Optical rain gauge vs tipping bucket rain gauge
Tipping buckets block, stick and under-read in heavy rain. Optical gauges have no moving parts. The comparison, the limits of each, and how to retrofit.
A tipping bucket rain gauge measures rainfall by collecting water until a small bucket tips and records a fixed volume. Optical rain gauges measure precipitation by analysing raindrops as they pass through an optical sensing area, eliminating moving parts altogether. This avoids issues such as blocked funnels, sticking buckets and reduced accuracy during intense rainfall, while providing reliable measurements throughout the year.
The three failure modes of a tipping bucket
- Blockage. A funnel is an open collector. Leaves, insects, grit and bird lime all reduce or stop the catch, and the gauge reports dry weather. On a remote or unvisited site the record can be lost for weeks before anyone notices.
- Sticking. The bucket pivots on a bearing. Grit, corrosion or ice stops it tipping cleanly, which under-reports.
- Under-reading in heavy rain. While the bucket is in motion it is not catching, and the water that falls during the tip is lost. The error grows with intensity, so the gauge is least accurate in exactly the storm the record was there to capture.
What an optical gauge does instead
TruMet PW100 measures liquid precipitation optically, using an infrared sensing area of 66 by 20 mm. There is no funnel to block and no bucket to stick, and nothing moves.
| Property | Figure |
|---|---|
| Intensity range | 0 to 200 mm/hr |
| Accuracy | 5% |
| Resolution | 0.1 mm |
| Output | One pulse per 0.1 mm, which replicates a tipping-bucket gauge |
| Protection class | IP66 |
| Operating temperature | -35 to +70 degrees C |
| Measurement temperature | 0 to +70 degrees C |
| Weight | 0.67 kg |
| Mounting | Fits a 44.45 mm, 1.75 inch, pole or mast |
| Warranty | 24 months |
The retrofit point that makes this easy
The output is deliberately one pulse per 0.1 mm, which is what a tipping bucket produces. So it connects to any system already expecting a tipping-bucket rain input: the logger, the SCADA point and the historic scaling all stay as they are.
That is the whole argument for a retrofit. You remove the maintenance and the blockage risk without touching the data pipeline or breaking the continuity of the series.
Where it goes, and where it does not
- MetConnect, through the contact-closure rain input
- MaxiMet GMX550 and GMX551, the two models with a tipping-bucket input
- Any third-party logger expecting a tipping-bucket pulse
- MaxiMet GMX603 carries TruMet rainfall integrated, so it is the product rather than an add-on
The honest limit: TruMet measures liquid precipitation only, with a measurement temperature range of 0 to +70 degrees C, so it does not measure snow or hail. On a site where frozen precipitation has to be quantified, an optical liquid gauge is the wrong instrument whatever its maintenance advantage. Do not offer a TruMet add-on on MaxiMet models other than the GMX550 and GMX551, because they have no tipping-bucket input.
What to take away
- Tipping buckets block, stick, and under-read as intensity rises.
- TruMet PW100 is optical, 0 to 200 mm/hr at 5%, 0.1 mm resolution, no moving parts.
- Its pulse output replicates a tipping bucket, so a retrofit needs no change to the logger.
- Liquid precipitation only, 0 to +70 degrees C measurement range, so not for snow or hail.
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