Patent ID: 7823446
Filing Date: 2010-11-02
Classification: G01F

Abstract:
1. A method for determining a process variable of a product in a tank based on a time delay of electromagnetic waves propagating between a predefined reference and a surface of said product, said method comprising: generating a transmission signal in the form of a first pulse train, generating a reference signal in the form of a second pulse train, wherein each pulse in said first and second pulse trains has an essentially identical waveform, pulses in said first and second pulse trains being delayed in relation to each other by a predefined and varying time delay, guiding said transmission signal into the tank as electromagnetic waves, and allowing said waves to propagate towards said surface, receiving a reflected signal reflected from the tank, forming a measurement signal comprising a sequence of values, each value representing a time correlation between a pulse of the reference signal and the reflected signal, said measurement signal comprising at least one amplitude peak indicating a surface echo, sampling and digitizing said measurement signal to form a digital signal, identifying a time window of said digital signal including said surface echo peak, determining a relative time period between a reference time corresponding to said predefined reference and a beginning of said time window, time-to-frequency transforming said digital signal in said time window to obtain a phase spectrum, determining a relative phase shift of said spectrum and using said relative phase shift to calculate a corresponding time shift, and determining said time delay by adding said relative time period and said time shift.