Rain Gauge Calibrator

Rain Gauge Calibrator

Project Lead: Manuel Lopez

Overview

The objective of this project was to develop and validate a freely downloadable, open-source, 3D printed rain gauge calibrator that can be adjusted for a wide range of gauges. The proposed calibrator provides for applying low, medium, and high intensity flow, and allows the user to modify the design to conform to unique system specifications based on parametric design, which may be modified and printed using computer-aided design (CAD) software. Currently available devices for this purpose tend to be designed for specific rain gauges, are expensive, employ low-precision water reservoirs, and do not offer the flexibility needed to test the ever more popular small-aperture rain gauges. To overcome the fact that different 3D printers yield different print qualities, we devised a simple post-printing step that controlled critical dimensions to assure robust performance. Specifically, the three orifices of the calibrator are drilled to reach the three target flow rates. Laboratory tests showed that flow rates were consistent between prints, and between trials of each part, while the total applied water was precisely controlled by the use of a volumetric flask as the reservoir.

Objectives

• Provide a rain-gauge calibrator delivering volumetrically precisely (<0.1% error) volumes

• Provide a rain-gauge calibrator delivering a wide range of applications rates to allow verification of rain gauge function under environmental fluxes.

• Provide a parametric design easily adapted to any range gauge, which can be produced on most 3D printers anywhere in the world.

Outcomes

In our initial test, the five and ten minute settings were consistent to within 3%, and the thirty minute setting to within 6.93%. Each delivered 500ml of water. Drilling the slowest setting became really hard as it required very small drill bits. It was concluded that twenty minutes was the slowest setting with highly reproducible results and therefore changed our target time. The following graphs show the performance tests that we ran on a series of calibrators; more tests were performed but only these are shown for demonstration purposes.

This is the data of the performance of a series of calibrators. It shows the first testing phase which included 5, 10, and 30 minutes settings.

After testing various diameters pairs, we were able to achieve consistency to within 4.75% of our target time. This performance was a major improvement upon our initial time. The following graphs shows the performance test that we ran on a couple of 20 minute setting calibrators.

This is the data of the performance of a series of 20 minutes setting calibrators.

It can be noticed that the performance of the calibrator was more consistent than the previous 30 minute target. We also pushed forward with reducing the time because the smaller the drill bit get the more expensive they are. This creates a barrier for anybody trying to make a calibrator.

Conclusion

When the small apertures in the device are drilled post-printing, the multi-rate rain gauge calibrator performs to our specifications and can be readily modified, printed and employed in the field. We demonstrate that without post-printing drilling, small orifices will have enough variability between prints and between printers to yield unacceptable performance. We also note that the most critical aspect of calibration of the rain gauge, that a known total volume of water is applied, is guaranteed to less than 0.1% deviation through use of a part-by-part calibrated volumetric flask, purchased for this effort for under $5 and made from high-impact polypropylene.

Publication

• Lopez Alcala, J. M., Udell, C. J., & Selker, J. (2019). A User-Printable Three-Rate Rain Gauge Calibration System. Frontiers in Earth Science. https://doi.org/10.3389/feart.2019.00338

Resource List

• CAD

• Data

Tutorials

• Calibrator Tutorials

Keywords

Rain Gauge Calibrator, Calibrator, Mariotte Stopper, Fusion360

References

• Bergmann, H., H. Breinhälter, O. Hable, & R. Krainer. (2001). Calibration of tipping bucket hyetographs, Phys. Chem. Earth, Part C Solar, Terr. Planet. Sci., 26(10), 731–736, doi:10.1016/S1464-1917(01)95017-2. Habib, Emad, Alemseged Tamiru Haile, Yudong Tian, & Robert J. Joyce. (2012). Evaluation of the High-Resolution CMORPH Satellite Rainfall Product Using Dense Rain Gauge Observations and Radar-Based Estimates. Journal of Hydrometeorology 13 (6): 1784–98. doi:10.1175/JHM-D-12-017.1.

• Humphrey, M. D., J. D. Istok, J. Y. Lee, J. A. Hevesi, & A. L. Flint. (1997). A New Method for Automated Dynamic Calibration of Tipping-Bucket Rain Gauges. Journal of Atmospheric & Oceanic Technology 14 (6): 1513.

• Kintel, M. OpenSCAD; ver 2015.03, Date page accessed October 20, 2016 athttp://www.openscad.org/index.html

• Mariotte, E. (1679). Essais de Physique : ou Mémoires pour servir à la science des choses naturelles, Paris.

• Vasvári, V. (2005). Calibration of tipping bucket rain gauges in the Graz urban research area, Atmos. Res., 77(1), 18–28, doi:10.1016/j.atmosres.2004.12.012.

• Volumetric Flask, accessed November 5, 2016 at http://www.ebay.com/itm/500ml-Lab-Volumetric-Flask-Measuring-Bottle-with-Cap-Graduated-Container-Plastic/291800957642?_trksid=p2047675.c100011.m1850&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%26asc%3D39827%26meid%3Da10abdb9e22d454bb7697b855ab0df1e%26pid%3D100011%26rk%3D2%26rkt%3D11%26sd%3D221946006304