Review of “Calibrating low-cost rain gauge sensors for their applications in IoT infrastructure to densify environmental monitoring network” by Krüger et al

Review by Rolf Hut

The authors calibrated a collection of off the shelf low-cost rain gauges to test if they are usable in scientific applications with the factory calibration. Given the amount of projects that aim to use Personal Weather Stations (PWS) to supplement professional networks, this is a valuable addition to the literature. I do have, however, some suggestions to in my opinion improve the paper (and its usability by the scientific community) before publication.

Overall comments

1. The manuscript as written hinges on two thoughts: on the one side a lab and field calibration of low-cost rain gauges and on the other side an overview of IoT hardware and cost needed to use low cost sensors in general and rain gauges in particular. This last part (IoT hardware) is worked out in far less detail compared to the first part (rain gauge calibration). In literature a large collection of articles reviewing state of the art development boards (including Arduino and Raspberry Pi) for use in environmental sensing in general and weather stations, is available. I would suggest that the authors focus on the rain gauge calibration and remove, or move ta an appendix, paragraphs 2.1 until and including 2.2.4. In the main text the authors can cite relevant literature on IoT hardware reviews. (a quick search on google scholar already resulted in these DOIs, there is much more: https://doi.org/10.3390/ijerph17113995, https://doi.org/10.1016/j.cosrev.2021.100364, https://doi.org/10.1016/j.procs.2014.07.059, 9734/AJRCOS/2021/v9i130215)
2. In calibrations of rain gauges the crucial question is always: “what do we use as ‘the truth’ and the authors have three reference devices available. They choose to use the Hellman gauge as reference without further justification. I would ask the authors to substantiate why the Hellman, compared to the other devices, should be considered “reference” (or “thruth”).
3. Continuing on point 2: the authors only report on the difference between the different gauges, both within the groups of low-cost gauges and between the low cost gauges and the reference gauges. However, they do not quantify if these are significant in the light of uncertainties, either inherent in their way of measuring, or inherent in the nature of rainfall. Given the large amount of low-cost rain gauges they use, it should be straightforward to indicate if the values of the the reference gauges are significantly outside the distribution of low-cost gauges. If the authors have access to a long time series from the reference devices (which I assume they have), they could use triple co-location to estimate the uncertainties in the three reference devices. This would than allow for a two-way comparison between the reference devices and the low-cost gauges. There is a wealth of literature on (how to do) comparisons between raingauges, including the statistics involved. I suggest starting at Lanza 2009 (https://doi.org/10.1016/j.atmosres.2009.06.012)
4. In the lab calibration it is extremely important that the rain gauges are placed perfectly horizontal. I assume the authors made sure of this. I would suggest to add a few sentences on how this was done. Furthermore, it is important to know if all raingauges were oriented exactly the same direction on the table. If the table was even slightly tilting, having all rain gauges in the same orientation would result in a bias towards a certain direction and could explain the left-right difference observed?
5. The analyses done within the discussion is, in my point, central to the manuscript. I would suggest to move the results of the comparison of the field and lab experiment to the result, explain in the methods which (statistical) methods are used to compare the two datasets and in the discussion only reflect on the result, not present new ones.

Specific comments

1. All figures need more detail in their captions to understand what is shown.
2. In figure 1 I would add a vertical (red?) line at 0.20 mm to indicate where the factory calibration is.
3. Figure 6 could use the Hellman data as crosses or points. Especially on the one hourly data it is interesting to look at the uncertainties of the three reference devices (see above).

Overall I think this is a highly relevant paper given the focus on citizen science projects to use Personal Weather Stations to supplement professional networks. With the above suggestions implemented I would be happy to recommend publication in GI.

Rolf Hut

The paper addresses the calibration of low-cost rain gauges in laboratory and field evaluation with referenced ones.

It is of interest for field application to enrich dataset and reducing instrumentation cost.

The part concerning low-cost data acquisition system could be shorten, as different publications are available in literature. Anyway, a paragraph focus on time synchronization versus the reference system used in the field test would be an interesting complement.

Concerning the laboratory calibration, the lecturer would appreciate to have a plot of the distribution of results obtained for all your rain-gauges, supposed to be Gaussian. Is the median close to your mean value?

During laboratory experiments, the flatness and horizontality was supposed to be controlled, what about the field conditions.

What was the confidence interval during your laboratory calibration ?

The referenced station for field trials are not positioned at the same spatial location, how do you controlled the spatial homogeneity? It could add uncertainty to your field results to be taken into account for the analysis.

As the final aim is to enhance the amount of sensor using low-cost sensor, using opportunistic data from private owner of rain stations can be discussed by comparison with the knowledge acquired during your experiments.

The research work presented is of importance for field experiments.

I hope that these few suggestions will help authors to improve their paper for publication in GI Journal.