August 21, 2019

Temperature influence in flow and volume measurement

In mechanical ventilators, the inspiratory and expiratory volume (Vti, Vte) and minute volume (Vi and Ve) must be precisely checked with a gas flow analyzers or a ventilator tester.

Ventilator testers, from different manufacturers, can measure flow and volume. However, flow accuracy is strongly temperature dependent and needs to be temperature compensated. This is particularly important because in turbine- operated ventilators raise the gas temperature above the ambient temperature.

A turbine driven homecare ventilator (Resmed Stellar shown above) raises the temperature of the respiratory gas to 30.2°C due to the turbine. (Settings: IPAP = 30cmH2O, EPAP = 7cmH2O, rate = 15bpm)

The IMT Analytics FlowAnalyser measures the flow with an accuracy of 1.75% of reading and the CITREX H5 (shown above) measures the flow with an accuracy of 1.9% of reading.

A Fluke Biomedical VT650 or VT 900 measures the flow with an accuracy of 1.70% of reading ONLY if gas temperature is between 19…27°C. That means at a gas temperature of 30°C, a VT650 or VT900 measures flow with an accuracy of 2.3% of reading (1.7% + 3 x 0.2%) because per degree Celcius the flow accuracy must be corrected by 0.2% (see users manual on page 41).

Graph shows Flow Accuracy vs. Temperature of different Ventilator Testers

The temperature influence of flow and volume measurement on ventilator testers must be taken into account. This is especially important when testing turbine-driven ventilators that deliver warm air. All IMT Analytics Gas Flow Analyzers feature a very complex patented temperature compensation of the flow measurement, which guarantees a constant flow accuracy over a wide temperature range.


Ken Racen

August 22, 2019

Mr. Friberg,

Excellent information. Without this type of analysis, many people would not make the correlation between the gas temperature and accuracy.

This will help BC Group sell more IMT products and provide better service support.

Ken Racen
Head of Service

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