###### Abstract
These variables offer valuable information on the physiological state of microbial cultures; additionally, they allow for the analysis of biomolecular interactions, quantification of nucleic acids and proteins, and monitoring of cellular processes [23]. Among these variables, absorbance and fluorescence are the most widely used, not only for monitoring cellular processes in bacterial cultures and quantifying biomolecular interactions [23] but also as core techniques in various laboratory analytical procedures [24]. Although bioreactors can quantify various characteristics of a bacterial culture, these measurements are often expressed in arbitrary units, making them not directly comparable to those obtained from other instruments or bioreactors. As a result, the characteristics of the culture are not interchangeable between them. To overcome this limitation, the use of standardized measurement units enables consistent, quantitative comparisons across different instruments and experimental setups. Standardized measurement units enhance comparability across experiments, laboratories, and measurement systems, improving reproducibility and reliability [25; 26; 27; 28]. Moreover, the engineering of biological systems in a rational way, as pursued in synthetic biology, relies on the use of mathematical models for the design process. These models ideally require the use of parameters and process variables defined in precise standard biophysical units [25; 29; 30; 31]. An approach for the standardization of measurement units is expressing absorbance in particle-based and fluorescence intensity in Molecules of Equivalent Fluorescein (MEFL), as shown in works [19; 28]. This approach is valuable because it provides a clear physical and biological interpretation of optical measurements. In this way, absorbance measurements, typically recorded as optical density (OD), can be interpreted in terms of particle concentration, under the assumption that the particles have optical properties and scattering behavior comparable to an _E. coli_ cell [26]. In addition, fluorescence measurements can be expressed in terms of Molecules of Equivalent Fluorescein (MEFL), a standardized unit that correlates raw fluorescence measurements with the number of Molecules of Equivalent Fluorescein. MEFL units offer a molecular-scale interpretation of fluorescence intensity, enabling the estimation of the average number of Green Fluorescent Protein (GFP) molecules per cell [28]. Although standard units are documented at a small scale, typically employed in microplates of 96-well plates with small volumes, 200 \(\upmu\)L per well, they have only occasionally been applied at larger scales, particularly in bioreactors. The successful implementation of standard units such as Particles and Molecules of Equivalent Fluorescein (MEFL) in the context of microplates demonstrates their potential utility and scalability. With the aim of enabling a physical and biophysical interpretation of optical measurements in experimental setups involving larger culture volumes, in this work we apply the standardized units' approach to calibrate a Chi.Bio mini-bioreactor [20]. In addition, these standardized units can serve as input measurements for downstream applications, such as the estimation of the growth rate and protein synthesis rate. This type of information is particularly relevant in fields like microbiology, synthetic biology, and biotechnology, where precise control of cellular behavior enhances both experimental reproducibility and industrial process performance [32]. Since growth and protein synthesis rates cannot be directly measured, indirect estimation methods must be employed [33; 34]. The growth rate can be estimated using on-line software sensors (i.e., mathematical observers) that track the accumulation of particles, which reflects the biomass concentration in the system [35; 36].