Beginner's Guide to Differential Pressure Level Transmitters

The Not-So-Straightforward Basics of This Measurement Technique

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By David W. Spitzer

GIGO means "garbage in, garbage out." This phrase applies in industrial automation because using faulty measurements can fool even the best control system. One remedy that can help avoid a GIGO scenario is to understand the measurement technique and its limitations to the extent that its application can be reasonably evaluated. Differential pressure level measurement is one of those key measurements you need to understand to avoid the dreaded GIGO.

The importance of level measurement cannot be overstated. Incorrect or inappropriate measurements can cause  levels in vessels to be excessively higher or lower than their measured values. Low levels can cause pumping problems and damage the pump, while high levels can cause vessels to overflow and potentially create safety and environmental problems. Vessels operating at incorrect intermediate levels can result in poor operating conditions and affect the accounting of material.

The level of a liquid in a vessel can be measured directly or inferentially. Examples of direct level measurement include float, magnetostrictive, retracting, capacitance, radar, ultrasonic and laser level measurement technologies. Weight and differential pressure technology measure level inferentially. All have problems that can potentially affect the level measurement.

Differential pressure level measurement technology infers liquid level by measuring the pressure generated by the liquid in the vessel. For example, a water level that is 1000 millimeters above the centerline of a differential pressure transmitter diaphragm will generate a pressure of 1000 millimeters of water column (1000 mmWC) at the diaphragm. Similarly, a level of 500 millimeters will generate 500 mmWC. Calibrating this differential pressure transmitter for 0 to 1000 mmWC will allow it to measure water levels of 0 to 1000 millimeters.

Note that this example presumes that the liquid is water. Liquids with other specific gravities will generate other differential pressures and cause inaccurate measurements. Continuing with the previous example, the same 500-millimeter level of another liquid with a specific gravity of 1.10 at operating conditions in the above vessel will generate 550 mmWC of pressure at the transmitter. As such, the differential pressure transmitter calibrated for water would measure 50 millimeters higher than the actual 500 millimeter liquid level. Conversely, if the liquid has a specific gravity that is lower than that of water, this transmitter will measure lower than the actual level. This example illustrates that differential pressure technology does not measure level, but rather infers level.

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  • <p>I've always been interested about pressure vessel. I just came across this site and found this very interesting guide in differentiating pressure level transmitter. Thanks for sharing these useful measurement technique. </p>


  • <p>God reward you for sharing this piece of knowledge...</p>


  • <p>Please tell me how to calibrate transmiiter for boiler durm level transmit</p>


  • <p>i facing problem with calibrate range for application closed tank under vacuum zero level is 840mm and span is 2840 from transmitter and vacuum not constant from 0 to 277mbar what can i do for find correct level note this system connected to plc and i think must be used equation for correct the level value according the pressure </p>


  • <p>how is the calibration of differential pressure transmitter for level measurement different from that of flow measurement?</p>


  • <p>can someone explain me that why in last example the specific gravity is changed from 1.10 at the higher end to 1.05 at the lower end? And also how 3 bar is taken as pressure,why not the any other value for it? </p>


  • <p>5800 mm tank of napatha differantial level range calculations</p>


  • <p>pls tell me the calculation range for the tank of napatha 0-5800 mm?</p>


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