## 3 Physiological Measurements  
In addition to a routine electrical evaluation, physiological measurements in dry and immersed conditions were performed using two commercial potential-based ECG sensors and our current ECG. Commercial sensors were the Noraxon Desktop DTS (Noraxon U.S.A., Inc., Scottsdale, AZ, USA) as a Gold-standard reference system for dry measurements and the Shimmer2 (Shimmer Research Ltd., Dublin, Ireland) ECG sensor for underwater measurements. The Shimmer device allowed the attachment of extended electrode cables (up to 1.5 m) allowing comfortable water access. The measurements were performed either with metal-plate electrodes or with Ag/AgCl adhesive foam electrodes. We recorded data from five different adult subjects (two male, three female). All recorded data showed the same characteristics as described in the following. We therefore use one of those datasets, from a healthy male participant, to demonstrate the signals and our findings. We tried several different electrode positions for the measurement electrode (e.g., on the wrist or all typical 12-lead ECG electrode positions). However, most showed similar signal characteristics (Figure 4). In the dry environment, measurements using the wide range configuration showed accurate morphology of the ECG. However, the signal was strongly overlapped with 50 Hz power line interferences (Figures 5(a-II) and 4), however, those can be easily removed using a notch filter in a post-processing stage. This was not done for these figures in order to clearly show the difference between various electrode positions (Figure 4) and the overall impact of noise on the circuit. In contrast, the small range configuration showed very little noise but due to the small bandwidth several morphological ECG events were filtered as well. Current-based measurements in immersed conditions corresponded to those obtained in dry conditions. The wide bandwidth configuration still showed power line artifacts, however, the signal-to-noise ratio was much higher than outside the water (see Figure 5). One particularly influencing noise characteristic in underwater measurements, which is also visible in Figure 5, is a heavy baseline wandering, which is very rapid and unpredictable. In severe cases, this caused the potential ECG's ADC to go into saturation. While also still visible in the current ECG, it is not as susceptible to those problems. The small bandwidth configuration depicted almost no additive noise. The QRS complex was correctly amplified. In order to prove the accurate working of the current-based ECG, the sensor was compared to two commercial potential-based ECGs. To compare the current-based sensor, measurements were performed simultaneously using both current and commercial ECG sensors. Figure 5a demonstrates the simultaneous measurement of both sensors. Due to the internal circuit of the commercial sensor, a greater amplitude occurs in comparison to the current-based ECG senor. The measurement displays an accurate reproduction of the ECG signal using the current-based sensor and no mutual influence of both when measured simultaneously. QRS complexes seen in the potential-based ECG sensor correspond to those identifiable in the signal of the newly developed current-based sensor. However, the commercial sensor showed an increase of interferences in immersed condition. The reference potential-based sensor was not able to record an accurate morphologically interpretable ECG. Figure 4: Comparison of measured signal (**a**) and power spectral density (PSD); (**b**) of that signal from our device using different electrode placements. In configuration (I), the measurement electrode was placed on the proximal backside of the right arm wrist. In configuration (II), the measurement electrode was placed on the chest skin at the third intercostal space between the sternum and the midclavicular line. In configuration (III), the measurement electrode was placed on the chest skin at the sixth intercostal space at the midclavicular line. All three configurations show a typical PSD.