The purposes of this article are to describe the process of building, programming and testing of the microprocessor based Assmann psychrometer. Studies were conducted based on a thesis in Institute of Metrology and Bioengineering, Faculty Mechatronics of Warsaw University of Technology. The main task was to implement modern components to a conventional construction of the psychrometer, as a way to increase its functionality. Psychrometers were designed for measuring relative humidity. The measure of humidity is based on the difference of indication of two sensors – dry bulb thermometer (ambient conditions) and wet bulb thermometer (100% humidity conditions). Under the influence of air movement, water evaporates from the thermometer cooling down the surface of the wet bulb sensor. Using mathematical transformations of the temperatures and making additional measurements (air flow, atmospheric pressure), the device is now able to calculate the relative humidity. The main component in the device is a microcontroller – KAmduino Uno which is connected to two thermometers, an atmospheric pressure sensor, an universal DTH22 sensor and a LCD display. The microprocessor allows not only immediate calculation but also archiving data using a serial port monitor. Calculations are based on the Sprung and Buck models. The device monitors the presence of critical errors such as sensor disconnection or over-measuring. Finally, calibration of thermometers, tests, measurements and validation of the device were conducted. © Springer International Publishing AG, part of Springer Nature 2018.