I went to Retired_Navy's link and sent the question.
I just got an answer back, so I thought I would share what I found out.
"Thank you for contacting The Coca-Cola Company, Mr. Short. We appreciate
the opportunity to answer your questions about carbonation.
Carbonation is measured in "volumes." The traditional method for
measuring CO2 levels in soft drinks is as follows: You must first use a device
called the Zaha-nagel Device. The bottle closure or can closure is pierced and
a probe is placed into the head space of the package. The package is then
shaken and the pressure in the head space is measured and recorded. Then the
temperature of the product is measured and recorded. Next you must refer to
the "Table of Solubility of CO2 in Water." Find the temperature and
pressure measurements on the table and that will give you the corresponding
volumes of CO2.
Carbonation is the incorporation of carbon dioxide into water. In soft drinks,
carbon dioxide produces the distinctive "sparkling" quality and a
"bubbling" effect as the drink is poured from the container. The
bubbling of carbonated drinks has to do with the surface tension. Surface
tension is a physical attraction between the surfaces of two substances. The
presence of ice increases the surface area, thereby causing more bubbling.
CO2 is a gas that is dissolved. A gas escapes whenever it is agitated or moved
around. The temperature of the beverage will determine how much carbonation
can be dissolved into the beverage. More gas will remain in solution in a cold
beverage than in a warm or hot beverage. Cans are filled under pressure to
keep the beverage carbonated at a specific level.
Bubbles of gas can form along the surface of the can (the number of bubbles
depends on many things including the microscopic smoothness of the can). If
there are a lot of bubbles in the liquid as compared to the headspace, foaming
will occur when the can is opened. Tapping the can may release these
microscopic bubbles and they will either go back into solution in the beverage
or become part of the headspace gas.
The target level of carbonation in Coca-Cola classic is 3.2 - 4.2 (measured in
volumes).
We hope this helps to answer some of your questions. If you have additional
questions or comments, please visit our Web site again.
Sheila
The Coca-Cola Company
Industry and Consumer Affairs"