Magnetic flap levelThe advantage of being able to intuitively reflect the actual state of liquid level in tanks without the need for other means has been increasingly recognized by people. Various manufacturers have used additional technical means to enable it to be applied to actual production control. The measurement accuracy of large range (2m or more) magnetic flap level gauges is often not lower than that of differential pressure transmitters, so users are paying more and more attention to their use. However, due to the actual conditions on site, the advantages of maintenance free claimed by various manufacturers cannot be reflected, and the demand for testing has also increased.

　　Presentation of the Problem

According to the national verification regulations, level gauges below 2 meters need to be calibrated using a standard water tank device, and those exceeding 2 meters need to be calibrated using simulation methods. However, due to the limitations of the size of the large range magnetic flap level gauge itself, there are problems with the installation of level gauges exceeding 2 meters in the laboratory, and the regulations do not provide specific instructions for simulation verification methods. Can this type of liquid level gauge be calibrated online? In summary, the following issues need to be addressed:

(1) The density range of the medium measured by the liquid level gauge is often wide, generally 0.8-1.2g/cm3. How to correct the influence of density on the measurement error of the liquid level gauge;

(2) The on-site storage tank is like a black box, and how to determine the liquid level reference point for calibration is a key issue;

(3) The installation of the liquid storage tank cannot be vertical, so it will be very important to choose a measuring standard and how to measure it.

This article takes the side mounted liquid level gauge with live remote transmission signal (4-20mA) as an example to explore a simple and effective on-site calibration method, in order to achieve the goal of measuring the liquid level error and uncertainty of the entire system under on-site installation conditions, which has practical significance.

　structural principle

　　Magnetic flap level gaugeIt is developed based on the principle of buoyancy and magnetic coupling. When the liquid level in the tested container rises or falls, the magnetic float in the liquid level gauge body tube also rises or falls accordingly. The y-long magnetic steel inside the float is transmitted to the magnetic flip column indicator through magnetic coupling, driving the red and white flip columns to flip, thereby achieving clear indication of the liquid level. By opening and closing the built-in reed switch contacts, current or voltage signals can be transmitted. Magnetic flap level gauges are usually calibrated through simulation methods (not specified in this method specification) at the factory to ensure that they match the actual medium when supplied.

Specific steps for on-site calibration:

Taking the side mounted liquid level gauge as an example, as shown in Figure 1. Figure 1 Basic schematic diagram of on-site calibration pipeline connection and testing

A is the zero marking line of the liquid level gauge; B is the full range marking line of the liquid level gauge; C is the flange or reserved port for the connecting pipe on the tank body;

D is the liquid level transmitter; E is a manual ball valve with a lower connecting flange; F is the flange connecting the tank and the level gauge; A is the horizontal line of the lower pipe mouth; B is the zero point level line of the liquid level; C is the measured water level line

　　(1) Determine the density of the medium

The medium density can be measured using a standard density meter or retrieved based on specific information provided by the user. The medium density needs to be recorded and filed to ensure that it meets the requirements of the liquid level meter user manual. Although theoretically the density of the medium has an impact on the reading of the liquid level gauge, in practical use, the zero and full values of the liquid level gauge can be directly adjusted through a potentiometer.

　　(2) Determine the reference zero point

a) Measure the inner diameter D of the connecting pipeline with a vernier caliper, and determine a standard liquid level measuring point at the upper part of the tank. If possible, it can be polished into a groove to prevent the depth gauge from swinging and marked;

b) Manually inject water into the storage tank without pressure. Stop injecting water when the water level is slightly higher than the inlet pipe of the level gauge. Open the manual ball valve E at the lower connecting flange and loosen the connecting flange F between the tank body and the calibrated level gauge (do not remove it to prevent water flow from surging). When there is no surging flow in the pipeline, close E and remove the flange. When the liquid in the tank is stable, open E and wait for it to drip for 1 minute (if necessary, drain the water through the drainage valve to improve detection efficiency);

c) Measure the distance ha from the measuring point to the water surface with a depth gauge steel tape, and the actual zero position clearance h0=ha-D/2. This state islevel gaugeMeasure zero point

(3) Calibration of each liquid level point

a) Install the flange, close E, and continue to inject water into the tank until the main scale indicating the level to be calibrated is reached. After the water surface stabilizes, measure the output current Ii and the water level height hi. The actual liquid level is H0=h0- hi=ha-D/2-hi;

b) Continue measuring other points until full range is reached.

　　(4) Calibration of liquid level zero and full degree

At the same time as determining the reference zero point, adjust the zero potentiometer so that the output electrical signal is displayed as 4mA; adjust the full-scale to the upper limit of the standard liquid level, increase the full-scale potentiometer so that the output electrical signal is displayed as 20mA. If there is a deviation in the output during the down stroke measurement, refer to this method for adjustment. On site calibration requires repeating the above steps for three return measurements.

　Conclusion

The instruments used for on-site calibration are simple, practical, and highly operable, solving the problem of a large number of instruments that cannot be detected in the laboratory and having practical value. In situations where direct contact with liquid media is not possible, if the inclination of the tank body is small and the surface of the measuring point is horizontal, a laser rangefinder can also be used instead of a depth measuring steel tape. However, on-site testing still has certain limitations, and this method is not suitable for liquid level calibration of pressurized containers or volatile media.