.

Software (free online) will do all these things for you but the maths is basic.

 

[jascarver]

Hi,

I had problem with my first two beer and ended up evaporating way too much water. In the recipe I did there is an OG you have to achieve, but you have to measure it to know if you reached it. I measured it a few time in the boil to figure out at which time I would achieve the desired gravity (linearly). The problem is I think evaporation speed isn't constant in time so I end up coming back after the calculated time and there is too much boiled off volume. Evaporation in my setup is fast and it seems like it speed up during the boil. I look at it, not much water has evaporated for the given time, I go away for a short while and come back to see that it evaporated too much.

I am trying to mark my kettle inside so I know what volume there is inside just by looking. If I achieve the volume I want I will be able to achieve an ok OG and final volume. It is quicker and safer just to take a look inside. I will know right away if I am close to my OG and if I have to stay around.

I think there is a problem with your equation because the mass on the left isn't equal to the mass to the right. A gravity is the density of something divided by the density of water. If you mutplity a gravity by the density of water, you will get a density. A density is a mass divided by a volume. If you multiply the density by the volume, you will get a mass.

The total mass at the beggining of the boil is equal to the mass at the end of the boil and the mass of the evaporated water. Your equation states that the mass at the beggining of the boil (divided by a constant which is the density of water) is equal to the mass at the end of the boil (divided by the some constant which is the density of water). This gives OG*volume=OG2*volume2 (density*volume *cte= density 2* volume 2 *cte). This is not true because water was evaporated, so the mass in your kettle at the end is less than at the beggining.

The interface of brewing software is kind of an hassle to deal with and not that useful. I would only like to know if the approach in my original post is valid. The problem would be that evaporated water don't have a density of 1 water (not pure water). It would fluctuate with the composition of the wort and that would be plausible. But is it a good approximation? Does anyone do the same approach?

 

[Gavin C]

Why not just measure the boil off in an hour. Fill pot with volume of water x

Boil for 60 mins

Measure volume y

boil off/per hour is x-y

Once you measure your system's details right and plug them into the software it is tremendously accurate. It does all the calculations for you. All you have to do is feed it the correct info.


Good pont on my simple equation. You are spot on there. The masses do not equal. These things are usually measured in gravity points. That is what remains constant not mass as you are losing mass via the boil.

I use Beersmith FWIW and find it to be invaluable. Accurate measures of volume, gravity and temperatures are crucial if you really want to nail down accurate data with a view to improving the process.

You need to work out
boil off rate
trub loss
dead space loss in kettle/chiller/mash tun
fermentor losses
grain absorption
mash tun material will have a bearing on strike temperatures

These are all just details that in many cases you can measure exactly prior to doing your next brew. Others take some trial and error and may be subject to external factors.

 

[MusicalBrewer]

Why not just measure the boil off in an hour. Fill pot with volume of water x

Boil for 60 mins

Measure volume y

boil off/per hour is x-y

+1 to this.

You don't need Einsteins theory of relativity...just measure, boil, and measure. Quite simple really

 

[pricelessbrewing]

The boil off rate is linear as it depends almost solely on the heat applied and the surface area of the liquid.

just do a test run with some water to find out the boil off rate per hour.

 

[maximus4444]

I did a test boil as well. It was my first time doing a full-volume boil. So I just needed to figure out my pre-boil water volume. I did the 1 hour boil of just plain ole water. I lost about 1.2 gallons. (Starting with 6.5)

 

[brewmesomewater]

Why not just measure the boil off in an hour. Fill pot with volume of water x

Boil for 60 mins

Measure volume y

boil off/per hour is x-y

Once you measure your system's details right and plug them into the software it is tremendously accurate. It does all the calculations for you. All you have to do is feed it the correct info.


Good pont on my simple equation. You are spot on there. The masses do not equal. These things are usually measured in gravity points. That is what remains constant not mass as you are losing mass via the boil.

I use Beersmith FWIW and find it to be invaluable. Accurate measures of volume, gravity and temperatures are crucial if you really want to nail down accurate data with a view to improving the process.

You need to work out
boil off rate
trub loss
dead space loss in kettle/chiller/mash tun
fermentor losses
grain absorption
mash tun material will have a bearing on strike temperatures

These are all just details that in many cases you can measure exactly prior to doing your next brew. Others take some trial and error and may be subject to external factors.

+1
If you can add,subtract,multiply, and divide you can make good beer. Test boil lol