This paper concerns a theoretical analysis and a case-study about noise in chemical processes using discrete equivalents. Many studies include a zero process noise input in a plant model or Gaussian noise only. Here, the R-wpsd and R-vpsd that represent a physical process are obtained, and it is confirmed that pure white noise does not exist in nature. The process noise magnitude R-w and the measurement noise magnitude R-v are calculated for discrete equivalents choosing a bioreactor as a model. Finally, an algorithm for the measurement noise is presented.