The easiest approach to monitoring pressure difference is to set the outlet relative pressure to zero. Monitoring a pressure in the model is then monitoring the pressure difference with the outlet. To stop the solver based on monitor, you can use Monitors –> Convergence Monitors –> Convergence Manager. The convergence manager facility allows you to set convergence conditions on the solution that are based on the values from surface, volume, lift, drag or moment monitors. Convergence check for the monitors can be independent of that for the residuals or both convergence monitor and residuals can be checked.