Test & measurement
In the following, the most relevant methods for fluid measurement are discussed.
potentiostAt
The most fundamental and commonly used measurement principle is based on a potentiostat. As shown in Figure 1, a potentiostat measures and controls the voltage between a working electrode (WE) and a reference electrode (RE). The potential of the WE is kept constant in relation to the reference electrode through adjustment of the current flowing through the counter, or auxiliary, electrode.
AmperometriC The simplest form of amperometric measurement applies a bias voltage to the sensor and measures the response current. Here, a constant voltage is applied between the RE and the WE and the current profile is converted to a digital signal using a current-to-voltage converter and an analogue- to-digital converter (ADC). This current profile depends on both the sensor and the measured variable. The circuit is illustrated in Figure 2 by means of the ADuCM355.
Figure 3. Electrochemical impedance analysis.
patient can adjust his or her medication or diet. In the future, development should move away from the individual measurement and toward a continuous measurement method for constantly monitoring the blood sugar level. Highly accurate, power-saving impedance measurements are urgently needed here. Another application of fluid measurement is
dialysis. With chronic renal failure, the blood has to be filtered. The dialysis fluid conductivity measurement is also done via impedance analysis. In this way, for example, pH, conductivity, composition, and saturation can be measured. Finally, faeces and urine from patients are
also measured. Here, the bodily excretions are investigated for the purposes of drawing conclusions about diseases and irregularities. This is a relatively new field in medicine with numerous different approaches and a wide variety of methods. However, at the foundation is impedance measurement via electrodes, which enables conclusions about various diseases to be drawn. Here, for example - in addition to the pH measurement - conductivity measurements are performed. The measurements previously described are, of
course, not exhaustive. There are still many more fluid measurements that are of use for medical technology for humans and animals, for example, hormone measurements or measurements of pharmaceutical agents. Here, too, an impedance method is important. Although all measurements determine different
parameters, they are always based on impedance analysis. Despite their many faces, they all have one thing in common - the need for a power- and space-saving solution for supporting wearable
Figure 4. Block diagram of the ADuCM355. Instrumentation Monthly January 2022
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devices is huge. In the following, different impedance measurement methods are described. They are used partly in combination and partly on an individual basis to enable complete analysis.
Different impeDAnCe meAsurement prinCiples While the basic principle behind impedance measurement remains the same for all applications, there are still great differences between the individual measurement capabilities.
CyCliC VoltAmmetry Voltammetry measurements function electrochemically, whereby the potential of an electrochemical cell is slowly increased and then linearly decreased. Thus, the potential follows a triangular waveform while the current flowing through the WE is measured. Voltammetry is used, for example, to measure the half-cell reactivity of an analyte. This method is a form of electrolysis, and the resulting current is a result of reduction and oxidation. With it, the sample can be investigated qualitatively and quantitatively.
ConDuCtiVity meAsurement Conductivity measurements are based on the
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