Improved accuracy and extended flow range for a Fleisch pneumotachograph
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A large linear flow range and a small instrumental dead space volume are incompatible properties for a pneumotachometer (PTM). The linearity of a Fleisch number 2 PTM is studied for flows up to 6 litre s−1 (nominal range 0–2 litre s−1) with various up- and downstream geometries. It is hypothesised that using an array of calibration factors (conductance; flow/pressure), instead of a single calibration factor over the entire flow range, could improve accuracy and also extend the applicable flow range. The conductance against pressure characteristics are calculated with a previously described weighted averaging technique based on multiple strokes from a precision syringe. A single conductance value gives stroke volume errors in the range of −5 to 3% (0–2 litre s−1) and −6 to 11% (0–6 litre s−1) for validation using the same geometry as for calibration. The pressure dependent conductance improves accuracy to within −3% and 1% independent of flow range. However, for validation using a different geometry than for calibration, errors range from −5% to +8%. The degree of non-linearity varies between the geometries (range 3–15%) and is highest when using a one-directional valve upstream of the PTM and a Y-shaped connector. In conclusion, a pressure-dependent conductance improves accuracy and can also be used to extend the applicable flow range up to at least three times the nominal flow range.
KeywordsConductance Flow calibration Flowmeter Extended flow range
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