"Never open the firing chamber during a vacuum phase. Rapid pressure equalization will crack the muffle and damage the heating element."
For zirconia frameworks, the manual allows you to program a long, slow cool (segment 4 or 5). Set rate to 10 °C/min down to 500°C to prevent framework cracking. This reduces residual stresses by 70%. Ivoclar Programat P90 Manual
The printed manual included with your P90 is comprehensive, but digital copies are available from: "Never open the firing chamber during a vacuum phase