APPLICATION OF WEIGHTED LEAST SQUARES TO CALIBRATE A DIGITAL SYSTEM FOR MEASURING THE RESPIRATORY PRESSURES José Leonardo Ferreira, Nadja Carvalho Pereira, Marconi de Oliveria Júnior, João Lucas da Silva Flávio Henrique Vasconcelos, Carlos Julio Tierra-Criollo Department of Electrical Engineering, UFMG, Av. Antônio Carlos 6627, Belo Horizonte, Brazil joseleo@cpdee.ufmg.br , carjulio@cpdee.ufmg.br Raquel Britto, Verônica Franco Parreira Department of Physiotherapy,UFMG, Av. Antônio Carlos 6627, Belo Horizonte, Brazil Keywords: Calibration, measurement uncertainty, respiratory pressures measuring, weighted least squares. Abstract: This article presents the results associated to calibration and evaluation of the measurement uncertainty of a digital respiratory pressures measuring system developed at the Biomedical Engineering Research & Development Laboratory (NEPEB) of the Department of Electrical Engineering – UFMG. The proposed method uses the least squares weighted regression to establish the measurement model and to evaluate the uncertainty. The standard expanded uncertainty estimated by the model was 1.4 kPa. 1 INTRODUCTION The qualitative and quantitative characterization of the result of a measurement is a demand of metrology organisms, providing knowledge about the reliability, important to quality control of products and services. In Brazil, the National Institute of Metrology, Standardization and Industrial Quality (INMETRO) is the local NMI (national measurement institute) and as such is organism responsible for metrology politics and for establishing criterions and general normalization relative to expression of uncertainty (INMETRO, 2007). Brazilian Ministry of Health (MS) established the compulsory certification for medical and hospital equipment (MHE), not only attributing conditions for a major credibility of use, but, also causing a huge demand for calibration services in the country. A non-calibrated medical equipment can lead clinic diagnostic errors, wrong therapies and iatrogeny in patients. Today, regulation of certification process of MHE quality in Brazil is defined by two normative documents: MS and ANVISA (1999) and MIDIC and INMETRO (2006). The former defines the strategies, while the latter describes the technical details for certification. A digital system was developed at NEPEB (Silva, 2006) in order to measure the maximum pressures exerted by the muscles of the human respiratory system. It was designed as equipment to be used in diagnostic and therapy and, therefore, must be submitted to metrological evaluation according to the standards of the MS. Thus, the measuring system must be certificated, and this process requires instrument calibration and the evaluation of the measurement uncertainty. The measuring system includes a signal acquisition module for pressure and an analogical to digital signal conversion module that allows the digital data to be acquired using a personal computer (PC) through an USB interface. 2 OBJECTIVES The signal acquisition module of pressure, denominated manovacuometer, uses two sensors of differential pressure, model MPX5050 by Freescale Semicondutor Inc. (Austin, Texas). The sensor possesses two pressure sides (figure 1) and the measured value corresponds to the differential 220