Systems in Package (SiP) contain an increasing number of MEMS components such as resonators for time references or RF filtering. These resonators are packaged at wafer level and the cavity quality has to be guaranteed in terms of pressure and moisture level by the final manufacturing test. Therefore, environmental sensors could be integrated in the cavity to monitor these parameters and/or to calibrate the MEMS resonator with an electronic loopback. This paper presents the design of a low-cost Pirani pressure sensor for such application. First, the sensor principle is explained together with the fabrication process. Then, the closed-loop conditioning circuit, used to keep the sensor temperature constant, is described. It has been implemented on a first prototype that demonstrates a pressure measurement range extending over three decades, from 0.2 to 200 mbar. This range matches the cavity pressure level reached by some vacuum packaging techniques. In the end, the experimental results are used to develop a behavioral Matlab/Simulink model of the closed-loop sensor. This model provides simulation results in very good agreement with the experimental ones and it could be used to optimize the sensor design to cover an extended measurement range.