**Francesco D'Amato 1, Silvia Viciani 1,\*, Alessio Montori 1, Marco Barucci 1, Carmen Morreale 2, Silvia Bertagna <sup>2</sup> and Gabriele Migliavacca <sup>2</sup>**


Received: 26 November 2020; Accepted: 17 December 2020; Published: 21 December 2020

**Abstract:** In order to assess the limits and applicability of Pitot tubes for the measurement of flow velocity in narrow ducts, e.g., biomass burning plants, an optical, dual function device was implemented. This sensor, based on spectroscopic techniques, targets a trace gas, injected inside the stack either in bursts, or continuously, so performing transit time or dilution measurements. A comparison of the two optical techniques with respect to Pitot readings was carried out in different flow conditions (speed, temperature, gas composition). The results of the two optical measurements are in agreement with each other and fit quite well the theoretical simulation of the flow field, while the results of the Pitot measurements show a remarkable dependence on position and inclination of the Pitot tube with respect to the duct axis. The implications for the metrology of small combustors' emissions are outlined.

**Keywords:** laser flow meter; Pitot tube; flow speed; time of flight; dilution method; flow simulation; flow turbulence; gas sensing applications
