## 1 Introduction  
A recent report from the World Health Organization, dated March 2023, states that hypertension affects 1.28 billion adults aged 30-79 years worldwide, that an estimated 46% of adults with hypertension are unaware that they have the condition, that less than half of adults (42%) with hypertension are diagnosed and treated and that hypertension is a major cause of premature death worldwide [1, 2, 3, 4, 5, 6]. Thus, blood pressure measurement (BPM) has a real impact on the welfare of the worldwide population, on people's quality of life and on the reduction in the large costs associated with the control and treatment of this disease. Regarding BPM accuracy, it is important to note that the current gold standard is through intra-arterial measurement, but this is an impractical method in ambulatory conditions due to its highly invasive nature. Therefore, practically all clinical institutions rely on indirect alternatives, and office BP measurements are mostly performed using automatic oscillometric devices with a brachial cuff. However, most of the automated blood pressure measuring devices (ABPMDs) do not prove accurate readings according to international validation standards [7], and their regular test and calibration are essential. Moreover, currently, there are many thousands of unique automated BPMDs from different manufacturers, each one using proprietary and empirical algorithms to estimate BP parameters [8; 9; 10]. Thus, each manufacturer creates its own pressure curves or envelopes for NIBP measurement, which are sometimes referred to as O\({}^{\prime}\) curves, that are specific to a particular NIBP monitor. Those curves, obtained during the deflation phase of the cuff, are a graphical representation of the sensed pressure oscillation amplitude variations during the linear decreasing cuff air pressure [11; 12]. This paper proposes a low-cost NIBP calibrator that creates specific envelope profiles that provide an algorithm-independent calibration of ABPMDs using a common envelope profile for which the different ABPMDs give the same measurement results if they use the same BP evaluation algorithm type, being it the fixed ratio or the maximum slope algorithm. In this way, the proposed NIBP calibrator prototype, which will be presented in this paper, enables the evaluation of the fixed high ratios implemented by different ABPMD manufacturers and also enables us to check the differences between different devices in terms of implemented algorithms. Regarding comparisons with similar studies, it will be worth highlighting the EU-funded research project adOSSIG [13] that aims to improve the reliability and accuracy of blood pressure (BP) measurements by developing an interesting oscillometric signal generator (OSG). In this project, the hardware solution that is presented for an oscillometric signal generator is capable of generating oscillometric blood pressure pulses that pretend to be indistinguishable from real physiological human signals [14]. However, the research that is developed by adOSSIG is more centered in hardware issues than in software issues. The proposed simulator is developed considering a large database of physiological signals recorded from healthy patients and people with heart diseases and aims to achieve a reliable reproduction of those signals. Thus, the referred research project does not present any new methodologies to test ABPMDs but proposes specific oscillometric pulse envelopes to obtain an independent algorithm calibrator of the empirical algorithm used by each ABPMD. There are also a significative number of papers [15; 16; 17] that present other solutions for the implementation of an ABPMD calibrator, but almost in all of them, the performance evaluation of the proposed solutions requires clinical validation and the usage of a significant group of persons and trained medical staff. These requirements are expensive, not immune to different error types and time consuming in terms of data acquisition.