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Home Brew Forums > Home Brewing Beer > Brew Science > Calibrate boil point based on pressure or altitude?
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Old 03-01-2012, 03:40 AM   #1
afr0byte
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Default Calibrate boil point based on pressure or altitude?

Hey, so I've been having some attenuation problems (a few points high on the FG). Anyways, I'm recalibrating my thermometer. I looked at the local weather as reported by a weather station in my town. The weather station says the elevation is 700 feet, which is probably about where my house is as I live on a ridge. However, the most recent pressure readings have said about 29.95 inches of mercury. I'm assuming I should calibrate my boiling temperature based on the current pressure as opposed to the reported elevation. Is this what you folks do?
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Old 03-01-2012, 03:53 AM   #2
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Use absolute pressure; barometric readings should be fine, IMHO.
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Old 03-01-2012, 03:35 PM   #3
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You want the barometric pressure at your 'station' (QFE). Weather reporting entities get barometric pressure data from the nearest airport. These actually report 'altimeter' (QNH) but some report sea level pressure (QFF). For example, if you live near Burlington the current METAR looks like
KBTV 011549Z 13007KT 2SM -SN BKN017 BKN024 OVC036 M01/M04 A2979 RMK AO2 SLP092 P0001 T10061039
Here A2979 means QNH is 29.79 (1008.8 mb) while SLP092 means the station pressure reduced to sea level (by unfathomable rules) is 1009.2 mb) i.e. not much difference. Use either as either is representative of pressure at sea level). The ratio of pressure at 700 feet to that at sea level is 0.9750 so you can convert a QNH or QFF report to QFE by multiplying by that ratio. Thus, using the QFF value QFE = 1009.2*0.9750 = 983.9. The last step is to put the formula
" = 1013.25*(10^( 8.07131 - 1730.63/(233.426+ A1)))/760"
into a cell (other than A1) in an Excel spreadsheet. Now type temperatures ( C) into cell A1 until the cell with formula reads 983.9 (or use the Solver find the value of cell A1 which does that). Doing that for 983.9 would give you a temperature of 99.19 C as the temperature at which water would boil at 700 feet MSL if located near BTV at 15:49Z.

To convert mmHg to mB use mb = 1013.25*mmHg/29.92. To get the METAR from your nearest airport take the airport code (e.g. BTV), stick a K in front of it (KBTV) and do a search on 'KBTV METAR'.

You might want to look at wunderground.com/wundermap and interpolate barometer readings to determine the pressure in your area. Try to interpolate between airport or other 'official' stations. You may find an amateur station close to you but the odds that the guy has corrected his barometer correctly are pretty slim. Of course < 1C isn't that much. It's probably not the source of your problem.
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Old 03-01-2012, 07:00 PM   #4
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Much easier to buy a thermapen.
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Old 03-01-2012, 07:27 PM   #5
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Quote:
Originally Posted by wildwest450 View Post
Much easier to buy a thermapen.
Why? Do they automatically calibrate based on altitude/pressure?
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Old 03-01-2012, 07:46 PM   #6
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Quote:
Originally Posted by ajdelange View Post
You want the barometric pressure at your 'station' (QFE). Weather reporting entities get barometric pressure data from the nearest airport. These actually report 'altimeter' (QNH) but some report sea level pressure (QFF). For example, if you live near Burlington the current METAR looks like
KBTV 011549Z 13007KT 2SM -SN BKN017 BKN024 OVC036 M01/M04 A2979 RMK AO2 SLP092 P0001 T10061039
Here A2979 means QNH is 29.79 (1008.8 mb) while SLP092 means the station pressure reduced to sea level (by unfathomable rules) is 1009.2 mb) i.e. not much difference. Use either as either is representative of pressure at sea level). The ratio of pressure at 700 feet to that at sea level is 0.9750 so you can convert a QNH or QFF report to QFE by multiplying by that ratio. Thus, using the QFF value QFE = 1009.2*0.9750 = 983.9. The last step is to put the formula
" = 1013.25*(10^( 8.07131 - 1730.63/(233.426+ A1)))/760"
into a cell (other than A1) in an Excel spreadsheet. Now type temperatures ( C) into cell A1 until the cell with formula reads 983.9 (or use the Solver find the value of cell A1 which does that). Doing that for 983.9 would give you a temperature of 99.19 C as the temperature at which water would boil at 700 feet MSL if located near BTV at 15:49Z.

To convert mmHg to mB use mb = 1013.25*mmHg/29.92. To get the METAR from your nearest airport take the airport code (e.g. BTV), stick a K in front of it (KBTV) and do a search on 'KBTV METAR'.

You might want to look at wunderground.com/wundermap and interpolate barometer readings to determine the pressure in your area. Try to interpolate between airport or other 'official' stations. You may find an amateur station close to you but the odds that the guy has corrected his barometer correctly are pretty slim. Of course < 1C isn't that much. It's probably not the source of your problem.
Wow, thanks for all the info. Apparently, having looked at the METAR data on the Weather Underground page, the pressure data appears to be from KMPV, which is Knapp State Airport in Barre/Berlin. EDIT: Also, I agree that the temperature probably isn't my problem, but I like to do my best to eliminate variables. My pH on the last beer was 5.42, so that's definitely not the problem. I use pure O2 for aeration, so that's not it. The recipe I used for the last batch was for 70% efficiency, but I forgot to convert the proportions of special malts when I converted to 75%, so that'd have some effect. Also, I used 60L caramunich instead of crystal 40, so that might be slightly less fermentable. I'd imagine the biggest problem with this last one was just the temperature swing overnight the first night after pitching (The next day I got it in to a fermentation chamber with my temperature controller.). The other possibility, I suppose, is that all three batches used yeast from my LHBS...maybe the yeast wasn't handled very well. Also, I haven't been wicked focused on getting an accurate OG, so perhaps I was a few points high to begin with (The more I think about it, the more I think this could be it.). Anyways, thanks for your info.
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Old 03-01-2012, 07:49 PM   #7
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Quote:
Originally Posted by afr0byte View Post
Why? Do they automatically calibrate based on altitude/pressure?
No, but they're super accurate, and at your altitude, your attenuation problem is not height related.
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Old 03-01-2012, 07:52 PM   #8
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I think WW is suggesting you buy a thermapen to use as a reference to calibrate your current thermometer. They are suppose to be very accurate. http://www.thermoworks.com/blog/video-training/

Edit. WW beat me to it
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Old 03-01-2012, 08:16 PM   #9
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Quote:
Originally Posted by Dan View Post
I think WW is suggesting you buy a thermapen to use as a reference to calibrate your current thermometer. They are suppose to be very accurate. http://www.thermoworks.com/blog/video-training/

Edit. WW beat me to it
Well, I don't think I'll be spending $89 on a thermometer anytime soon, as I have a kid that's due soon. Certainly I would love to have a thermapen.
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Old 03-01-2012, 08:24 PM   #10
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Quote:
Originally Posted by wildwest450 View Post
No, but they're super accurate, and at your altitude, your attenuation problem is not height related.
How can you know that my problem isn't elevation related, since you don't know what my thermometer currently reads at boiling?

EDIT: Nevermind, I'm tired, I didn't completely think that one through.
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