# Noninvasive Magnetic-Marking-Based Flow Metering with Optically Pumped Magnetometers
## 4 Conclusions  
We presented a noninvasive, magnetic-marking-based flow metering procedure, which uses an OPM operating in the ZULF regime at magnetic-field strengths lower than 100 nT as the signal detector. The detection method was based on a time-of-flight measurement, which makes the procedure calibration-free.  
In the frame of the experiments, a macroscopic polarization of 300 pT was created in the water samples using a 1 T Halbach magnet. Applying short RF pulses as \"markers\" on the water flow resulted in polarization transients with amplitudes between 10 and 80 pT, which were detected with the OPM with good signal-to-noise ratio and used to derive the flow velocity of the medium.  
In the current demonstration setup, the useful flow velocities ranged between 12.7 cm/s and 23.3 cm/s. Utilizing the procedure presented, we successfully demonstrated measuring the water flow with an average accuracy of 3% in the given velocity interval. Comparing the measured results to the pump rates of the syringe pump showed that the data were consistent with expected values.  
The current experiments demonstrate the feasibility of a marker-free magnetometry-based flow metering, enabled by the outstanding performance of commercially available optically pumped magnetometers. A number of steps will be necessary to move from our \"proof-of-concept\" experiment to a metering system applicable in practical situations by answering the following questions:  
What signals are still detectable in a certain background situation? How much shielding effort is required? Which level of pre-polarization and RF pulsing is required? What is the resulting time accuracy? What RF pulse form is best suited?  
The mentioned aspects must not be considered isolated from each other. Addressing them is the topic of experiments currently under way.  
Figure 8: Extract of the experimental results with the predicted signal for comparison. The OPM data shows the peak of Figure 6 highlighted with a red asterisk. The predicted signal is longer than the experimental data and has a full width at half-maximum (FWHM) of 0.67 s. The experimental data has a FWHM of 0.39 s. In a first approximation, we assumed the magnetic field of the RF coil is symmetric, which is not confirmed by the data. Thus, we refrain from a detailed analysis.  
**Author Contributions:** [PERSON], [PERSON] and [PERSON] conceptualized the research. [PERSON] conceived the initial idea of the procedure, performed the experiments, and wrote a draft version of the manuscript. [PERSON] and [PERSON] designed the experimental setup. All authors analyzed the data and revised the final version of the manuscript. All authors have read and agreed to the published version of the manuscript.  
This work was supported as a Fraunhofer LIGHTHOUSE PROJECT (QMag). In equal parts, this work was funded by the Ministry of Economic Affairs, Labour and Housing of the State of Baden-Wurttemberg, Germany.  
The data is available from the corresponding author upon reasonable request.  
We acknowledge [PERSON] of the Medical Centre of Freiburg, Germany for his helpful discussions on this research.  
The authors declare no conflict of interest.