Creating a
Calibration
Curve
The volume of a calcimeter reaction cell determines the relationship
between the pressure increase and the amount of CO
2
released. This rela-
tionship is constant for a given reaction cell. The calibration curve and cal-
culated calibration factor are used to convert the amount of pressure
released into a percentage of calcium carbonate. All points on the calibra-
tion curve represent 100% CaCO
3
(for that sample weight). Any number of
samples can be used to construct the calibration curve. The following are
recommended for accuracy.
1. Prepare five sets of duplicate specimens with the following masses of
CaCO
3
:
0.2 ± 0.01 g
0.4 ± 0.01 g
0.6 ± 0.01 g
0.8 ± 0.01 g
1.0 ± 0.01 g
2. Load a calibration sample.
Perform the procedure outlined in the “Test Procedure”, step 3.
3. Tip the cell and allow acid to run out of the cup onto the sample. Swirl
the cell gently and continuously until a constant pressure is obtained.
This will take at least 10 minutes. Keep the reactants in the lower part
of the cell to avoid getting acid into the pressure gauge or pressure
transducer. As soon as the reaction has started, observe the rapidly
rising pressure. Record the peak pressure to the nearest 0.1 PSI (0.5
kPa) as the CaCO
3
pressure for the sample weight used.
If a mechanical shaker is available, it may be used to agitate the cell
rather than swirling the reactants. Agitate the sample for 10 minutes.
4. Repeat steps 2 and 3 for each of the remaining samples you prepared
in step 1.
5. For each sample create a plot on linear graph paper. On the x-axis plot
grams of CaCO
3
and on the y-axis plot the final pressure in PSI. Draw
a straight line through the average of the points. The graph on page 9
shows a sample calibration curve.
6. Because the relationship between pressure and sample size is linear,
you may assume the curve to be a straight line with a constant slope.
As the graph on page 9 illustrates, the slope of the curve is 2 PSI / .1 g
CaCO
3
, or 20 PSI / 1.0 g CaCO
3
, resulting in a slope of 20. Therefore,
the calibration factor is .05 × 100 = 5. This number is the slope or aver-
age slope for the equipment. It is a function of the volume of the reac-
tion cell.
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9
Tip
