Manual measurements of distribution of gas velocity in conduits of flue gas installations using systems with differential pressure sensors of velocity are often performed for the requirements of determining emissions of dust pollutants from industrial process plants to the atmosphere. The aim is to determine an axial velocity profile. Flows in measuring sections are not always coaxial along the run of the duct; they are characterized by different directions of the velocity vector at various measuring points. The determination of actual directions of vectors of local velocities giving a guarantee of an accurate calculation of the axial velocity is often not possible from the technical point of view and the measurement of the velocity is carried out with the parallel setting of the sensor head in relation to the axis and the walls of the conduit. Then the knowledge of the directional sensitivity of the applied velocity sensor allows either to eliminate the axial velocity measurement error or to take it into account by the uncertainty of this measurement. For specific situations of two-dimensional variation of direction of the velocity vector, the directional sensitivity characteristics and in consequence the characteristics of error have been determined for three sensors adopted to tests: a zero pressure dust sampling probe with the anemometric function as an element of the gravimetric dust sampler and comparatively - two commonly used Pitot tubes: types S and L.

Aspiration dust probes of some isokinctic samplers have such a construction that enables them to measure the gas velocity at the point they arc placed in a duct, the measurement being required for maintaining isokinctic conditions and in determining the dust mass flow rate in the duct. The gas velocity is correlated with a probe specific pressure difference, a quantity measured directly when the train is in operation. This relationship is a mclrological characteristic of a given probe, being established in a calibration procedure. Two types of the above-mentioned probes, namely a pressure balance-type and an in-stack filtration probes (in two versions) combined with a type S velocity sensor, have been tested. The behaviour of the characteristics of the probes was studied in the flows of different turbulence at a laboratory stand. The achieved results, presented in the paper, show that the turbulence state of gas flow is the factor that shapes the mctrological characteristics. Facing the fact that the turbulence of gas flow in industrial installations is not identified during the dust concentration gravimetric measurements, this parameter constitutes a source of uncertainty of the gas velocity measurement. For the probes under test, the values of this uncertainty, varying with the gas velocity, were determined. The test and calculation procedures were described.