Linearity of Microbalances Used for Gravimetric Method Calibration of Piston Pipettes, Determined by the Method of Small Mass Increments

Abstract

Measuring nonlinearity (linearity error) in high-resolution electronic balances, including microbalances, is challenging. Today's electronic balances are usually characterized by a small linearity error (with tolerance limits for nonlinearity specified by manufacturers in the order of several elementary divisions), and all criteria for measuring nonlinearity must meet or even exceed this tolerance limit. Linearity is determined over the full range of the balance at several measurement points (usually at least five points). The use of balances in the calibration of piston pipettes requires the determination of the linearity error within a very small working range. The paper describes a new method for testing the linearity of electronic balances used for the calibration of piston pipettes using the gravimetric method with small, constant mass increments, which corresponds to the process of calibrating piston pipettes. Studies have shown that the method of determining linearity using small mass increments gives more accurate results — yielding smaller linearity errors in the measuring range corresponding to pipette calibration. When incorporating the linearity component of the balance in the measurement uncertainty model for calibrating piston pipettes, the new method allows for reducing the uncertainty of pipette calibration. This is important for laboratories that calibrate pipettes, as it enables improvement in the calibration and measurement capability parameter, and for users of piston pipettes who receive pipettes with more accurate calibration.