Do barometric pressure changes affect boil temperatrue?

Yes. 212°F (or 100°C) is only at sea level, and 760 mmHg (I'll bet you learned this in high school chemistry and forgot).

As you go up in elevation, the atmospheric pressure drops. For example, in the "mile-high city", Denver, water will boil at 203°F (95°C). The water in an open pot will not get any hotter than this. So, for many things (vegetables), you would need to boil longer.

Of course, with weather changes, the barometric pressure changes and will also affect the temperature at which water will boil.

If you've used a pressure cooker, you might then know that under pressure water will boil at a much higher temperature.

Regarding brewing, the main reason for boiling is 1) drive off some unsavory compounds (DMS), and 2) isomerization of the hops (this is simply making the alpha acids soluble in the wort). I just found this. Might want to dig out the calculator. Maybe some kind soul here would plot this in Excel and post a jpg of the graph.

From Zymurgy Vol. 20 # 4 Special 1997.

Michael L. Hall writes in his article "What's Your IBU" that:

The isomeration reaction rate depends on temperature, so the boiling-point temperature at your elevation can make a big difference... Garetz* gives a correction factor for this effect:

Fbp = 1 / (1 + Eft / 27500)

where Fbp is the Boiling-Point Factor

and

where Eft is the elevation in feet.


* Garetz, Mark. Using Hops - The Complete Guide to Hops for the Craft Brewer, HopTech, 1994b.​

Click to expand...

Hex said:

So, should brewers in Denver adjust their home brew recipes for the altitude?

It could also be a factor when calibrating your thermometer with 'boiling' water.

And every batch could be different. Do the pro's compensate for barometric fluctuations?

Click to expand...

zepolmot said:

Lets also remember that disolved solutes in water cause freezing point depression and boiling point elevation (this is the reason salting your driveway in the winter works). So i'm going to say no. Even the brewers in denver are probably boiling at 212 because of this.

Click to expand...

passedpawn said:

From Zymurgy Vol. 20 # 4 Special 1997.

Michael L. Hall writes in his article "What's Your IBU" that:

The isomeration reaction rate depends on temperature, so the boiling-point temperature at your elevation can make a big difference... Garetz* gives a correction factor for this effect:

Fbp = 1 / (1 + Eft / 27500)

where Fbp is the Boiling-Point Factor

and

where Eft is the elevation in feet.


* Garetz, Mark. Using Hops - The Complete Guide to Hops for the Craft Brewer, HopTech, 1994b.​

Click to expand...

Thanks for that, I never put it into numbers before but I have noticed that I had to add extra hops to get a well balanced beer.

Click to expand...

passedpawn said:

...Maybe some kind soul here would plot this in Excel and post a jpg of the graph.

From Zymurgy Vol. 20 # 4 Special 1997.

Michael L. Hall writes in his article "What's Your IBU" that:

The isomeration reaction rate depends on temperature, so the boiling-point temperature at your elevation can make a big difference... Garetz* gives a correction factor for this effect:

Fbp = 1 / (1 + Eft / 27500)

where Fbp is the Boiling-Point Factor

and

where Eft is the elevation in feet.


* Garetz, Mark. Using Hops - The Complete Guide to Hops for the Craft Brewer, HopTech, 1994b.​

Click to expand...

Can someone rewrite this formula in terms of boil temp vs. hops isso?

Other factors including water profile are not necessary in delta boil if you take an actual reading each boil, and compensate, but by what percent? Longer boils in Denver on a rainy day? to short boils in the summer in Death Valley?.

Click to expand...

passedpawn said:

I think the only thing that might be affected is how well the hop acids are liberated from the hops.

Here, I plotted that formula from above.

Click to expand...