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Using PCB technology for calorimetric water flow sensing in HVAC systems - a feasibility study

Optimal operation of heating, ventilation, and air conditioning (HVAC) systems can benefit from proper monitoring of the flows in the air ducts and water pipes. For large installations, this requires a large number of networked, Internet of Things (IoT) inspired sensors performing coordinated data acquisition. Cost-effectiveness and robustness are major issues for the design of such sensors. This paper investigates calorimetric flow sensors based on printed circuit board technology. While such sensors have been shown to be feasible for air flow measurement, the applicability for water flows has not been addressed yet because the thermal short circuit due to the high thermal conductivity of the fluid strongly influences the performance of the sensor. The sensor in this paper uses a modified calorimetric principle with thermoresistive heat transfer, where the sensing elements are implemented by simple copper leads. The sensor is scaled so as to fit across the diameter of the water pipe and thus have an averaging effect rather than a spot measurement. Finite element simulations as well as experimental results demonstrate that the flow- and temperature-induced chance of the resistance values is small, but sufficiently large to be measurable.