| 09 Jan 2026
An Adjustable-Rate User-Printable Rain Gauge Calibrator
Abstract. Accurate precipitation measurement is essential. However, calibration of field-deployed rain gauges remains a challenge. Many methods require laboratory conditions, costly commercial equipment, or instruments designed for specific rain gauges, which cannot accommodate smaller-diameter gauges.
We introduce the Adjustable-Rate 3D Printed Rain Gauge Calibrator (AR3D), a low-cost, open-source device designed for in situ calibration of rain gauges.
The AR3D introduces three innovations: (1) an adjustable screw valve, enabling flow rate tuning from 0.15–16 mL min⁻¹, accommodating the lower flow rates needed for smaller rain gauges; (2) a compact and durable design developed with low-volume reservoirs and elimination of degradable parts; and (3) an integrated pyranometer cover to generate automatic calibration event signals.
We evaluated the AR3D using gravimetric tests, constant-rate stability tests, device-to-device equivalence tests, and field comparisons. Laboratory gravimetric tests of the AR3D demonstrate its ability to deliver volumes of water accurately with an average error of 0.11 % (comparable to ISO Class B tolerances for plastic volumetric flasks) at flow rates within ±5 % equivalence across independent devices, with a coefficient of variation (CV) below 5 %. The AR3D has been successfully deployed to weather stations in both Kenya and the United States. Field validation of the AR3D with a single setting resulted in a flow rate CV of 8.7 %. The AR3D enabled identification of properly functioning rain gauges, as well as an under-reporting station. The total cost of material and labor to build an AR3D is approximately USD 12.
In this manuscript, the authors present an adjustable‑rate, printable calibrator for field calibration of low‑cost rain gauges - a concept I find both elegant and meaningful. They describe modifications to a previous calibration device and report various test results for the modified calibrator with adjustable flow rates. Overall, the manuscript reads more like a technical note to me than a full research article; whether this fits the journal’s scope is, in my view, a decision for the editor(s). With this being said, I find the methods and results interesting and the manuscript in general suitable for publication. However, I have a few points for (minor to major) revision that should be addressed before final publication.
My main concern relates to the field evaluation in Section 4.7. Fist of all, it would be informative to specify the temporal resolution used for field calibration and whether the ATMOS 41 provides bucket‑tip counts per time interval or exact tip timestamps: Details on how this gauge is calibrated (presumably via a screw that adjusts the tipping point) could also be provided. Furthermore, demonstrating that the gauges in Kenya performed well while the one in the USA did not mainly shows that the latter was poorly maintained or malfunctioning, rather than providing a robust field validation of the AR3D. A more informative approach would be to test a set of well‑maintained, calibrated gauges under multiple predefined flow rates, thereby establishing the performance range under ideal conditions. This would be useful for identifying gauges in the field that require maintenance and/or recalibration. Extending such tests to other types of low‑cost gauges could also yield useful insights into the applicability and robustness of the AR3D.
Minor comments
- Sections 2 and 3 contain very short subsections and/or paragraphs, often limited to a single sentence or a table; these should be merged into longer, cohesive sections. Tables currently placed in Sections 2.3 and 2.4 could be moved in the Appendix.
- The “pyranometer cover” findings (4.6) do not require a separate subsection, this can be integrated into the results.
- Please check bibliographic entries for complete details and consistent citation formatting (including years).
Specific edits
- p. 1, line 13 - Convert the flow rates (ml min⁻¹) to equivalent rainfall rates.
- p. 3, line 1 - Replace “document” with “article” or “manuscript”.
- p. 6, line 141 - Use SI units consistently.