ISO pressure unit conversion to Newtons
Q: How can pen pressure values be converted to Newtons for storing in ISO format?
It is not possible to convert precisely from digitizer units to standard units of pressure because of the variability of the pen characteristics.
The digitizer values are not a linear function of the applied force, and furthermore there is hysteresis, meaning that the result at a given load will be different depending on whether the pen is being loaded or unloaded.
For verification purposes it is better to compare normalized force curves rather than absolute values.
Please note that ISO 19794-7 is entitled "Information technology – Biometric data interchange formats".
It is intended for use when biometric data is to be exchanged between systems from different vendors - it isn't primarily a storage format and there is no obligation to store biometric data in this way.
If you do want to use it then the uncalibrated data can be stored in ISO format by using the native digitizer units and ignoring the optional scaling value in the channel description.