Precipitation Gauge Intercomparison

2 Day Gauge Intercomparison

7 Day Gauge Intercomparison


Sensors

Sensor Model Mechanism Type Data Type Data record interval Frozen Precipitation
Apogee 6-Inch Apogee SG-400 sensor with a 6-inch funnel Weighing Filtered weight calculation 1 min Antifreeze
Apogee 8-Inch Apogee SG-400 sensor with an 8-inch funnel Weighing Filtered weight calculation 1 min Antifreeze
Met One Met One Model 385 Tipping Count of tips 3 min Heated
Pluvio OTT Hydromet Pluvio2 version 400 Weighing Filtered weight calculation 1 min Antifreeze
PreciBal Hyquest Solutions PreciBal 314 Weighing Filtered weight calculation 1 min Antifreeze

Terminology

What measurement mechanisms are used in rain gauges?

Two types of measurement mechanisms are used by the gauges included in this comparison:

  • Tipping rain gauges - funnel precipitation into a tipping mechanism that tips when filled to its calibrated level. The signal is a pulse to the data logger -- each pulse indicating an amount of rainfall
  • Weighing rain gauges - have a bucket mounted on a weighing platform that measures the change in weight as precipitation is added. Signals from these sensors are typically serial measurement data. Many models contain electronics to provide filtered data that minimize the influence of evaporation, vibration (from wind), and temperature change.

Weighing gauges are typically more accurate for measuring all forms of precipitation, particularly at high rates. Maximum rate specifications for tipping bucket gauges are typically in the range of 12 to 20 mm per min (720 to 1200 mm per hr), and the uncertainty of the measurement is often unspecified beyond 8 mm per min. Conversely, maximum rate specifications for weighing gauges are in the range of 30 to 120 mm per min (1800 to 7200 mm per hr), and uncertainty is specified across the entire measurement range. Weighing gauges can also measure smaller amounts of precipitation. Depending on the model, tipping bucket gauges require 0.1 mm or 0.25 mm of precipitation to trigger a tip of the bucket, in addition to precipitation needed to wet the funnel. Weighing gauges are more sensitive, where events as low as 0.025 to 0.05 mm can be detected. (Quoted from the Apogee SG-400 Manual)

Why are the data series lines not all the same?
  • As was mentioned above, the resolution for sensors can be different. For example, a weighing gauge may have measured a certain amount of precipitation before a tipping bucket receives enough to wet the funnel and a tip to occur. So the tipping gauge series may appear more jagged.
  • With sensors that provide filtered data, there can be a lag of up to 5 minutes between when the precipitation occurs and when the filtered precipitation is reported. This can cause the charts for these sensors to lag slightly.
  • Measurement data for the compared sensors is interpreted and stored in data logger tables that have specified intervals for records. Periodically, the data tables are collected into a database and used to generate the charts on the screen. In the charts on this page, not all of the data records were recorded on the same interval, so if a series is using data collected on longer intervals, the data point may be missing from the pop-up list or the data points may appear if you zoom in on the chart.
  • Precipitation events are sporadic. You may have observed times when it is raining or snowing in your front yard and not in your back yard. Precipitation is also influenced by wind and obstructions. So it's entirely possible that two sensors right next to each other, actually receive different amounts of precipitation from the same event.
How is frozen precipitation mitigated?
  • For tipping rain gauges, frozen precipitation must be melted by a heated funnel.
  • For weighing gauges, the bucket is usually filled with an antifreeze mixture such as propylene glycol to melt the frozen precipitation. Typically, mineral oil is added to minimize evaporation of bucket contents so the weight stays consistent between precipitation events.