What is the accuracy of the hot air pre-heaters 853, FR-820, and FR-830?

The hot air pre-heaters do not have a temperature accuracy specification. Please see KB Article 10340 - INFO: Calibration and Hot Air Rework Station Temperature for a detailed explanation of measuring hot air. As analog stations, there is no precise way to set the temperature and then measure for adjusting accuracy. If you need to adjust the setting versus your measured temperature, please see the user's manual for further instructions.