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GLEON / LakeMetabolizer

Collection of Lake Metabolism Functions
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DO.sat conversion from ml/L to mg/L #101

Closed aappling-usgs closed 9 years ago

aappling-usgs commented 9 years ago

DO solubility is computed by Weiss, Garcia-Gordon, etc. in milliliters per liter (ml/L), but we need this value in mg/L. Several coefficients exist in the literature for this conversion.

Staehr et al. 2012 cite Weiss 1970 for a coefficient of 1.423.

USGS #81.11 memo (http://water.usgs.gov/admin/memo/QW/qw81.11.html) says,

The DO solubility is calculated in milliliters per liter (ml/L) which must be multiplied by the constant 1.4276 to convert to milligrams per liter (mg/L).

A more recent USGS memo (https://water.usgs.gov/admin/memo/QW/qw11.03.pdf) corrects the previous one, writing,

The Weiss equations compute a result in units of mL/L, and a factor was used to convert that result to mg/L. Unfortunately, the factor used by USGS since 1981 was based on an incorrect assumption that the Weiss results were for an ideal gas at standard temperature and pressure (STP: 1 atm, 0 °C). In fact, Weiss intended his results to be expressed as the volume occupied by oxygen as a real (non-ideal) gas at STP (see Benson and Krause, 1980). Therefore, the 1.4276 factor used by USGS in Technical Memorandum 81.11 to convert mL/L to mg/L is incorrect; that factor should have been 1.42905 mg/mL.

Based on this progression of recommendations, I suggest that LakeMetabolizer should adopt 1.42905. I can include this change in my next pull request.

rBatt commented 9 years ago

Nice. We might want to include some of that information in the help pages. Perhaps simply as a reference, note, detail. Whichever.

On Fri, Aug 28, 2015 at 11:26 AM, Alison Appling notifications@github.com wrote:

DO solubility is computed by Weiss, Garcia-Gordon, etc. in milliliters per liter (ml/L), but we need this value in mg/L. Several coefficients exist in the literature for this conversion.

Staehr et al. 2012 cite Weiss 1970 for a coefficient of 1.423.

USGS #81 https://github.com/GLEON/LakeMetabolizer/pull/81.11 memo ( http://water.usgs.gov/admin/memo/QW/qw81.11.html) says,

The DO solubility is calculated in milliliters per liter (ml/L) which must be multiplied by the constant 1.4276 to convert to milligrams per liter (mg/L).

A more recent USGS memo (https://water.usgs.gov/admin/memo/QW/qw11.03.pdf) corrects the previous one, writing,

The Weiss equations compute a result in units of mL/L, and a factor was used to convert that result to mg/L. Unfortunately, the factor used by USGS since 1981 was based on an incorrect assumption that the Weiss results were for an ideal gas at standard temperature and pressure (STP: 1 atm, 0 °C). In fact, Weiss intended his results to be expressed as the volume occupied by oxygen as a real (non-ideal) gas at STP (see Benson and Krause, 1980). Therefore, the 1.4276 factor used by USGS in Technical Memorandum 81.11 to convert mL/L to mg/L is incorrect; that factor should have been 1.42905 mg/mL.

Based on this progression of recommendations, I suggest that LakeMetabolizer should adopt 1.42905. I can include this change in my next pull request.

— Reply to this email directly or view it on GitHub https://github.com/GLEON/LakeMetabolizer/issues/101.

aappling-usgs commented 9 years ago

Good idea, @rBatt. I will update the roxygen2 block for o2.at.sat with the new citations and a longer description paragraph detailing the the choices made in the code.