Draw-off temperature
Where to set the draw-off: the boiling point of water at your sugarhouse today, plus the boiling point elevation for the density you finish to.
01 · Inputs
Enter the boiling point of water you measured today, or estimate it from your elevation and the barometric pressure, then pick the density.
Boiling point of water
Boil pure water at the sugarhouse and read it after the thermometer has been in at least 3 minutes. Use the same thermometer you draw off with, so a steady error in it cancels out.
Finish to
Draw-off at nearby densities
Add the boiling point of water to fill in the draw-off column. The elevations come straight from the manual’s table.
| Density | Above boiling water | Draw off at |
|---|---|---|
| 65.5°Brix | 6.9°F | – |
| 66.0°Brix | 7.1°F | – |
| 66.5°Brix | 7.3°F | – |
| 66.9°Brix(interpolated) | 7.46°F | – |
| 67.0°Brix | 7.5°F | – |
03How this is calculated
Syrup boils hotter than water, and the gap grows with density. The North American Maple Syrup Producers Manual tabulates that gap, the boiling point elevation, for pure sucrose: 7.1 °F (3.9 °C) above boiling water for 66.0 °Brix, 7.3 °F for 66.5, and 7.5 °F for 67.0. The table has no row for 66.9, so the calculator interpolates between 66.5 and 67.0 and adds about 7.46 °F. The draw-off temperature is today’s boiling point of water plus that figure.
Water boils at 212 °F only at 29.92 inHg. Elevation and weather move it, which is why the manual says to boil water at the sugarhouse each day and not assume 212. Near syrup density, temperature is sensitive: the manual notes that going from 64.5 to 67.0 °Brix raises the boiling point only about 1 °F, so a thermometer one degree off puts the syrup about 2 to 2.5 °Brix light or heavy. Measuring the water with the same thermometer you draw off with cancels a steady error in it.
The estimate takes the barometric pressure the weather service reports, which is reduced to sea level, and works back to the pressure at your elevation with the standard atmosphere the National Weather Service uses for station pressure. It then finds the temperature at which water’s vapour pressure equals that pressure, from the NIST equation for water. At 29.92 inHg and sea level that gives 212.0 °F, and on a standard day water boils about 1.8 °F lower for each 1,000 ft. Some services report a sea-level pressure reduced a little differently, so treat the result as a starting point.
The table is for pure sucrose and maple syrup is not quite that, so the draw-off temperature gets close, not exact. Confirm the finished density with a hydrometer or refractometer once the syrup has cooled to room temperature.
Sources
- Ch. 7, boiling point elevation method (draw-off offsets, measuring the day’s boiling point of water)NAMSPM 3rd ed. (2022)p. 7-21
- Ch. 7, Table 7.1 "Boiling point elevation in pure sucrose solutions"NAMSPM 3rd ed. (2022)Table 7.1, p. 7-22
- Ch. 8, pure maple syrup definition and legal density (66.0 to 68.9 percent soluble solids; 66.9 °Brix minimum in Vermont and New Hampshire, footnote 1)NAMSPM 3rd ed. (2022)p. 8-2
- Water, phase change data: Antoine equation parametersNIST Chemistry WebBook, NIST Standard Reference Database 69 (U.S. National Institute of Standards and Technology)Antoine table, log10(P/bar) = A − B/(T/K + C), row 344 to 373 K: A = 5.08354, B = 1663.125, C = −45.622 (coefficients calculated by NIST from Bridgeman, O.C., Aldrich, E.W. (1964), Vapor Pressure Tables for Water, J. Heat Transfer 86(2):279–286, doi:10.1115/1.3687121)
- Station Pressure (formula for the station pressure script)National Weather Service, WFO El Paso, weather calculatorstation pressure from the altimeter setting (inHg) and the station elevation in metres (h_m = 0.3048 × h_ft), standard atmosphere with 288 K at sea level, a 0.0065 K/m lapse rate and exponent 5.2561
- Pressure definitions: station pressure, altimeter setting, mean sea level pressureNational Weather Service, WFO Denver/Boulderaltimeter setting is the reading heard in broadcasts, not the true pressure at a station but reduced to sea level with the standard atmosphere; mean sea level pressure uses a mean temperature for the preceding 12 hours; station pressure drops about 1 inHg per 1,000 ft
- Ch. 11 "Aircraft Performance", Figure 11-2 "Properties of standard atmosphere"Federal Aviation Administration, Pilot’s Handbook of Aeronautical Knowledge (FAA-H-8083-25)p. 11-3, Figure 11-2: 29.92 inHg at sea level, 28.86 at 1,000 ft, 27.82 at 2,000 ft, 26.82 at 3,000 ft, 25.84 at 4,000 ft, 24.89 at 5,000 ft; p. 11-2, pressure falls about 1 inHg per 1,000 ft
- Lesson 1, "Probing Pressure"Pennsylvania State University, Department of Meteorology and Atmospheric Science, METEO 101: Understanding Weather Forecasting (course materials by S. Seman)nearly all sea-level pressures lie between 950 and 1050 millibars, most between 980 and 1040; long-term average about 1013 mb; standard mercury column 29.92 in
Keep the boil on the record
Sapline keeps syrup production and boil logs with the syrup Brix and grade of each batch, linked to the sap collections that went into it.
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