::: Measuring technology
The True Measure of CO2
Beverage carbon dioxide analysis based on "total
pressure and temperature” measurement is the most
widely used method worldwide. The vast majority of
beverage labs applies this method many times per
day with great success.
An article was published in Beverage World
International (May/June 2001 edition) under
the title "The Truth about CO2” which states:
A number of beer and soft drink manufacturers use instruments that measure the "total
pressure and temperature” to determine how
much CO2 is in the drink. This method to calculate the amount of CO2 is risky because it
assumes the only gas present in the drink is
CO2, which is often not the case.
The truth is: modern "total pressure and temperature” based systems do assume the presence of other gases in the beverage. They minimize the influence of other gases thoroughly in
order to arrive at accurate CO2 results, as will
be proven here.
There are two principles of minimizing the
influence of other dissolved gases on the "total
pressure and temperature” method.
Both principles utilize the fact that these other
gases, which are oxygen and nitrogen, have a
much lower solubility in the beverage than carbon dioxide.
Why is that so?
Due to their very low solubility in beverages,
about three quarters of the total amount of
oxygen and nitrogen contained in a bottle or
can is actually in the headspace. Only about
one quarter is dissolved in the beverage. The
major part of the nitrogen and oxygen in the
headspace is therefore removed when releasing
the headspace pressure and does not contribute to the measured total pressure.
2. In modern style CO2 analyzers, both for online and lab applications, a very effective
method of eliminating the impact of dissolved
oxygen and nitrogen is applied:
A measurement chamber is completely filled
with sample, sealed and then expanded by e.g.
10%. Pressure equilibrium is achieved by vigorous stirring or other means prior to the pressure and temperature measurement. This volume expansion has a very distinctive effect: the
contribution to the total pressure virtually
vanishes for all other gases except CO2.
Why is that so?
1. With conventional lab instruments, the
bottle or can is pierced or opened and the head
pressure released into the surroundings.
Releasing the headspace pressure prior to performing the measurement removes most of the
oxygen and nitrogen. The remaining small
quantity contributes to the measured total
pressure to a much lesser extent and is normally insignificant for the CO2 analysis result.
Again, it is a consequence of the very low solubility of oxygen and nitrogen in beverages. A
volume increase of the measurement chamber
causes the pressure of oxygen and nitrogen to
drop dramatically. There is simply not enough
of these gases present in the beverage to build
a reasonable pressure in the gas phase generated by the volume expansion of the measurement chamber. Only CO2 can build up signi-
ficant pressure in the gas phase, because its
solubility in the beverage is so much larger.
The pressure which is finally measured can be
considered the sole carbon dioxide pressure.
We will demonstrate the impact of high and
low gas solubility on the gas pressure measured
after expansion of the measurement chamber
in the following picture. For comparison reasons we assume that there is 1 bar of saturation
pressure for both carbon dioxide and nitrogen.
It should be noted that normally the saturation
pressure of nitrogen in beverages is much lower
due to degassed water being used for beverage
production, while the carbon dioxide saturation pressure is typically higher.
The picture on the second page shows the fol
lowing: When one liter of beverage is saturated
with 1 bar of carbon dioxide, about 0.75 liters
of carbon dioxide are actually dissolved. If that
beverage is in a sealed chamber which is then
expanded by 10%, the carbon dioxide pressure
will decrease by 12% to 0.88 bar.
If the same beverage is saturated with 1 bar of
nitrogen, only 0.015 liters of nitrogen are actually dissolved due to its poor solubility. If the
chamber volume is expanded by 10%, the
nitrogen pressure will drop by 87% to 0.13
bar!
For a better understanding of what is the true
result of the CO2 measurement using "total
pressure and temperature” we have to go into
more detail. The above mentioned article presents two "practical” examples with two cans of
beverage, one containing 4 V/V of CO2, 1.1
ppm N2 and 0.5 ppm O2, and the other con-
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taining 3.8 V/V of CO2, and the exceptionally high content of 5.5 ppm N2 and 2.5 ppm
O2. Both cans show the same total pressure.
Therefore the author of that article wrongly
assumes that a "total pressure and temperature” based measurement shows the same CO2
result for both cans, although the actual carbon
dioxide contents are quite different.
When applying a volume expansion of 10% to
the measurement chamber of a modern "total
pressure and temperature” based carbon dioxide analyzer, the following will happen:
• the CO2(partial) pressure will drop from
5.34 to 4.70 bars in the first and from 5.04
to 4.45 bars in the second example,
• the N2 pressure will drop from 0.06 to
under 0.01 bar in the first and from 0.30 to
0.04 bars in the second example and
• the O2 pressure will drop from 0.02 to 0
bars in the first and from 0.08 to 0.02 bars
in the second example.
The "total pressure and temperature” based
analyzer will measure the total pressure after
volume expansion. The measured pressure is
automatically corrected for the volume expansion and converted to V/V of carbon dioxide
by multiplying by its solubility.
So the true CO2 result for the first example
will be:
Measured pressure:
PM = 4.70+0.01+0.00= 4.71 bar
Pressure correction for volume expansion:
PC = PM*(1+0.1/0.75) = 5.34 bar
CO2 result: CO2 = PC*0.75 = 4.0 V/V.
The true CO2 result for the second example
will be:
Measured pressure:
PM = 4.45+0.04+0.02 = 4.51 bar
Pressure correction for volume expansion:
PC = PM*(1+0.1/0.75) = 5.11 bar
CO2 result: CO2 = PC*0.75 = 3.8 V/V
(or 3.83 V/V with 2 digits after the point).
This is the truth about CO2 measurement:
Modern CO2 analyzers based on "total pressure and temperature” measurement are virtually
unaffected by the dissolved oxygen and nitrogen in the beverage. They measure the true
CO2 content very accurately. Unlike older
models of this kind, they are independent of
altitude and weather conditions. They are not
only highly accurate, but also
• drift-free,
• less affected by process conditions such as
line pressure and flow rate,
• faster and
• more durable and reliable
than membrane based CO2 sensors. Because of
the absence of any membrane, "total pressure
and temperature” based systems do not require
the complex membrane exchanges, elaborate
sensor calibrations or frequent re-adjustments
which are necessary for membrane based CO2
sensors. So in truth, "total pressure and temperature” based CO2 analyzers are the better
option for real life CO2 measurement and control.
Gerhard Murer and Josef Bloder
Anton Paar GmbH
Anton-Paar-Str. 20
A-8054 Graz
Tel: +43 316 257-300
E-mail: [email protected]
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