1. Conventional calibration methods

As the radar level meter is generally tested at a distance of 2-10m or more than 10m, it is difficult to simulate the distance needed for calibration in the laboratory, so the radar level meter is mostly calibrated online. Radar level meter online calibration using on-site process conditions, through the change of liquid level, compare the radar level meter output value and the actual scale measurement value or other forms of liquid level measurement value. According to the "Liquid Level Meter Measurement and Verification Regulations" for radar level meter multi-point or single point (later detailed description of the feasibility of a single point) of the calibration, and then through the debugging software for its debugging, so that it meets the requirements of the measurement accuracy.

Conventional online calibration of radar level meter can ensure that the requirements in accordance with the regulations, while the online calibration of the process interference and installation error factors at the same time for the calibration, the actual measurement accuracy of the radar flow meter should be higher than the laboratory calibration. But online routine calibration to use the site process conditions, during the calibration of the site process equipment - storage tanks or reaction tanks to stop production, seriously affecting the site process production. At the same time the site level change by the process conditions, the stability time is too long, so online routine calibration time is usually longer. Check scale measurement or other forms of level measurement are also subject to site safety and technical constraints.

2. Reference point calibration method

Since on-line conventional calibration is too much affected by process conditions, we explore the reference point calibration method. Reference point calibration is a single point calibration of the radar level meter by setting a reference reflection point. The actual operation is as follows: set up one reflector at the full scale position of the radar level meter beam, and the distance between the reflector and the radar level meter is measured precisely. During calibration, the radar level meter is calibrated by comparing the actual measurement of the reference point by the radar level meter with the known value, and the radar level meter is debugged and calibrated by the comparison value.

3. Setting of reference points

There are two advantages to being located at full scale:

1) Generally the process level will not reach full scale, so the calibration can be done without affecting the normal production; 2) The distance at the full scale is short and the reflection intensity is high, so that the reflecting original can be made small enough to reduce the interference of reflecting objects 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 interference with the normal liquid surface reflection wave intensity. From the measurement principle of radar level meter, we can see that: the distance error is caused by T or △f, set the reference point in the full scale position, then T or △f should be the smallest measured value, that is to say, the measurement is the most difficult, and the corresponding error should be the maximum value in the full scale. Therefore, it is possible to meet the full-range error accuracy requirement by correcting the value/factor of the electrical original to obtain T at the reference point. At the same time, we consider that the electrical detection of the original in the full range of measurement is reflected in the linear error, then in the reference point position of the single-point debugging, data correction is feasible. Just as the pressure gauge in a single point fixed value on the full range of linear error correction effect, a single reference point value correction in the calibration of radar type flow meter is feasible.

To do reference point calibration of radar level meter, it is very necessary to set the reference point at full scale. Generally intelligent instrument has full scale and zero adjustment, we can use the full scale adjustment option to calibrate the reference point of radar type flow meter without adding other functions. At the same time, using the display function and filtering function of the intelligent instrument, after the calibration is completed, the reflected wave of the reference point will be shielded or marked, so that it does not have an impact on the reflected wave of the process interface.

Reference point online calibration can be carried out without affecting production, reducing the impact on the process and calibration time. However, reference point calibration is a single-point calibration, which only has a numerical correction effect on the linear error. Reference point calibration cannot calibrate process fluctuations, bubbles and other non-linear errors, so the calibration should consider the site process conditions.