The "Syringe Potentiometer": A Low-cost Solution For Accurate and Precise Calibration Of Pneumotachographs

Abstract

PURPOSE: To modify a standard 3-L calibration syringe to provide a valid reference flow signal.

METHODS: A standard 3-L syringe was modified to incorporate a 3D-printed linear-slide mechanism affixed to the plunger. A string potentiometer was attached to a fixation point on the linear-slide-plunger assembly which provided a voltage signal in proportion to the plunger length and, consequently, volume. The first derivative of this signal provided a “reference” flow. Nonlinear K curves were obtained via the weighted averaging (WA) and polynomial (POLY) approaches, and from dividing the “reference” flow by the analog signal from the pneumotachograph (REF). The flow accuracy (average error) and precision (standard deviation of errors) of each method were compared over low (<1 L·s-1), medium (≥1-3.0 L·s-1), high (≥3.0-6.0 L·s-1) and very high (≥6.0-16.0 L·s-1) flow rates. Over 180 strokes were recorded from the syringe.

RESULTS: The accuracy of the WA, POLY, and REF methods displayed high accuracy (<±25 ml·s-1) over the low, medium and high flow ranges. However, the WA and POLY methods yielded modest underestimates of flow (75-100 ml·s-1) in the very high flow range, while the REF method remained within ±25 ml·s-1. The precision of each method was <25 ml·s-1 for low to high flow rates. At very high flows, the REF method displayed the best precision (60 ml·s-1) compared with the POLY (80 ml·s-1) and WA (>120 ml·s-1) methods.

CONCLUSIONS: While all 3 calibration methods displayed comparable performance up to 6.0 L·s-1, the “syringe potentiometer” and, by extension, the REF method provided the best accuracy/precision at very high flow rates (i.e., ≥6.0-16.0 L·s-1).