Two-point calibration procedure of the forced oscillation technique.

The forced oscillation technique is usually calibrated by loading the measuring device with a known impedance. A correction function is calculated, relating the measured and reference impedances at each frequency. However, this one point calibration procedure does not account for transducer asymmetry. A procedure has previously been presented to circumvent this problem: in addition to one known reference impedance, the calibration was repeated with the system occluded (infinite impedance). The aim of the present study was to evaluate a variant of this procedure, in which instead of resorting to an extreme condition imposing high requirements on the flow measuring system, two reference loads of 4 and 50 hPal-1 s were measured, thus covering the range of impedances observed in children and infants (a two-point procedure). The calibration procedure was performed with these two impedances and evaluated with a third impedance of approximately 17 hPal-1 s. The results of three calibration procedures were compared: one-point, two-point and a previously reported calibration procedure. Impedances consisted of sintered glass and mesh wire screens mounted in glass or polyvinyl tubes. For low impedance values, in the range of 4 to 17 hPal-1 s, measured and predicted values were similar for the three calibration procedures at frequencies from 4-52 Hz, although with the one point calibration procedure there was some underestimation above 44 Hz. With the highest load, especially above 32 Hz, marked discrepancies between measured and predicted values were observed with the one-point calibration procedure and the previously reported calibration procedure. Under these circumstances the two-point procedure is preferred.