On-site calibration method and specific operation steps of magnetic flap level gauge

Compared with other new instrument types, the measurement method of magnetic flap level gauge is more traditional. It works based on the buoyancy principle and magnetic coupling principle. It can intuitively reflect the actual state of the liquid level in the tank without the help of other means. Its intuitive measurement advantage is very prominent. By selecting relevant types of equipment and technical means, its measurement can be more widely used in actual production. According to the calculation of the technicians of Runzhong Instrument, the measurement accuracy of large-scale (more than 2m) magnetic flap level gauge is often comparable to that of differential pressure transmitter. However, due to the constraints of actual on-site conditions, the maintenance-free advantages advertised by various manufacturers cannot be reflected, and the demand for detection has also increased. More and more types of magnetic flap level gauges have appeared in the market. Here I would like to share with you the calibration methods of magnetic flap level gauges used on-site.
 
1. Problem statement
   According to the description of the national calibration regulations, level gauges below 2m need to be calibrated by standard water tank devices, and those above 2m need to be calibrated by simulation methods [1]. However, due to the limitation of the size of the large-scale magnetic flap level gauge itself, there are problems in the installation of the magnetic flap level gauge with a length of more than 2m in the laboratory, and the regulations do not specify the simulation verification method. Magnetic flap level gauge So can this type of level gauge be calibrated online? In summary, the following problems need to be solved:
(1) The density range of the medium measured by the level gauge is often wide, generally 0.8 to 1.2g/cm3. How to correct the influence of density on the measurement error of the level gauge;
(2) The on-site storage tank is like a black box. How to determine the liquid level reference point for calibration is a key issue;
(3) The installation of the magnetic flap level gauge storage tank cannot be absolutely vertical, so how to choose the measurement standard and how to measure will be very important.
   This article takes the side-mounted magnetic flap level gauge with electric remote transmission signal (4 ~ 20mA) as an example to explore a simple and effective on-site calibration method, in order to achieve the purpose of measuring the liquid level error and its uncertainty of the entire system under on-site installation conditions, which has practical significance.
 
2. Structural principle
The magnetic flap level gauge is developed based on the buoyancy principle and magnetic coupling. When the liquid level in the measured container rises and falls, the magnetic float in the body tube of the level gauge also rises and falls. The permanent magnet in the float is transmitted to the magnetic flip column indicator through magnetic coupling, driving the red and white flip columns to flip, thereby achieving a clear indication of the liquid level. The transmission of current or voltage signals is achieved by opening and closing the built-in reed switch contacts. The magnetic flap level gauge is generally calibrated by a simulation method (not described in this method specification) when it leaves the factory to ensure that it matches the actual medium when it is supplied.
 


III. Specific steps for on-site calibration
(1) Determine the medium density
   The medium density can be measured with a standard density meter. The magnetic flap level gauge can also be obtained based on the specific information provided by the user. The medium density must be recorded and filed to ensure that the medium density meets the requirements of the level gauge instruction manual. Although in theory the medium density has an impact on the indication of the level gauge, in actual use the zero and full scale values ​​of the level gauge can be directly adjusted using a potentiometer.
(2) Determine the reference zero point
a) Use a vernier caliper to measure the inner diameter D of the connecting pipe. The magnetic flap level gauge determines a lower scale point of the standard liquid level on the upper part of the tank body. If conditions permit, it is best to grind it into a groove to prevent the depth gauge from swinging, and make a mark;
b) Manually add water to the storage tank without pressure in the tank. When the water level is slightly higher than the water inlet pipe of the level gauge, stop adding water. The magnetic flap level gauge opens the manual ball valve E at the lower connecting flange and loosens the connecting flange F between the tank body and the calibrated level gauge (do not remove it to prevent water from over-rushing). When there is no surging flow in the pipeline, close E, remove the flange, and open E when the liquid in the tank is stable. Wait until it is in a dripping state and stabilize for 1 minute (if necessary, drain the water through the drain valve to improve detection efficiency);
c)  The magnetic flap level gauge uses a depth measuring steel tape to measure the distance ha from the measuring point to the water surface. The actual zero position height h0 = ha-D/2. This state is the measurement zero point of the level gauge.
(3) Calibration of each liquid point
a) Install the flange, close E, and continue to fill the tank with water until the flap indicates the main scale of the liquid level to be calibrated. 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 to measure other points of the magnetic flap level gauge until the full scale.
(4) Adjustment of liquid level zero point and full scale
   While determining the reference zero point, adjust the zero point potentiometer so that the output electrical signal is displayed as 4mA; the full scale adjustment is carried out at the upper limit of the standard liquid level, and the full scale potentiometer is increased so that the output electrical signal is displayed as 20mA. If there is a deviation in the output during the downstroke measurement of the magnetic flap level gauge, refer to this method for adjustment. On-site calibration requires repeating the above steps for three return measurements.
 
IV. Conclusion
   The instrument used for on-site calibration of the magnetic flap level gauge is simple and practical, with strong operability. It solves the problem that large-scale liquid level instruments cannot be detected in laboratories and has more practical value. In the case of no direct contact with the liquid medium, if the magnetic flap level gauge has a small inclination and the surface of the measuring point is horizontal, a laser rangefinder can be used instead of a depth measuring steel tape. However, on-site detection still has certain limitations, and this method is not suitable for level calibration of pressurized containers or volatile media.
 
   In addition, with the improvement of the productivity of domestic magnetic flap level gauges, the magnetic flap level gauges produced are becoming more and more high-end, and the measurement accuracy is also getting higher and higher. Magnetic flap level gauges are not only used in industry, but are also very popular in some chemical and biological research.