Abstract
Once the metal surface makes direct contact with the electrolyte body fluid, there forms an ionic double layer, which results in the interface characteristics between metal and fluid for the signal pass. The degree of signal distortion through this layer is dependent upon the frequency and voltage or current of the signal, fluid temperature, impurities contained in the metal, conditions of the pretreated metal surface and the concentration of the electrolytes. By using the Ag-AgCl or platinized platinum, the Warburg impedance may be rendered. Bright platinum, gold and silver, however, exhibit the dominance of the frequency dependency of the R part, which is inversely proportional to the signal frequency, and the C part is relatively constant against the frequency. For the emf system, the stableness of this interface characteristics is important to minimize, the drift in zero flow level in the long duration of time. In the sine-wave excitation, the Wagner's grounding is successfully applied. In the practical applications, zero drift is also originated from the mechanical slide or slip of the vessel wall on the electrode due to pulsation and/or respiration. The degree of zero stability may be evaluated from the signal to noise ratio, where square-wave system generates over ten times as large a signal by the same velocity as the sine-wave system against the same noise caused by the electrode characteristics. Uneven distribution of the eddy current, however, may cause more zero drift in the high frequency square-wave system than seen in sine-wave system.
