Boil off

[knarfks]

Has anyone noticed a difference in boil off volumes in the winter. I just did an oatmeal stout, boiled low like I usually do. There was 6.5 gallons in the beginning of the boil, then somehow I boiled off 3 gallons in 60 minutes. I typically boil off a gallon, at the most 1.5 gallons in the summer, fall, spring. Its about 35 degrees and raining so I figured the lack of humidity couldn't have done it.

I would have had an imperial oatmeal stout at 1.080 If I didn't add a gallon of water, still pretty stong. I'm just not understanding how the boil off could have increased so much.

 

[Bromley]

I've been experiencing similar losses on my last couple of boils. I chalked it up to dry winter weather and have adjusted since then in ProMash. That theory is of course blown all to hell if you were boiling in 100% humidity. Not sure what to tell you.

 

[FlyingHorse]

A lower atmospheric temp and higher humidity would both serve to lower your evap rate, all other things being equal. Something else is at work here besides the change of season.

 

[drayman86]

As I understand the physics, atmospheric pressure (i.e. altitude) affects boil off more than temp. or RH.

In Denver for example, wort would boil at a lower temp. than in Miami and one could expect more of a boil off in the Mile High City than on the Atlantic shores, no?

 

[knarfks]

Maybe a bizarre low front from the rain or something. It was just bizarre. I hate to add water, but I don't want an 8% oatmeal stout. Will have to try and adjust next time.

 

[budbo]

If the air is Dryer and colder your boil rate will be faster. I saw that when I lived in Nebraska, The Midwest winters get very dry and when the pressure drops you boil cooler which also contributes.

The guys in Phoenix probably have it Best/worst Best in that their shouldn't see a change and worst because they are probably losing to evaporation in the mash

 

[Surly]

I need some help and advice with a boil off issue. On Saturday I brewed a five gallon batch. It was 25F with a light snow.

Using BeerSmith, I should have started my boil with 5.83 gallons of wort. In reality i started with six.

At the end, I had to add three quarts to top off my fermenter to five gallons.

How can I do the math to adjust my boil off rates in BeerSmith. And, should I be concerned about this? Will this likely be a one time issue? If not, do I need more data, ie, more boils to determine an average rate?

ty

 

[Davidmush]

I've been experiencing similar losses on my last couple of boils

The season influences somehow

 

[Bobby_M]

Cold air is usually drier and more "accepting" of moisture. If you're down at 10% humidity, it's going to practially suck the water out of your boiling wort.

 

[Turkeyfoot Jr.]

I had a similar problem when I brewed 3 weeks ago, temps were in the teens. My boil started with 6.5G and I ended up with 4G after just 60 minutes. I was astonished to say the least.

 

[archmaker]

Gammon you can adjust your boil off rate under Tools->Options->Brewing Tab->Advanced.

I experienced the same issue as everyone else and I bumped the number up to 11% from the default, when I am outside brewing on a cool (40's) windy day. If it is warm (70's) and windy then I will drop that down to 10%, and warm and still I will probably drop it to 8 or 9%.

Not exact but it does get me closer than if I just left it alone.

 

[uwmgdman]

Why do you experience a higher evaporation rate in colder temperatures even if the humidity is 100%? Here's an explanation without being too technical.

Relative humidity (the humidity seen on weather reports) is exactly that, relative. Relative humidity is qualitatively how much water vapor that is in the air relative to the total amount of water vapor the air can hold at a given temperature.

A reasonable approximation is to say cold air can hold less water vapor than warm air. So an airmass with 100% relative humidity on a 30F degree day has much less water in it than an airmass with 100% relative humidity on a 80F degree day. Why that's important in second.

But if the air is saturated, that is the relative humidity is 100%, how can I still evaporate anything?

The relative humidity of 100% is at a given temperature, the temperature from the weather report. When you boil, generating heat, the cold ambient air and hot air coming off your boil mix, resulting in air that is warmer than the ambient weather report. Now remember the warm air can hold more water vapor than cold air. Your mixed warmer air that was saturated at 30F (relative humidity 100%) is now say 90F, is far from saturated (low relative humidity, in fact if you added no moisture to that air, the relative humidity would be 11%). But that mixed air doesn't have 11% relative humidity, it 'picks up' moisture from your brew pot via evaporation.

Now if your ambient air was 70F there is less of a difference in the total amount of water vapor the ambient air and heated air can hold and there will be less moisture 'picked up' from your brew pot....aka lower evaporation rate.

Bottom line, the larger the difference between the ambient air temperature and your boiling temperature the larger the evaporation rate will be.

Yes more wind will increase evaporation rate some due to more efficient mixing of air. The relative humidity also has an impact (greater impact at warmer temperatures, when ambient and boiling temperature difference is less), but overall the RH impact is far less than the difference attributed to the temperature difference between the ambient air and the boiling temperature.


A couple items to chew on:
100% relative humidity at 30F = 11% relative humidity at 90F

Consider two airmasses with 50% relative humidity at 40F and 90F.
The one at 90F as 600% more water vapor in it than the one at 40F.

*All this assumes sea level pressure. There are altitude concerns.........ie lower pressure in Denver, that has an impact too, but we'll let those Mile Highers out there worry about that.

 

[Bombo80]

I guess I wouldn't be too concerned about boiling off too much water. You can always add it back in, when it goes into the primary.

My issue was I had 10 gallons of wort to boil down to 5. It took 3 hours to do it, and I hit my first hop addition pretty close. After the hotbreak, I idled back the turkey fryer, so it maintained a nice rolling boil, without having to worry about scorching in the keggle. (first time I used one) I also was boiling in the breezeway, which has lots of glass, and not much insulation, but it still went very well. Plus being in Minnesota, it's still pretty cold out there.

 

[knarfks]

thanks for the explaination uwmgdmn, now it makes sense.