Caratterizzazione di un Pitot S per flussi gassosi di natura industriale con anemometro laser-doppler

The aim of this paper is to determine and to evaluate an innovative, more accurate and reliable procedure for the calibration of an S-type Pitot tube (SPT) by means of Laser Doppler Anemometry (LDA). The SPT is used for measuring the flow of industrial stack emissions with good accuracy and at reasonable costs. Due to its coarse structure, the SPT is employed in aggressive environments so requiring an accurate and periodic characterization. Among the different information provided by the measure, the accurate pollutant content in the exhaust gas released from the stack into the atmosphere could be crucial in order to fulfil local and national environmental regulations. So, the instrument needs a suitable calibration to be made by means of more accurate technique. The calibration procedure for a Pitot tube consists of determining its calibration coefficient through flow rate measurements by applying Bernoulli law. Currently the SPT calibration is made against an L-type Pitot tube (which has to meet construction and geometrical requirements stated by ISO 3966 standard). Its procedure is described in ISO 10780 (1994) and earlier in “Guidelines for type S-Pitot calibration” EPA (1977). This procedure consists of comparing the flow velocity measurements made by the reference Pitot tube and SPT. Under unchanged flow conditions, it is possible to relate the STP coefficient to the know calibration coefficient of the reference instrument by applying the corrected Bernoulli equation. This method though its easiness and immediacy suffers several drawbacks: first, the reference instrument and the SPT measure flow velocities of different points, although they are installed in the same cross section under test, because of their different geometries; secondly, the replacement itself prejudices the repeatability and reproducibility of entire calibration procedure; thirdly, the lack of an accurate uncertainty characterization of the calibration factor has to be mentioned. In this paper a different approach is proposed based on the use of LDA as reference instrument. LDA offers several advantages as high accuracies and spatial resolution and absence of load errors (however an optical access is required). The measure have been made in a wind tunnel were the air flow is generated by a variable speed fan. Many solutions have been implemented in order to achieve better evaluation of SPT calibration factor. The measurement volume of the laser is located upstream the stagnation opening of SPT at enough large distance from it such that the velocity sensed by LDA is not influenced by the instrument obstruction. To take into account the relatively large inlet opening of SPT, the LDA measurement volume has been moved to several points in the cross section under test in order to get information of more streamline velocities (at the same flow rate). The temporal repetition and the spatial replications made by LDA have been statistically treated by means of techniques such as ANOVA one-way in order to validate the samples acceptability.