The positive and negative drift issue of differential pressure transmitters
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“ Positive and negative drift: After the zero point undergoes positive or negative drift, the absolute values of the upper and lower limits of the measurement range shall not exceed 100% of the upper limit of that measurement range. (Intelligent type: range ratio of 15:1) The maximum positive drift is 500% of the minimum calibrated range ； The maximum negative migration is 600% of the minimum calibration range. ”How should this sentence be understood? For example: I currently have a liquid level. When the liquid level is 0, a value of 375 can be used to detect that the negative pressure at the transmitter is 184.40 Kpa. The transmitter shows a liquid level of 80%. After I adjusted the zero point to 184.40, the transmitter outputted 4 mA. My transmitter’s range is 0–226 KPa; when the tank is full, the pressure of the medium is 226 KPa. So, after adjusting the zero point, what will be the transmitter’s output when the tank is full? Is it 20mA? What has this range become? Is it -184.40~41.60?
The last edit to this post was made by zzzi on 2010-11-10 at 15:17. It would be simple enough to change the range directly to –184～42 KPa; there’s no need for such complexity. The transmitter shows a liquid level of 80%, and after I adjusted the zero point to 184.40, the transmitter outputted 4 mA. Why do we need to do this? ! 4MA is output only when the liquid level is empty; how can it be transferred when there is a liquid level present?
The actual liquid level before migration was 0; therefore, a reading of 80% at that time was an incorrect value. Only after migration did the correct level of 0 appear. I’m not sure whether it’s my description that is incorrect or your understanding that is off.
The range is the upper and lower limits of measurement; migration involves changing both the upper and lower limits simultaneously, while the range remains unchanged. If the transmitter’s measurement range is 0~226 kPa and its full scale is 226 kPa, after migration the measurement range becomes -100 kPa~126 kPa, while the full scale remains 226 kPa ； The migration due to liquid level changes the measurement range of the actual liquid level. Without level measurement, there is a negative pressure of 184.40 kPa, while the transmitter’s measurement range is 0–226 kPa; this indicates that a negative pressure exists in the negative diaphragm chamber, meaning vacuuming has occurred. It is incorrect to perform migration at this time.
When the liquid level is empty, if the transmitter gauge is installed below the pressure tapping on the negative pressure side! So the method used by the original poster to adjust the range is correct! The poster’s approach is negative transfer! The core of migration is: changing the zero point of measurement, but the overall measurement range remains unchanged!
Replying to floor 5: The range of negative migration volume has not changed. I understand. Thank you