The syringe potentiometer: a low-cost device for pneumotachograph calibration

Abstract

The purpose of this report was to 1) detail the construction of a low-cost device that provides a "reference" flow waveform for pneumotachograph (PNT) calibration, i.e., the "syringe potentiometer" (SP), and to compare the 2) accuracy and 3) practical performance of "calibration curves" obtained with the SP device to other more established methods of PNT calibration, i.e., the weighted averaging (WA) and polynomial least-squares (PolyLS) methods. Volume and flow waveforms obtained via the SP device were validated against a motion capture system and were deemed accurate surrogates of actual syringe volume and flows. The SP device was used to construct a calibration curve of a PNT by dividing the flow waveform of the SP by the analog output of the PNT amplifier. A total of 187 inspiratory and 187 expiratory strokes were collected. When the entire data set of expiratory strokes was used, the SP, WA, and PolyLS methods together demonstrated acceptable volume and flow errors as per American Thoracic Society/European Respiratory Society recommendations (less than ±3.5% and less than ±5.0% errors, respectively). The "practical" performance of each method was assessed with a nested subsampling procedure, whereby volume and flow errors were evaluated as the number of strokes was increased (in blocks of 5 strokes). To this end, the SP method demonstrated practical performance superior to that of the WA and PolyLS approaches, whereby acceptable volume and flow errors were achieved after only 5 calibration strokes; the WA and PolyLS methods required 15 and 20 strokes, respectively, to achieve the same level of volume and flow accuracy.NEW & NOTEWORTHY This report describes the construction and validation of a low-cost device for the purposes of pneumotachograph (PNT) calibration: the "syringe potentiometer" (SP). Calibration of a PNT with the SP device yielded acceptable volume and flow errors (<3.5% and 5%, respectively) across a wide range of flows (<0.5 to 15 L/s). The SP device offered superior "practical performance" over other established PNT calibration methods, whereby acceptable volume and flow errors were achieved after only five calibration strokes.

Keywords: calibration; pneumotachograph; potentiometer; respiratory airflow; syringe.

Fig. 1.

Illustration of the syringe potentiometer assembly.

Fig. 2.

Frequency distribution of syringe flows through the middle third of inspiratory and expiratory calibration strokes. A total of 187 inspiratory and 187 expiratory strokes were obtained.

Fig. 3.

Measurement agreement of instantaneous syringe volume (A and B) and flow rate (C and D) obtained via the syringe potentiometer device and motion capture system. The gray points in A and C represent observed values of syringe volumes and flows measured via the syringe potentiometer device and motion capture system. The dashed lines in A and C denote lines of identity (i.e., perfect agreement) for each relationship. The vertical dashed lines in B and D illustrate the 95% confidence percentiles for the distribution of errors incurred by measuring syringe volume and flow via the syringe potentiometer device (whereby values obtained via motion capture are taken as the reference).

Fig. 4.

Calibration curves obtained via the syringe potentiometer (SP), weighted averaging (WA), and polynomial least-squares (PolyLS) methods of pneumotachograph (PNT) calibration. k, Calibration constant at the corresponding voltage output from the PNT amplifier. The solid lines represent calibration curves obtained via the 3 methods with the entire data set of calibration strokes. The gray points represent empirical values of k obtained by dividing the reference flow waveform (from the syringe potentiometer) by the analog voltage output of the PNT amplifier.

Fig. 5.

Hyperparameters for the weighted averaging (WA) and polynomial least-squares (PolyLS) calibration methods as a function of increasing numbers of calibration strokes. Values are presented as means ± 95th percentile confidence interval.

Fig. 6.

Syringe volume errors produced by syringe potentiometer (SP), weighted averaging (WA), and polynomial least-squares (PolyLS) calibration methods as a function of increasing numbers of calibration strokes. Values are presented as means ± 95th percentile confidence interval (CI_95%). The gray shaded region denotes the ±3.5% error margin of volume errors as recommended by the American Thoracic Society and the European Respiratory Society (11). Note that CI_95% of volume errors produced by the SP and WA methods fall within the recommended error margin after only 5 calibration strokes, whereas it takes 20 strokes with the PolyLS method to achieve the same level of volume accuracy.

Fig. 7.

Flow errors produced by syringe potentiometer (SP), weighted averaging (WA), and polynomial least-squares (PolyLS) calibration methods as a function of increasing numbers of calibration strokes. Values are presented as means ± 95th percentile confidence interval. The gray shaded region denotes the ±5.0% error margin of volume errors as recommended by the American Thoracic Society (ATS) and the European Respiratory Society (ERS) (11). Note that flow errors produced by the SP method fall within the ATS/ERS-recommended error margin across all ranges of flow rates after only 5 calibration strokes.

Fig. A1.

Method for quantifying the instantaneous displacement of the syringe plunger via motion capture. m₁–m₄, passive-reflective markers 1–4. The instantaneous displacement of the syringe plunger is calculated as the perpendicular distance between m₄ and the planar surface formed by m₁, m₂, and m₃.