Converting % Dissolved oxygen to mg/L

Does anyone know how to convert % dissolved oxygen to milligrams per liter?

[JohnS]

There are tables or formulas that express the maximum (saturated) O2 as a function of temperature, pressure, and salinity in mg/L. Detrmine saturated value, then multiply by the given percentage.

This Wikipedia article is a place to start (skip over the blood section), The formula for water considers salinity and temperature, but not pressure. Follow the link to the USGS site for a pdf document with tables. They include pressure. http://en.wikipedia.org/wiki/Oxygen_saturation

Much like water vapor in air for humidity calculations, it is not a trivial conversion

[JohnS]

Here is an "official" (web copy) of the USGS memo on calculating this. http://water.usgs.gov/admin/memo/QW/qw81.11.html
It is the only method accepted for USGS work. It contains typos probably related to conversion to the web. (missing minus signs, superscripts, etc)

This document is a "readme" for an Excel spreadsheet that implements the method. The author mentions the typos above and formula corrections he made (see Table 1) to get results in agreement with USGS tables. http://science.nature.nps.gov/im/uni...tor_Readme.doc

I think between the two, you can duplicate the math.

Perhaps it's just me as I'm not seeing it. Although the links that you've provided are great, but the equation(s) are for calculating DO saturation, using atmospheric pressure, temp, and salinity, and didn't show anything on how to convert DO mg/L to DO saturation.

I know this is an old questions, so hopefully you'll come across it and provide some additional insight.

Thanks :-)

[JohnS]

Originally Posted by Unregistered

Perhaps it's just me as I'm not seeing it. Although the links that you've provided are great, but the equation(s) are for calculating DO saturation, using atmospheric pressure, temp, and salinity, and didn't show anything on how to convert DO mg/L to DO saturation.

I know this is an old questions, so hopefully you'll come across it and provide some additional insight.

Thanks :-)

You have to calculate the saturated value for the conditions, then

Percent_Saturation = actual_DO/saturated_DO * 100%

or actual_DO = (Percent_Saturation/100) * saturated_DO

Saturation percentage is somewhat like relative humidity; it is a fraction of what the medium is capable of holding. The saturation value must be calculated.

Originally Posted by Unregistered

Does anyone know how to convert % dissolved oxygen to milligrams per liter?

0.115686275 mg/L per 1%

P1V1=P2V2

hope this formula helps.

Originally Posted by Unregistered

0.115686275 mg/L per 1%

how reliable is this conversion?

[JohnS]

Originally Posted by Unregistered

how reliable is this conversion?

It is perfect when saturated_DO is exactly 11.5686275 mg/L (about 9 °C, 760 mm) and wrong the rest of the time. Saturated DO is a complex function of temperature, pressure and salinity. See the tables referred to in the above links to see how much it varies with these parameters. There is no shortcut.

I need to convert a DO reading in mg/L to a percentage.