the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 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.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2025-5744', Anonymous Referee #1, 10 Apr 2026
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AC1: 'Reply on RC1', Matthew Tippett-Vannini, 12 May 2026
Dear Reviewer,
Thank you for your thorough and constructive review. We appreciate the time and care taken in evaluating this manuscript. Although the revised manuscript cannot be uploaded until a second reviewer is chosen, we responded to the first reviewer's comments and modified the original manuscript. Below we address each of the comments (reviewer’s comments are the section headers and authors’ responses are the main body):
Section 4.7: Temporal resolution and bucket tip counts
The METER group’s ATMOS 41 is a drip counter gauge, not a tipping bucket gauge. Nonetheless, we have added the 5-minute logging interval to the methods (Section 3.2) and calibration run times to the results (Section 4.2) in the revised manuscript.
Section 4.7: Details on how the gauge is calibrated
Because the ATMOS 41 is a drip counter gauge, it cannot be adjusted or recalibrated in situ. Adjustments can only be made by cleaning the measurement probes and funnel, factory recalibration, or data post-processing. We have added a brief description of the drip counter mechanism and clarified that the AR3D, for this type of gauge, serves as a diagnostic tool rather than a recalibration tool.
Section 4.7: Robust field validation and testing under multiple predefined flow rates
We believe that performance testing under controlled conditions using predefined flow rates was addressed in the laboratory results (Sections 4.1 in the revised manuscript), which characterize volumetric precision, flow rate range, rate constancy, single-device repeatability, and device-to-device equivalence. The field deployment (Section 4.2) demonstrates the AR3D intended diagnostic ability. The AR3D was tested on four independent gauges, under field conditions and successfully identified three functioning stations and one station requiring factory recalibration. We have revised the manuscript language to clarify this distinction between laboratory performance validation and field diagnostic demonstration.
Section 4.7: Extending tests to other low-cost gauges
We agree that extending tests to other gauge types would yield valuable insights. The AR3D's open-source design files are provided to facilitate adaptation to other gauges, though its adjustable rate makes it print-ready for many smaller gauges, without modifications. We have noted more explicitly, in the revised manuscript, that future work could include systematic evaluation across gauge types in a dedicated study.
Sections 2 and 3: Combine short subsections
Short subsections in Sections 2, 3, and 4 have been merged into cohesive sections in the revised manuscript.
Section 4.6: Pyranometer cover does not require a separate subsection
The pyranometer cover results have been integrated into the main results sections rather than presented as a standalone subsection.
Tables 1 and 2: Move to Appendix
Tables 1 and 2 have been moved to the Appendix in the revised manuscript.
Bibliographic entries
References have been checked and corrected for completeness and consistent formatting.
1, line 13: Convert flow rates to equivalent rainfall rates
Equivalent rainfall rates have been added. Flow rates in mL min⁻¹ were retained as well, since flow rate is independent of gauge funnel diameter.
3, line 1: Replace "document" with "article" or "manuscript"
Corrected.
6, line 141: Use SI units consistently
Corrected.
We thank the reviewer again for their comments, which improved the clarity of the manuscript. We look forward to continued discussions.
Best regards, Matthew Tippett-Vannini, on behalf of all co-authors
Citation: https://doi.org/10.5194/egusphere-2025-5744-AC1
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AC1: 'Reply on RC1', Matthew Tippett-Vannini, 12 May 2026
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RC2: 'Comment on egusphere-2025-5744', Anonymous Referee #2, 20 Jul 2026
The AR3D is a promising open-source calibration tool, but the manuscript needs revisions in scientific rigor, environmental context, validation depth, and presentation before it meets the standard of a full scientific article.
Technical Achievements
The AR3D's principal contribution is the introduction of an adjustable set screw enabling tunable flow rates from 0.15 – 16 mL/min, substantially lower than its predecessor, the 3D.1. Applied to the ATMOS 41 (9.3 cm funnel diameter), the new minimum corresponds to ~1.32 mm/hr, well within the sensor's high-precision range (≤50 mm/hr, i.e., 5.7 mL/min), a marked improvement over the 3D.1's slowest equivalent rate of ~148 mm/hr, roughly three times the upper limit of the ATMOS 41's ±5% accuracy band. This shift brings the calibrator's operating range into environmentally realistic rainfall intensities, aligning calibration conditions with the light-to-moderate rain rates most commonly encountered in the field rather than the extreme intensities imposed by the 3D.1.
Scientific Rigor and Environmental Considerations
Despite these advances, the manuscript is overly optimistic regarding the AR3D's readiness for dynamic field calibration, and several critical points deserve deeper treatment:
Flow rate drift: The authors report a median downward drift of -8.5% across constant-rate tests, largely attributed to air-bubble entrapment near the set screw. This is more than a minor artifact - dynamic calibration relies on a stable, known flow rate to correct tipping-bucket spillage errors, and drift of this magnitude undermines that premise. The recommended "gentle squeeze" of the flask bulb is a useful field workaround but should not substitute for a critical engineering discussion. The manuscript should more explicitly acknowledge the implications for calibration accuracy and elaborate on the proposed mitigations (wider/longer flow paths, needle-valve-style set screws).
Temperature sensitivity (currently absent): The manuscript grounds the constant-flow behaviour in the Hagen–Poiseuille equation, yet does not discuss the temperature dependence of dynamic viscosity, which changes by roughly 3% per °C for water. A 10 °C offset between laboratory calibration (reported at 22 °C) and field conditions could translate into ~30% error in the intended flow rate. An effect that dwarfs the ±5% device-equivalence bounds the authors claim. This omission is significant given that the AR3D is explicitly promoted as an in situ, field-deployable device used in Kenya and the U.S.. The authors should either provide correction factors, recommend an operational temperature range, or at minimum characterize the sensitivity experimentally.
Depth of validation: The field campaign, three sites in Kenya and one in the U.S., primarily serves to diagnose a malfunctioning station (the U.S. gauge under-reported at 8.33, 26.13, and 39.64 mL against a delivered 100 mL) rather than to rigorously validate the AR3D itself. A stronger validation would test the AR3D against well-maintained, independently calibrated reference gauges at multiple predefined flow rates spanning the 0.15–16 mL/min operational range, providing a traceable performance envelope rather than a single-setting field CV of 8.7%.
Engineering rationale for hole diameters: Table 1 specifies water- and air-hole diameters of 1.6 mm and 0.8 mm, respectively, and a water-outlet capillary diameter of 1.1 mm, but no derivation is provided. Since the manuscript invokes the Hagen–Poiseuille equation, readers would expect a mathematical link between these dimensions and the target flow-rate range. Without it, the values appear empirically chosen and are difficult for other groups to adapt to different rain-gauge geometries.
Structural and Presentation Concerns
Fragmented subsections: Sections 2–4 rely on many short subsections (e.g., 2.2.2 "Adjustable Flow Rate" and 2.2.3 "Removal of O-rings") that read like a technical manual. These should be consolidated into a cohesive engineering narrative describing the evolution from the 3D.1.
Tables 1 and 2: Table 2 (Bill of Materials) itemizes trivial components such as an Allen key and a single set screw and would be better reorganized into broader categories (Material, Tools, Labor) or moved to an appendix. Table 1 would benefit from added columns or accompanying text linking each dimension to the Hagen–Poiseuille-derived target flow rate.
Unit conversions: The manuscript already presents Eqs. (2)– (3) for converting mL/min to mm/hr and gives one worked example for the ATMOS 41. All reported flow rates should be consistently accompanied by their equivalent rainfall intensities (mm/hr) for the relevant gauge diameter to give hydrologists immediate interpretive context.
Generalization figure: A line chart mapping funnel diameter (cm) to rainfall rate (mm/hr) across the full 0.15–16 mL/min range would visually demonstrate the AR3D's applicability beyond the ATMOS 41 — consistent with the authors' claim that the device "can be easily modified to fit any rain gauge“.
Recommendation
The manuscript demonstrates significant potential but requires substantial revisions to meet the standards of a scientific article. I recommend acceptance as a Technical Note following major revisions, with particular attention to the environmental (temperature) sensitivity analysis, a stronger multi-rate validation against reference gauges, an engineering justification for the orifice dimensions, and consolidation of fragmented subsections and granular tables.
Citation: https://doi.org/10.5194/egusphere-2025-5744-RC2 -
AC2: 'Reply on RC2', Matthew Tippett-Vannini, 10 Aug 2026
Dear Reviewer,
Thank you for your constructive and detailed review of the manuscript. We appreciate the time, effort and insight you have given to this work.
We have resubmitted a manuscript (and a tracked-changes version) with many of the changes suggested.
Below we address the comments provided:
Flow Rate Drift: The discussion now includes the explicit implications of bubble entrapment (slower rates due to smaller effective area) and elaborates the mitigation possibilities.
Temperature Sensitivity: We have added a temperature sensitivity analysis to the appendix. We also describe the temperature-dependent effects on flow rate in the testing section and in the discussion and describe the need to adjust flow rate to field temperature conditions before calibration. We also explain that an advantage of the AR3D is the ability to adjust the flow to adapt to variable field conditions. Eq. 1 and Appendix G allow an operator to apply a correction to flow rate, assuming no in-field adjustments are made; however, we recommend adjusting flow rate to field conditions.
Validation against multiple, independent gauges: We agree that testing against multiple independently calibrated gauges at predefined flow rates would be informative and would be the natural extension to this manuscript. We have added that to the suggested future work part of the discussion.
We offer two points for limiting the current scope. First, the volumetric accuracy of the AR3D is determined against two traceable methods: 1) gravimetrically with a 0.01 g balance and 2) from the volumetric flask with a ±0.3 % tolerance. Flow rate constancy, single device repeatability and device-to-device equivalence were characterized. These establish the performance of the calibrator itself, which is the claim this manuscript makes.
Our second point is that a test across multiple reference gauges characterizes the response of the gauges as much as the performance of the calibrator. The interpretation of any disagreement would require independent knowledge of each gauge’s state of calibration and methods of adjustment. Such a study would be a valuable contribution but would address gauge performance more than calibrator performance and we consider it beyond the scope of this study.
The field deployment we describe in the manuscript demonstrates the AR3D's intended use: to determine if a low-cost gauge is functioning correctly.
Engineering rationale for hole diameters: The engineering rationale was added to section 2.2. Flow rate, even the maximum, was determined by the set screw, not the water hole. Water hole size was chosen such that when the set screw was backed out to the maximum non-leaking state, the flow rate would be ~16 mL min⁻¹, near the 3D.1’s minimum rate. The Hagen-Poiseuille equation was included to explain the constant head “Mariotte bottle” effect; it was not actually used for the calculation of the water outlet hole.
Fragmented subsections: The fragmented subsections were condensed and the headers were removed.
Tables 1 and 2: The tables were both moved to the appendix.
Unit conversions: We added equivalent rainfall intensities to all appropriate flow rates.
Generalization figure (chart to visualize the relationship between funnel diameter and rain fall intensity): We created this figure and it added to the appendix.
Technical Note vs. full article: We acknowledge the reviewer’s concern and leave this decision to the editors.
We thank the reviewer again for their comments, which help to improve the quality of this manuscript, and we look forward to continued discussions.
Kind regards,
Matthew Tippett-Vannini, on behalf of all co-authors
Citation: https://doi.org/10.5194/egusphere-2025-5744-AC2
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AC2: 'Reply on RC2', Matthew Tippett-Vannini, 10 Aug 2026
Data sets
AR3D Cleaned Calibration Dataset (Version 1.0) Matthew Tippett-Vannini https://doi.org/10.5281/zenodo.17642189
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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.