What exactly is the positive migration and negative migration of transmitters?

Simply put, after the transmitter is installed, if its output is not at zero at the actual zero point, it is necessary to perform a calibration to shift the transmitter’s zero point to the actual zero point. If the transmitter output is greater than zero, positive migration is required; otherwise, negative migration is necessary.

This post was last edited by 1111111 on 2018-1-5 at 13:30. By “migration,” what is meant is adjusting the original reading of the pressure gauge by + or -1 unit, so that the calculated value from the pressure gauge matches the value you want to have. Do you understand? . How could you not understand? I’ll bite your pig’s mouth off! . . {:2_43:}

The purpose of migration is to ensure that the current value output by the transmitter corresponds to the actual range of process parameters. When the process parameter is zero, simply make the transmitter output a current of 4 milliamps – that’s all.

Most pressure transmitters and differential pressure transmitters are equipped with a \"zero shift\" function, which allows the zero point of the transmitter – that is, the starting point of its measurement range – to be shifted in either positive or negative direction to meet the various requirements of the production site. In practical applications, migration can be divided into three types: no migration, negative migration, and positive migration. The measurement range of the transmitter is equal to the sum of the scale range and the offset, that is, measurement range = scale range + offset. As shown in the figure, the range for A is 40 kPa, the migration amount is -40 kPa, and the measurement range is from -40 to 0 kPa ； The B range is 40 kPa, with no drift, and the measurement range is 0–40 kPa ； The C range is 40 kPa, the migration amount is 40 kPa, and the measurement range is 40–80 kPa. Zero shift of pressure transmitters: yunrun.com.cn/tech/549.html. As can be seen from the graph, the input and output characteristic curves for positive and negative shifts are those without any shift, shifted along the horizontal axis that represents the input value. Positive migration moves in the positive direction, while negative migration moves in the negative direction; the distance of movement represents the amount of migration. Therefore, the essence of positive and negative migration is to adjust the transmitter in order to change the upper and lower limits of the range, while the size of the range remains unchanged. It can also be remembered this way: when the input pressure is zero (0%), the output current signal is also zero (0%), indicating no migration ； When the input pressure is zero (0%), the output current signal is positive, above the operating zero point, indicating negative shift ； When the input pressure is zero (0%), the output current signal is negative, below the operating zero point, indicating positive shift. Not all transmitters have a migration function; for example, conventional temperature transmitters rarely have such a function, and even when they do, its capabilities are very limited. The applications that are most frequently migrated are pressure and differential pressure transmitters, which is determined by the usage requirements and the structure of the primary components. However, it is very difficult to migrate differential pressure transmitters with square root functionality. Why is it necessary to perform zero drift adjustment on pressure transmitters? Changhui Instruments will first show you some practical issues that arise during production. 1. The problem of measuring the liquid level in a closed container: When using a pressure transmitter to measure the liquid level in a closed container, if the gaseous medium tends to condense, then condensate will enter the pressure guiding tube of the transmitter as well as the negative pressure chamber. This causes the liquid level in the negative pressure chamber to change, resulting in measurement errors. Therefore, in general, a balance vessel is installed in front of the conduit above the negative pressure chamber, and this vessel is filled with condensate. For measuring harmful and corrosive media, to prevent these media from entering the transmitter, an isolator also needs to be installed, and the pressure conduit must also be filled with isolation fluid. In both of the above tests, the presence of condensate will cause the signal in the negative pressure chamber of the transmitter to be higher than that in the positive pressure chamber, making it impossible to conduct the test. If the method of reversing the positive and negative pressure ports of the transmitter can be used, the transmitter’s output decreases as the liquid level rises; this can lead to misunderstandings, and it also makes connection to the control system inconvenient. To solve this problem, negative migration should be used, so that the transmitter’s zero point starts from a negative differential pressure, which will meet the usage requirements. 2. Problems when the transmitter is located below the pressure tapping point: Generally, the transmitter is installed below the pressure tapping point, and the pressure conduit is filled with condensed water. Due to the effect of the hydrostatic pressure of the water column, the output current of the transmitter includes this hydrostatic pressure; the greater the vertical distance between the pressure measurement point and the transmitter, the larger the resulting error. The question of “how to correct the error caused by the static pressure of the liquid column in the measurements of pressure transmitters” is analyzed in detail here. When measuring the liquid level in an open container, if the transmitter is installed below the container, measurement errors can also occur due to the hydrostatic pressure of the liquid column being measured, with results that are similar to those mentioned above. To solve the above problems, positive migration must be used: adjust the transmitter’s output value to zero (4 mA), which effectively removes the static pressure of the liquid column. After positive migration, the output value of the transmitter represents the actual operating pressure. 3. Improve measurement accuracy by shifting the compression range. Example 1: There is an industrial boiler with a capacity of 35 t/h, and its gas consumption is always above 25 t/h. To enhance the accuracy of flow measurement, it is possible to change the measurement range of the display instrument to 20–40 t/h; the transmitter will then compensate for this by shifting its range in a positive direction. It is known that when G=40t/h, △P=60kPa; in this case, the following formula can be used for calculation: http://yunrun.com.cn/upload/201607/05/201607051501376772.png Here, △Pmax represents the maximum differential pressure that the instrument can handle ； △P is any differential pressure ； G is any mass flow rate ； Gmax is the mass flow rate of the instrument at the upper limit of the differential pressure. According to the calculation results from the above formula: when the flow rate G=20 t/h, the corresponding differential pressure △P is 15 kPa ； At a flow rate of G=40 t/h, the corresponding differential pressure is △P=60 kPa. At this point, the range of the transmitter is reduced to 0–45 kPa; a positive offset is applied so that the transmitter’s measurement range becomes 15–60 kPa, corresponding to a flow rate of 20–40 t/h. This improves the accuracy of the measurements. Example 2: In production, the range of the temperature being measured is 500–600°C. If a temperature transmitter with a range of 0–800°C and an accuracy class of 0.5 is used, its maximum measurement error will be ±4°C. However, if positive drift is used to compress the range and a temperature transmitter with the same accuracy in the 400–800°C range is selected, the maximum measurement error becomes ±2°C, thereby improving the measurement accuracy after applying the drift. It should be noted that only some temperature transmitters have the aforementioned functions, so care should be taken when selecting them. As can be seen from the above examples, due to the wide variety of measurement parameters and equipment in production environments, the requirements for measurements vary. This necessitates that transmitters have a calibration function, allowing users to make adjustments on-site according to actual conditions, in order to meet the various measurement needs in production.

Not bad, not bad, I’ve learned it!!