The following is an introduction to the calibration method of rain gauges:
1、 Basic requirements and cycles for calibration
The calibration of rain gauges must follow strict measurement standards to ensure that the measurement results can be traced back to the International System of Units. According to industry standards, the rainfall sensors of automatic weather stations are usually calibrated at least once a year, especially before the flood season. If the following situations occur, calibration should be carried out immediately: after creating or replacing equipment; After repairing the data collector; Abnormal measurement values (such as significant deviation from actual rainfall); After heavy rain or during monthly routine maintenance. In addition, it is recommended that the recalibration interval for weighing rain gauges should not exceed one year to ensure their accuracy.
2、 Preparation before calibration
The meticulous preparation before calibration directly affects the reliability of the results. Taking the tipping bucket rain gauge as an example, the outer bucket needs to be disassembled first to check the installation stability, whether the water flow channel is blocked, the flexibility of the tipping bucket rotation, and whether the reed switch signal is normal. For snow melting rain gauges, the power must be disconnected and the wiring sequence recorded to prevent restoration errors. The environmental conditions also need to be strictly controlled: the temperature should be between -5 ℃~35 ℃, the humidity should be ≤ 95% RH, the wind speed should be ≤ 6m/s, and it should be sunny without rain. At the same time, the calibrator needs to be leveled and the air in the pipeline should be eliminated to avoid measurement errors caused by bubbles.
3、 Calibration methods for different types of rain gauges
1. Bucket type rain gauge
Use a fully automatic calibrator (such as ARC100-M) for dynamic calibration. Accurately control the water injection rate through a constant flow pump (simulating the intensity of light rain to heavy rain), record the rainfall value corresponding to the number of bucket triggers, and calculate the indication error. If the relative error exceeds ± 4%, the adjustment knob below the tipping bucket needs to be adjusted: if the error is positive, rotate clockwise; if it is negative, rotate counterclockwise, and simultaneously observe the number of water droplets on both sides of the tipping bucket to balance the accuracy.
2. Weighing rain gauge
Operate according to the specifications. Firstly, check the inner diameter of the rainwater inlet (should be ≥ 200mm), the blade angle (40 °~45 °), and the sealing of the water collector. Calibration divided into stages of water injection: inject 10% of the full range water to zero, and then inject water at a constant speed in increments of 5-10mm and 10-15mm (rain intensity ≤ 4mm/min), repeating more than 6 times to calculate the average error. Laboratory calibration also needs to cover 50% and 80% of the range points to comprehensively evaluate linearity.
3. Radar based rain gauge
The focus is on hardware parameter adjustment and data comparison analysis. By emitting electromagnetic waves through a dual polarized antenna and receiving the echo signal, the system deviation is corrected by comparing it with the measured precipitation. For example, the difference between horizontal and vertical echo signals needs to be compensated for through algorithms to ensure the accuracy of precipitation particle morphology recognition.
4、 Key technical details in calibration
1. Error adjustment techniques
The mechanical adjustment of the tipping bucket rain gauge should take into account the synchronization of the knobs on both sides to avoid tilting caused by excessive rotation on one side. If the calibration instrument interface leaks water, it can be reinforced and sealed by wrapping the joint with adhesive tape. Weighing rain gauges need to pay attention to water temperature control (5-30 ℃) to prevent density changes from affecting mass conversion.
2. Data recording and processing
The raw data should include the standard values for each water injection, sensor readings, environmental temperature and humidity, and equipment numbers. Mobile measurement software can assist in real-time collection and storage, reducing human intervention errors. The final report should indicate the maximum allowable error (such as a level 1 accuracy of ± 0.2mm or ± 2%) and repeatability index (≤ 0.1mm or 1%).
5、 Daily maintenance and long-term management
Regularly clean the rainwater inlet, tipping bucket, and filter screen to prevent insects from building nests or leaves from blocking them. Check whether the leveling bubble is centered every quarter to ensure that the equipment is in a horizontal state; Every year, submit to the metrology institute for calibration and promptly eliminate instruments that exceed the tolerance.