Anritsu recommends allowing the instrument to warm up for approximately 10 minutes to typical operating temperature before performing a user calibration.

During operation, if the internal temperature moves outside the valid cal window, any applied user calibration factor will automatically be turned off – and the factory default 1‑Port ReadyCal applied, when applicable. In this case, the instrument will display: 1‑PORT ReadyCal (↕ °C) to indicate that the User Cal has been turned off due to a temperature change.

The User Cal turns back on automatically if the internal temperature moves back into the valid cal window. Alternatively, you can perform a new user calibration, which will then have a new cal temperature window based on the current ambient temperature ±10 °C.