After a recent rain event, I noticed that The Davis tipping bucket rain gauge was not reading correctly. For comparison purposes I have it mounted side by side with a standard 4″ manual gauge of known accuracy. The 4″ gauge registered 1.12″ of precipitation while the Davis gauge registered .88″ of rain. This is a significant difference for such a small amount of total rain.
After investigating possible causes such as a mechanical failure of the Davis gauge, I noticed that the tipping buckets held a quantity of water in each bucket after emptying. I assume that this was due to the surface friction of the water in combination with a dirty bucket surface, causing the bubble of water to remain on the tip of each bucket as shown in the photo below.
The added weight of this water on each tip of the bucket will eventually accumulate to a significant error in rain quantity measurements quickly. My usual cleaning routine is to remove the tipping bucket from it’s bracket and thoroughly wash it in soapy water and remove any debris buildup. Lastly I take a moment to rub a bit of paste wax onto the bucket surface and carefully buff the surface. The allows the water droplets to roll completely off of the bucket as it was designed, thus emptying the bucket completely on each tip. Since I had removed the rain gauge from the field and brought it inside the workshop for cleaning, I decided to re-calibrate the gauge to make sure everything was working accurately before returning it to service.
The calibration procedure is not difficult, but taking care to ensure accuracy is important. Each gauge manufacturer has a different formula for calibration depending on their funnel rim diameter. For the Davis gauge, 544 mL of water equals 1″ of rain reported. Other manufacturers may use a different quantity of water, so it is important to use the correct amount for your specific gauge. I have always used 544 mL and this is accurate enough to stay within Davis specifications.
Using a gram scale, I measured 544 grams of water in a container as water weighs almost 1 gram per mL. The challenge is to find a way to drip this known quantity of water in a controlled manner through the gauge. Too fast, and the Davis console will not correctly read the total amount as it will skip bucket tips if they occur too fast. My solution is to use a small plastic funnel. I tape the opening at the tip of the funnel with a piece of foil tape. Using a needle, I pricked a tiny hole in the foil which allows the water in the funnel to drip at a slow and controlled rate through the gauge. After a bit of trial and error I can usually get it to drip at around 4 inches of rain per hour on the Davis console which replicates a typical hard rainstorm event, and the Davis computer does not miss count any bucket tips at that rain rate. After 15 minutes or so, the full 544 ml of water has completely run through the gauge and I take note of the amount of rain reported on the Davis console.
To calibrate the gauge, you must turn the two adjustment screws under each bucket. Raising the screw will report more rain while lowering the screw will report less. It is important to try to keep each screw the same distance as long as your bucket is in balance. Just a fraction of a turn on the screw is usually all that is need for calibration. I eventually got consistent reports of 1.02″ rain after several calibration runs through the gauge. This is accurate enough for the Davis equipment design specifications.
I cleaned all of the internal parts and returned the rain gauge to the field for another season of work.
Only rarely will two different rain gauges read exactly the same quantity of rain, but striving for accuracy is important as this information is compiled in the station climatological record. Seasonal calibration of the gauges ensures accurate reporting.