In the previous part of the tutorial the graph showed that the fourth data point was the correct point at which to correct for compressibility. Enter the number 4 into the box Correct for compressibility using top .. points.
Then click the whole Correct for compressibility ... button to perform the correction and add a third, green curve to the graph. The aim of the correction is to bring the gradient at high pressure for the corrected green intrusion curve as close to zero as possible - i.e. as near horizontal as can be achieved, as shown in the next screenshot. For you own samples choose an appropriate number of points to achieve this - you may have to find the correct number by trial and error.
The Results dropdown menu on the home page is now fully populated with the physical characteristics calculated by PoreXY during the blank and compression correction of the experimental data.
The figure above shows three different values of porosity for the tutorial sample, and shows why it is important to correct for the compressibility.
The Compression section of the Results drop-down list provides a value of Bulk Modulus of 26 GPa and a compressibility of 0.03817 kPa-1. Note that this is the compressibility characteristic of the solid phase of the sample - not the porous sample as a whole. Also shown is the estimate of accuracy - which is low in this range - i.e. the actual bulk modulus could be a few GPa different. The result is also sensitive to the number of points chosen, as illustrated by the 20 GPa result for 5 data points as shown in the section Exporting data.
An example of the usefulness of this measure is in determining the structure of paper coatings comprising kaolin clay whitener mixed with latex binder. If the latex binder is between the individual kaolin particles, the solid phase is much more compressible than if the kaolin particles form a contiguous matrix with binder simply gluing that matrix to the fibre substrate.
The next stage of the tutorial shows how you can use the trimming function to remove possible edge effects, or a plateau at high pressure.