Issue of mismatch between the actual liquid level and the reading on the gauge after differential pressure level measurement range adjustment
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I have a double-flange level gauge with a measurement range of 0 to 800 mm. When it isn’t installed, it shows 0 mm; once installed, it shows -680 mm. I adjusted its measurement range to -680 to 120 mm so that 4 mA is output when there is no liquid level. However, the actual liquid level does not correspond to the display on the transmitter’s LCD. How can this be resolved? (The gauge displays -680mm when there is no liquid level.)
As a supplementary note, the transmitter is from Rosemount.
Problems like yours generally don’t occur; for a newly installed watch, one needs to wait until the capillary silicone oil stabilizes before proceeding with negative migration. When the negative value is set to 0mm, the output will be 4mA; otherwise, check the output module.
The gauge head does not match the actual liquid level; the range of the gauge head needs to be reset.
If the range is changed, it will result in the actual liquid level not corresponding to the output current. The user doesn’t want a percentage display; they insist on seeing the value in engineering units.
For Rosemount transmitters, when you place the two flanges at the same height and the transmitter displays 0, it means that the transmitter has not yet been calibrated. Transmitters generally display the pressure difference or a percentage; it’s quite rare to see them show the liquid level in millimeters. The migration value is calculated as PGH, which is the density of silicone oil multiplied by gravity acceleration, then multiplied by the range. For example, 0.95*9.8*0.8 = 7.448 kPa. In the case of negative migration, the original range was likely from 0 kPa to PGH (assuming water is being measured, with a density of 1). The value for this scenario is the density of the liquid multiplied by gravity acceleration, again multiplied by the range: 1*9.8*0.8 = 7.84 kPa. So the original transmitter’s range was 0–7.84 kPa. What needs to be changed are these upper and lower limits: the negative migration value of 7.448 kPa should be changed to -7.448 kPa, while the upper limit should remain at 0.392 kPa. The liquid level range should stay at 0–800 mm; this doesn’t change. What’s needed is to adjust the values related to negative migration. Use the transmitter to check what the upper and lower kPa limits of the transmitter are Overall decrease of 7.448 kPa
The range is the differential pressure value, which is calculated based on the density of the medium being measured. Are instruments these days so intelligent that they can be adjusted directly?
Rosemount can have the manufacturer perform the calculation based on the medium density at the time of production, allowing the height to be displayed directly in mm.
It showed 0 at the same height before installation, and the migration was carried out only after negative pressure was detected after installation.
It is recommended that you recalculate based on the actual medium density and the pressure measurement spacing, or you can provide those values so that others can help with the calculation.