1 THE OSMOMAT 070 VAPOR PRESSURE OSMOMETER ...

36 User Guide OSMOMAT 070 Version 1.1
4.1.6 Determining the cell constants
After you have rinsed the baseline again, the calibration is terminated. Click Finish to access the
Evaluation Dialog. The result of a calibration measurement is a cell constant that indicates how large
the measurement effect is if a solution was measured with a 1x10 -3 Mol/kg concentration. At least
three solutions with an increasing osmolal concentration must be measured to determine the
molecular weights of polymer substances. Because substances with an increasing concentration more
frequently exhibit less than ideal behavior, the measurement effect is not proportional to the
concentration. For this reason, the osmolal concentrations of calibration solutions should be in a
narrow range and should match the feasible osmolal concentration of the substance to be measured
as closely as possible.
The measurement results are calculated with a special evaluation method to eliminate the nonlinear
effect. This evaluation method opposes the concentration/measuring effect quotient to the particular
concentration. If the measurement effect behaves in proportion to the concentration, a horizontal curve
is generated. With polymers, a curve with a negative or positive increase is displayed. The curve is
extrapolated up to a concentration of zero and thus eliminates the effect of the concentration. The
software performs a linear regression. The intercept determined from the regression calculation is the
cell constant. A graph of the calibration and readings is displayed at the end of the calibration
measurement. The measured values can be seen here and you can click to suppress individual values
if you wish. These values are not used for calculation. Use the Lin. Regression button to toggle
between the graph and measurement values. The cell constant is accepted after the calibration has
been saved and is now available for the subsequent measurements.
At the start and finish of the measurement, the program calculates the drift from the baseline values
and corrects it. You can also switch off this drift compensation if you don’t want it.
It is important to perform the measurement under the same conditions (temperature, probes, sampling
time, solvent) as the calibration.
36 User Guide OSMOMAT 070 Version 1.1