Calibration of an electromagnetic flowmeter involves comparing the measured values of a standard flowmeter with those of the instrument being calibrated, correcting deviations to ensure measurement accuracy. The core aspects are selecting a suitable calibration method, strictly adhering to operating procedures, and verifying data. It is suitable for industrial fluid metering and other similar scenarios.

I. Preparation Before Calibration

Preparation before calibration directly affects the reliability of the calibration results and must be comprehensively implemented from three aspects: equipment, environment, and medium.

(I) Equipment and Tool Verification

1. Confirm the Qualification of the Standard Device: Select a standard flowmeter that has been verified and certified by a legal metrology institution, such as a standard volumetric tube or standard metering device. Its accuracy class should be at least two levels higher than that of the electromagnetic flowmeter being calibrated. For example, if the accuracy of the instrument being calibrated is 0.5, the accuracy of the standard device should be no less than 0.2.

2. Check the Completeness of Auxiliary Tools: Prepare a multimeter (for testing signal output), an insulation resistance meter (for measuring electrode insulation), a wrench, screwdriver, etc., ensuring that the tool range matches the parameters of the instrument being calibrated and that they are within their calibration validity period. 3. Verify the status of the instrument being calibrated: Check the electromagnetic flowmeter sensor housing for damage, the electrodes for scaling or corrosion, the converter display for normal operation, and the wiring terminals for tightness, looseness, or oxidation.

(II) Environmental and Media Preparation

4. Optimize the calibration environment: Control the calibration site temperature between 5℃ and 35℃, and the relative humidity to no more than 85%. Avoid direct sunlight and strong electromagnetic interference (e.g., keep away from high-power motors). Keep the ground dry and level for easy equipment installation and commissioning.

5. Adapt the calibration medium: Prioritize using a conductive medium consistent with the actual usage. If conditions are limited, a simulated medium prepared with distilled water and an electrolyte (e.g., potassium chloride) can be used. Ensure the medium conductivity is not less than 5μS/cm, and that it is free of impurities and air bubbles, with the temperature stable within the normal range.

6. Purge air bubbles from the pipeline: Flush the calibration pipeline with the medium circulation, opening the exhaust valve to purge air from the pipeline until the pipeline is full of medium and no air bubbles overflow, preventing air bubbles from affecting the accuracy of electrode signal acquisition.

II. Mainstream Calibration Methods and Operations

Depending on site conditions and accuracy requirements, electromagnetic flowmeter calibration mainly employs the standard meter method, volumetric method, and electrical parameter method, each with its own emphasis in the operation process.

Method 1: Standard Meter Method

The standard meter method involves connecting a standard flowmeter and the electromagnetic flowmeter being calibrated in series. The error is calculated by comparing the measured values of the two. This method is suitable for calibrating medium- to low-precision instruments and is simple and efficient to operate.

Step 1: Pipeline Connection

Connect the pipeline in series, in the following order: “Medium storage tank → Pump → Filter → Standard meter → Electromagnetic flowmeter being calibrated → Valve → Return to storage tank”. Ensure that the inner diameter of the pipeline matches the nominal diameter of the instrument, the joints are sealed without leakage, and the distance between the standard meter and the instrument being calibrated is not less than 5 times the pipe diameter.

Step 2: Parameter Setting

Set the parameters for both the standard meter and the electromagnetic flowmeter being calibrated. Input basic parameters such as medium density, temperature, and pipe diameter. Unify the measurement units for both (e.g., both set to m³/h) and set the sampling frequency to be consistent (1Hz recommended).

Step 3: Flow Point Testing

Turn on the media pump and control the flow rate by adjusting the valves. Select 5 calibration points in ascending order (including minimum range, common range, and maximum range). After each flow point has been running stably for 3 minutes, simultaneously record the instantaneous flow values of the standard meter and the instrument being calibrated. Repeat the measurement 3 times for each point and take the average value as the measurement result for that point.

Step 4: Error Calculation

Calculate the error at each point using the formula: “Relative Error = (Measured value of the instrument being calibrated - Measured value of the standard meter) / Measured value of the standard meter × 100%”. If the error is within the instrument's allowable range (e.g., error of a 0.5 grade instrument ≤ ±0.5%), the calibration is qualified; otherwise, adjustment is required.

Method 2: Volumetric Calibration

The volumetric method collects the media volume over a certain period of time, calculates the actual flow rate, and compares it with the measured value of the instrument being calibrated. This method is suitable for calibrating small and medium diameter electromagnetic flowmeters and offers high accuracy.

Step 1: Device Setup

Construct a calibration system consisting of a medium source, the electromagnetic flowmeter under test, a timing device, and a standard measuring vessel. The standard measuring vessel must be calibrated and its volumetric error must not exceed ±0.02%. The timing device accuracy must be no less than 0.1 seconds.

Step 2: Exhaust and Preheating

Start the system to circulate the medium. Open the vent port of the measuring vessel to expel air. After the measuring vessel is full of medium, close the vent port and allow the system to run for 10 minutes to preheat, ensuring stable medium temperature and flow rate.

Step 3: Measurement and Recording

After the flow rate displayed on the instrument under test has stabilized, simultaneously open the timing device and the inlet valve of the measuring vessel. When the medium in the measuring vessel reaches the preset scale, immediately close the valve and the timer. Record the volume value of the measuring vessel, the timing time, and the cumulative flow rate value displayed on the instrument under test. Repeat the measurement 3 times for each flow point.

Step 4: Result Calculation

Calculate the actual value according to the formula "Actual flow rate = Meter volume / Timing time", compare it with the cumulative flow rate value of the calibrated instrument, and calculate the relative error. If the error exceeds the allowable range, the parameters of the instrument converter need to be corrected.