Analysis of the Calibration Method of Radar Level Gauge

Radar level gauges are more and more widely used in the field of oil and gas measurement due to their wide application range, high measurement accuracy, strong reliability, and low maintenance. The radar level gauge is a general-purpose radar level gauge. It is a measuring instrument based on the principle of time travel. The radar wave runs at the speed of light, and the running time can be converted into a level signal through electronic components. 1 The principle of radar level gauge The radar level gauge adopts the working mode of transmitting-reflecting-receiving. The antenna of the radar level gauge emits electromagnetic waves. These waves are reflected by the surface of the measured object and then received by the antenna. The time from emission to reception of electromagnetic waves is proportional to the distance to the liquid surface. The relationship is as follows: D=CT/2D ——The distance from the radar level gauge to the liquid surface; C——the speed of light; T——the electromagnetic wave running time. The radar level gauge records the time elapsed by the pulse wave, and the transmission speed of the electromagnetic wave is constant, the distance from the liquid level to the radar antenna can be calculated, so as to know the liquid level of the liquid level. Since the radar wave propagates at the speed of light C=2.9979×108m/S, the round-trip time of the electromagnetic wave of 1m is 6.6nS. For a measurement object with a relatively small height such as an oil tank, extremely high resolution is required, so it is almost impossible to measure the reflection time. To this end, time extension technology is needed. At present, frequency modulation continuous wave technology is used to solve the problem. Obviously, the measurement of time difference is converted into measurement of frequency difference, which increases the technical difficulty of testing and increases the precision coefficient. 2 Conventional calibration method Because the radar level gauge generally has a test distance of 2-10m or more than 10m, it is difficult to simulate the distance required for calibration in the laboratory, so the radar level gauge mostly adopts the online calibration method. The online calibration of the radar level gauge uses the on-site process conditions to compare the output value of the radar level gauge with the actual gauge measurement value or other forms of liquid level measurement through the change of the liquid level. Carry out multi-point or single-point calibration of the radar level gauge according to the 'Measurement and Verification Regulations of Liquid Level Gauge' (the feasibility of a single point will be described in detail later), and then debug it through the debugging software to make it meet the measurement accuracy requirements. The conventional online calibration method of the radar level gauge can ensure that it is carried out in accordance with the requirements of the regulations. At the same time, the online calibration calibrates the process interference and installation error factors at the same time. Therefore, the actual measurement accuracy of the radar level gauge should be higher than that of the laboratory calibration. However, on-line routine calibration needs to utilize on-site process conditions. During the calibration, the on-site process equipment—storage tanks or reaction tanks must be stopped, which seriously affects the on-site process production. At the same time, the on-site liquid level change is affected by the process conditions, and the stabilization time is too long, so the online routine calibration time is usually long. Detector measurement or other forms of liquid level measurement are also restricted by on-site safety and technology. 3 Reference point calibration method Because the on-line routine calibration is too much affected by the process conditions, we discuss the reference point calibration method. Reference point calibration is to perform single-point calibration on the radar level gauge by setting a reference reflection point. The actual operation is: set a reflection element at the beam full-scale position of the radar level gauge, and measure the distance between the reflection element and the radar level gauge accurately. During calibration, the radar level gauge is calibrated by comparing the actual measurement of the reference point with the known value by the radar level gauge. The setting of the reference point at the full scale has two advantages: 1) Generally, the process liquid level will not reach the full scale, so normal production can not be affected during calibration; 2) The distance at the full scale is short and the reflection intensity is high, so that the original is reflected. It can be made small enough to reduce the interference of reflectors and ensure the actual measurement of the intensity of the reflected wave of the liquid level. The setting of the reference point should not be too much, too much will interfere with the normal liquid surface reflected wave intensity. From the measurement principle of the radar level gauge, it can be known that the distance error is caused by T or △f. If the reference point is set at the full-scale position, the measured value of T or △f should be the smallest, that is, the measurement difficulty is the greatest, and the corresponding error should be is the maximum value in the full scale. Therefore, the numerical/coefficient correction of the electrical components that obtain T at the reference point can meet the full-scale error accuracy requirements. At the same time, we consider that the electrical detection element reflects the linearity error in the full-scale measurement, so it is feasible to debug and correct the data at the reference point position at a single point. Just as the single-point fixed value of the pressure gauge corrects the full-scale linearity error, the single-reference point value correction is feasible in the calibration of the radar level gauge. It is very necessary to set the reference point at the full scale for the radar level gauge to be calibrated at the reference point. Generally, smart instruments have full-scale and zero-point adjustment. We can use the full-scale adjustment option to calibrate the reference point of the radar level gauge without adding other functions. At the same time, using the display function and filtering function of the smart meter, after the calibration is completed, the reflected wave of the reference point is shielded or marked, so that it does not affect the reflected wave of the process interface. In the display of normal and interference waveforms, the reflected waves at the reference point can be marked and eliminated through this function.
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