1.  Field of the Invention
Ultrasonic flow-measuring processes have been implemented with the aid of ultrasonic flowmeters ranging in design from built-in versions all the way to flexible clamp-on systems (clamp-on ultrasonic flowmeters).  Ultrasonic flowmeters are available for lines with diameters from a few millimeters to several meters and are typically employed for measuring the volumetric flow rate of low-viscosity mediums. 
The measuring concept in an ultrasonic flow-measuring process is based either on the entrainment effect or on the Doppler effect.  The entrainment effect derives from the fact that the runtime of an ultrasound signal traveling through a flowing medium between two ultrasonic transducers is either longer or shorter depending on whether the directional component through the measuring path runs with or against the direction of flow of the medium.  What is measured here is the mean flow rate through the measuring-path section of the line traversed by the ultrasonic signal.  The Doppler effect utilizes sound-scattering particles contained in the medium, with the flow rate of the medium being measured across the volumetric profile of two ultrasonic paths. 
The advantages offered by ultrasonic flowmeters essentially consist in the fact that no moving parts need to be provided within the line, that there is no cross-sectional narrowing or contraction of the line, that the method applied is essentially independent of the electrical conductivity and the viscosity of the flowing medium, that the available measuring range is quite broad, that there is virtually no pressure loss, and that in the case of a clamp-on design the line does not have to be opened.  On the other hand, precise and reproducible ultrasonic flow-rate measurements require unimpeded linear inlet and outlet sections for obtaining a stable flow profile.  Moreover, the flow rates determined in an ultrasonic flow-measuring process usually do not represent rates that are averaged across the full diameter of the line, thus necessitating corresponding corrections for a calculation of the volumetric or mass flow. 
Ultrasonic flow measuring techniques employing the aforementioned entrainment effect are based on the measurement of the runtime of an ultrasound signal traveling between two ultrasonic transducers mutually offset in the direction of the flow.  The run-time determined by the ultrasonic flowmeter in the ultrasonic flow-measuring process is composed of the runtime of the ultrasonic signal through the flowing medium, the run-time of the ultrasonic signal within the ultrasonic transducers and delay times in the measuring electronics of the ultrasonic flowmeter.  The components of the total runtime that differ from the runtime of the ultrasonic signal through the flowing medium, i.e. total runtime minus the runtime through the flowing medium, constitute the time lag or so-called dead time of the ultrasonic flow-measuring process, i.e. of the ultrasonic flowmeter.  Obtaining precise measuring results requires appropriate corrections for that dead time. 
2.  Description of the Prior Art
In conventional ultrasonic flow-measuring processes and ultrasonic flowmeters it has been common practice to perform a one-time calculation of the dead time for the ultrasonic transducers employed, or even for a specific transducer type only, relying in actual measuring operations on the correction made for that dead time on the assumption that the latter would remain constant rather than being subject to changes.  In practice, however, it has been found that the measurements obtained have in part been rather error-prone since during the measuring operation the dead time does not remain constant at all but tends to change.  This is because the dead time is affected by changes in the ultrasonic field, by the temperature-related properties of the ultrasonic transducers and by fluctuations in the measuring electronics which, as well, are essentially temperature-related.  The ultrasonic field, in turn, depends on the configuration of the ultrasonic transducers, the inter-transducer distance and the acoustic characteristics of the flowing medium. 
To avoid and at least partly compensate for the dead-time problem that is largely a function of thermal effects, various approaches have been proposed.  For example, it has been suggested to arrive at a dead-time correction by determining certain parameters of the measuring electronics or by directly measuring the runtime within the ultrasonic transducers, as described for instance in U.S. Pat.  Nos.  5,280,728 and 5,824,914.  However, these approaches do not make it possible to factor-in the effects that cause dead-time variations or that originate in the flowing medium itself, so that in the final analysis dead-time correction remains inadequate.