How to perform periodic verification and calibration on mechanical water meters

The Significance of Periodic Calibration
Mechanical water meters are the most fundamental measuring instruments in water supply systems. Their accuracy directly impacts user water billing and water supply company profits. Periodic calibration is essential to ensuring meter accuracy. National regulations and metrology technical specifications clearly stipulate the calibration interval for water meters, which typically ranges from two to six years, depending on the meter size and operating environment. Periodic calibration can promptly identify meters with excessive metering errors, prevent data distortion, and ensure the fairness and legality of water supply metering.

Preparation Before Calibration
Mechanical water meters require cleaning and a visual inspection before calibration. Cleaning removes sediment and scale deposited inside the meter to prevent it from affecting calibration data. The visual inspection includes checking the meter housing for cracks, ensuring the glass is clear and intact, and ensuring the integrity of the lead seal. Preparatory work also includes recording the meter model, diameter, production number, and operating time to provide a reference for subsequent calibration.

Calibration Equipment and Standard Devices
Periodic calibration of mechanical water meters relies on specialized standard flow devices. Common devices include volumetric and mass-based standard devices. The volumetric method uses a standard water tank to collect flow and compare the indicated volume with the actual volume. The mass method uses a high-precision electronic balance to measure flow and calculate the meter error. Calibration equipment must undergo regular calibration by a metrology agency to ensure its accuracy meets calibration procedures. Using a standard device ensures objectivity and consistency in calibration results.

Basic Calibration Process

The calibration process consists of three stages: low flow rate, normal flow rate, and maximum flow rate. The calibrator sets the standard flow rate according to the regulations, measures a certain volume of water through the water meter, and then reads the indicated volume. This reading is then compared with the actual measurement data from the standard device. Errors must be within the limits specified by national standards. For example, the error at normal flow rate must not exceed ±2%, and the error at low flow rate must not exceed ±5%. If the error exceeds this allowable range, the meter is deemed unqualified and must be removed from service or repaired.

Technical Requirements for Calibration

Different from calibration, calibration focuses on evaluating the performance of a water meter under actual operating conditions. The calibration process takes into account pressure fluctuations, water quality, and temperature within the water supply network. Calibration not only includes standard flow rate testing but also continuous operation and durability testing to verify the meter's long-term stability. Calibration is typically performed by a qualified third-party organization to ensure authoritative and traceable results.

Calibration Steps
Mechanical water meter calibration typically involves three steps. The first step is a flow rate test to verify the meter's accuracy at different flow rates. The second step is a durability test, where the meter is operated continuously at a specified high flow rate for a period of time to re-check whether its metering accuracy has changed. The third step is an environmental adaptability test, where the meter is operated under simulated high and low temperatures and water conditions containing impurities to verify its ability to maintain stable performance. Calibration results provide technical support for water supply companies in selecting and maintaining water meters.

Differences Between Verification and Calibration
Verification emphasizes legal validity and is a mandatory test of the meter's measurement accuracy. The results are used to determine whether the meter can continue to be used. Calibration focuses more on performance optimization and quality control, providing reference data for water supply companies. Periodic verification ensures the legality and accuracy of water meters, while regular calibration helps companies improve equipment management and extend the life of water meters.

Periodic Verification and Calibration Management
Water supply companies should establish a comprehensive water meter record management system when performing verification and calibration. This record should include the meter's verification date, calibration records, operating environment, and maintenance history. This data management allows for tracking meter operating conditions and enabling the proper planning of replacement and maintenance. For large-diameter mechanical water meters, a shorter verification cycle is recommended, as they operate under high flow conditions and are more susceptible to mechanical wear and error.