Boiling point of water at different elevations?

[BrewBarrymore]

I recently bought an electric homebrewing kit that a guy in my area builds. They are pretty well built, however I had a difficult time getting the boil kettle to reach 100c. It would get to 99.2c at max power and still not reach 100.

I asked him what was up with it and he told me that the boiling point in Ho Chi Minh City, Vietnam is 96. Indeed, the wort had a good roll to it, however, I wasn't able to find any information online that supported his statement. My main concern is that by not getting to 100c, perhaps I am not getting as good of a hotbreak as I could be. Is temperature not a good indication of boiling and instead a good roll a better measure?

Ho Chi Minh City is 19m above sea level but then then it is usually over 80% humidity and around 30c in the daytime. Can anyone confirm what he is telling me is true?

 

[mredge73]

Boiling point of water is 212F at sea level.
Every 500' up it drops 1F; so in Denver water boils at 202F.

At only 19m above sea level you should boil close to 100C. You probably have a faulty thermometer, you need to calibrate it.
As long as you have a good rolling boil you are getting good hotbreak and hop utilization; temperature independent.
Temperature is important to the Mash stage, not the boil stage. If you are a all grain brewer you need to get a better sensor.

 

[ajdelange]

If it's an electric kit you are probably reading the temperature as measured by at best an RTD and at worst a thermocouple. You can't really expect much better than 0.8° error from a thermocouple or RTD unless special care is taken. The error is actually a bit more than 0.8° as your wort's boiling temperature at sea level (or within 19m of it) is a bit higher than 100 °C because of its sugar content.

Another thing to keep in mind is that the barometric pressure at the time of the boil may have had an impact. Water boils at 100 °C at a pressure of 29.92 inches of mercury (sorry for the English units but those are the ones I remember). On a low pressure day this may drop by an inch or more. An inch of Hg corresponds to a change of about 1000 feet elevation (the pressure on a standard day at 1000' above sea level will be about 28.82 inHg) and using mredge73's ROT would lower the boiling point 2 °F (a bit over 1 °C). Thus if you did your boil during stormy weather you reading of 99.2 might have been spot on.

 

[BrewBarrymore]

Thus if you did your boil during stormy weather you reading of 99.2 might have been spot on.

Well, damn.. Rainy season did just start in South Vietnam and I do remember it coming down pretty hard for the better part of the day that day. O_O

 

[Brickman]

This phenomenon is known as density altitude and is very important if you are a pilot. You can take what your density altitude is and then calculate what the actual boiling point should be. I live at about 1573 meters above sea level and in a dry climate. My current boiling point is 95 C. If I figure it based on altitude density my boiling point is a degree cooler at 94 C as my current altitude density with the current weather conditions is 1826 metes above sea level. Yes I am a nerd but it impacts my cooking as I am a competitive cook and when I change altitude it changes my cooking times.

I can get an approximation based on altitude and pressure. You can calculate your boiling point here with a little unit conversion.

http://www.thermoworks.com/software/bpcalc

 

[Silver_Is_Money]

How come every time I look at a Denver weather forecast their barometric pressure looks pretty much main stream identical to the barometric pressure reports for coastal cities? Shouldn't Denver's barometric pressure be consistently about 2" (or more) lower?

 

[ajdelange]

That's because they broadcast QNH or ('Q - not here' as pilots sometimes jokingly say) which is sort of the sea level pressure at Denver which, as Denver is nowhere sea level is pretty immaterial but the Wx guys at Denver do an additional calculation (QFF) as to what the pressure would be at the bottom of a well dug down to sea level. QFF is reported separately in METARS for stations that report it - not all do but Denver is one). What is really important is that the pressure reported is such that if a pilot sets his altimeters to QFE the instruments will read field elevation when on the ground at Denver. QFE (Q -field elevation) is what they would set the altimeter to if they wanted it to read 0 when they landed (field elevation is the reference whereas with QNH sea level is the reference). QFE would be about 2" below QNH. QFE is not part of a standard METAR and if you requested that from a controller you would probably get a "Huh?"

 

[Silver_Is_Money]

That's because they broadcast QNH or ('Q - not here' as pilots sometimes jokingly say) which is sort of the sea level pressure at Denver which, as Denver is nowhere sea level is pretty immaterial but the Wx guys at Denver do an additional calculation (QFF) as to what the pressure would be at the bottom of a well dug down to sea level. QFF is reported separately in METARS for stations that report it - not all do but Denver is one). What is really important is that the pressure reported is such that if a pilot sets his altimeters to QFE the instruments will read field elevation when on the ground at Denver. QFE (Q -field elevation) is what they would set the altimeter to if they wanted it to read 0 when they landed (field elevation is the reference whereas with QNH sea level is the reference). QFE would be about 2" below QNH. QFE is not part of a standard METAR and if you requested that from a controller you would probably get a "Huh?"

Amazing! Effectively, all reported local weather forecasts are lying with respect to your locations barometric pressure. Why would they find it advantageous to normalize them to seal level? What benefit does the audience (you and I) get from that?

 

[ajdelange]

Well, damn.. Rainy season did just start in South Vietnam and I do remember it coming down pretty hard for the better part of the day that day. O_O

It is also worth mentioning that high humidity will lower the boiling point a little as water vapor is less dense than dry air and thus a column of moist air weighs less than a column of dry air and the the pressure associated with moist air is going to be lower than with dryer air. Ebulition commences when the vapor pressure over the hot water equals the total pressure.

 

[ajdelange]

Amazing! Effectively, all reported local weather forecasts are lying with respect to your locations barometric pressure. Why would they find it advantageous to normalize them to seal level? What benefit does the audience (you and I) get from that?

The primary benefit we get is that when you are on a flight heading into Denver under IMC (low visibility) and the pilot sets his altimeter to the tower's reported QNH if his altimeter says he is 200' above the ground he is 200 feet above the ground and not 1000 feet above thus lulling him into thinking he shouldn't be well into his flare....

The second benefit is that it allows weathermen to perceive weather patterns better. A high pressure area should read 30.00" across the area. If we reported station pressure we'd have 30.00" for DCA because it's at sea level and 29.90" inches at IAD and even lower at some airport in further west because they aren't leading us away from appreciating that the region is enjoying a widespread high pressure weather system. Thus when you see LAX, LAS and DNV all reporting close to 30" you see right away that they are in the same weather system. If LAX reported 30, LAS 28" and DEN 25" (WAG difference based on 1"Hg/kft) you wouldn't see that. The first thing you are told to do when setting up a new aneroid barometer is to calibrate it to the nearest airport's reported (QNH) altimeter setting. Having done that you are on the same page with everyone else. Another point is that the scale on such a product only need run from say 27.5" to 31.5" (very low SLP to very high SLP) not 21" to 33" in order to accommodate buyers in Denver.

In the old days when every science classroom in every school in the US had a Fortin barometer (no longer made because the sole remaining manufacturer got sick of having to deal with the tree huggers) the first step in taking a barometer reading was to read the station pressure. That is what it responds to. That was then corrected for temperature and finally for station height above sea level. The actual station pressure altitude is pretty useless for weather forecasting and reporting though it is useful to people like brewers (and precision cooks, evidently).

 

[Brickman]

Yeah the first time I went down in altitude I turned everything into mush. Didn't need teeth to eat it. But what really got me was when it was hot and humid that screwed with it too cause it slowed it down. My brother told me about density altitude as he is a helicopter pilot and low and behold it applied to cooking too. I do have to say I still learn a ton from this forum and not all brewing related.

 

[ajdelange]

Remember that density altitude is simply another way of stating pressure. It is pressure that causes the boiling point to vary. Density altitude is simply the altitude in the ICAO standard atmosphere at which a particular pressure is found. So higher density altitude means lower pressure - that's all. If you are at an airport on a standard day the density altitude will be the altitude of the airport. If the weather gets hotter or more humid or both pressure will go down and density altitude up. This is important to pilots because lift, thrust and engine power all depend on the density of the air. For the rest of us it's easier to just think in terms of barometric pressure.