Due to the ubiquitous spread of SARS-CoV-2 coronavirus infection, the demand for anaesthesia and respiratory devices has significantly increased since the beginning of 2020. In particular, artificial ventilation devices, inhalation anaesthesia devices, or oxygen inhalers. The respiratory support devices must include special medical devices designed to heat and increase the moisture content of the respiratory mixture supplied to the patient from the respiratory therapy apparatus. These medical devices are known to as humidifiers. The production of a humidifier for breathing mixtures “TEVLAR” – “UOMZ” according to TU 32.50.21-121-07539541-2018 with accessories was organized jn the territory of Joint Stock Company “Production Association “Urals Optical & Mechanical Plant” named after Mr. E. S. Yalamov”. The electronic measuring instruments unit of the device has been modified within the framework of the project for the modernization of the humidifier of breathing mixtures “TEVLAR” – “UOMZ”. In particular, in order to increase the accuracy of thermal stability of the heating element when heating the breathing mixtures to the target temperature, the software of the electronic measuring instruments unit was modernized by the Medical Devices Design Bureau, namely, the stable PID controller was implemented and tuned. Thus, this article provides an overview of the methods for tuning the PID controller, justifies the choice of PID tuning by the Ziegler-Nichols method and also provides the experimental results to confirm device operation stability.   Lipchak D. A., Dushkina A. A., Chupov A. A. External humidifier for ventilator: PID control based on the Ziegler-Nichols method. Ural Radio Engineering Journal. 2020;4(4):376–317. DOI: 10.15826/ urej.2020.4.4.002.