Not to be supplied in France, Germany and Great BritainA short electromagnetic pulse is sent along a rope and is reflected back once it reaches its end. If the rope is immersed into the product to be measured, in the point of contact air/product an echo is generated by this discontinuity. The echo comes back to the same rope and is detected as a surface of the product. A method of time elongation, allows the assessment of the time of flight which, by its nature, is particularly short, to be able to calculate with accurancy the distance and hence the level of the product to be measured. These are particularly suitable for use with powders, granules, bulk solids such as plastic chips in high and narrow silos, as well as in the food industry because these instruments do not require new calibrations when changing the type of product.