Elastic guided waves are carriers of information of the (change in) condition of plate-like structures like wind-turbine blades, airplane wings and road surfaces on bridges. To measure these guided waves we do not have to use piezos. Other sensors offer interesting benefits like contactless sensing, embedding, or measuring without electricity. However, quantitively comparing them is not trivial: the sensors all have different geometries, operating principles and are sensitive to different mode shapes of guided waves. We designed and performed an experiment to quantitatively compare the performance of five state of the art sensors (piezo, in-fiber interferometer, FBG, free-space interferometer, and ring resonator sensors) to measure So and Ao guided elastic waves. The measurements were performed on guided waves in an 8 mm steel plate, in the 60-150 kHz range. The dimensions of the plate and the positioning of the sources and sensors was chosen such that the So and Ao waves arrived in separate time windows. The in-fiber interferometer was the sensor that came closest to the piezo, that was used as reference sensor (-11 dB difference in SNR), the other optical based sensors have SNR values below -30 dB compared to the piezo. The measurements and simulations show that it is important to have two quantitative SNR measures for the performance to measure guided waves: one for the So and one for the Ao wave. For one sensor we found a difference of 22 dB between these two SNR measures.

Nanoparticles are applied in an increasing number of products (e.g. medicine, food and cosmetics). There is a need to real-time monitor the growth of nanoparticles during production inside the chemical reactor. However, standard particle sizing methods such as dynamic light scattering (DLS) and transmission electron microscopy (TEM) cannot do this. A suitable candidate may be ultrasonic transmission spectroscopy (UTS). Here, the sound speed and attenuation are measured of a dispersion and a physics based model is inverted and fitted to the measurements. The measurements have to be done with great accuracy: e.g. one should be able to measure frequency dependent variations in the speed of sound smaller than 0.05%. The measurements also have to be carried out fast enough compared to the time scale of the reaction. Moreover, the inversion has to be able to deal with the changing properties of the solvent during particle formation. Here, we report on the new ultrasound nanoparticle sizer (UNPS) that has these abilities and therefore could successfully monitor the size and concentration growth of nanoparticles during a reaction inside a reactor in real-time.