Physical and chemical parameters used for in-situ water quality monitoring of rivers and seas, are measured using sensors whose characteristics are highly sensitive with temperature. In this paper, the project, design and implementation of a testing bath with automated temperature control for environmental monitoring sensors characterization are presented. The paper also details the cooling and heating thermoelectric pump based on Peltier modules for the temperature control of the liquid, the tests conducted to obtain its optimal performance, the set of sensors and signal conditioning to measure the temperature and the proportional-integral-derivative (PID) algorithm implemented to control the bath temperature.