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.

Three Calculations

All is not lost because the calibration of the differential pressure transmitter can be modified to compensate for a different specific gravity. This technique used to calculate the new calibration is useful for both straightforward and more complex installations.

Figure 1 shows the vessel both at 0% and 100% level. The pressure generated by the liquid at the level transmitter diaphragm is the liquid height times the specific gravity. The pressure is 1.10*(0 mm) when the vessel at 0% and 1.10*(1000 mm) when the vessel at 100%. Therefore, the transmitter should be calibrated 0 to 1100 mmWC to measure liquid levels of 0 mm to 1000 mm.

A somewhat more complex application is illustrated in Figure 2. In this application, for process reasons, we need to take the measurement from 200 mm to 1000 mm above the nozzle. In addition, the transmitter is located 500 mm below the nozzle. Note that the technique of sketching conditions at both 0% and 100% level is the same as performed in Figure 1. At 0% level, the pressure at the transmitter is 1.10*(500 +200 mm), or 770 mmWC. At 100% level, the pressure at the transmitter is 1.10*(500+1000 mm) or 1650 mmWC. Therefore, the transmitter should be calibrated 770 to 1650 mmWC to measure liquid levels of 200 mm to 1000 mm above the nozzle.

Figure 3 illustrates the use of a differential pressure transmitter with diaphragm seals to sense the pressures at the nozzles in a pressurized vessel. In this application, the low-pressure diaphragm is located above the liquid to compensate for the static pressure in the vessel. Other complications include the densities of liquid and capillary fill fluid and 0% and 100% levels that do not correspond to the nozzle positions.

Using similar techniques as in the previous examples, at 0% level, the pressures at the high and low sides of the transmitter are {1.10*(200 mm) + (3 bar)} and {1.05*(1300 mm) + (3 bar)} respectively. Therefore, the differential pressure transmitter will subtract the high side from the low side and measure {1.10*(200 mm) + (3 bar)} minus {1.05*(1300 mm) + (3 bar)}, or -1145 mmWC.

At 100% level, the pressures at the high and low sides of the transmitter are {1.10*(1000 mm) + (3 bar)} and {1.05*(1300 mm) + (3 bar)} respectively. Similarly, the differential pressure transmitter subtracts the high side from the low side to measure {1.10*(1000 mm) + (3 bar)} minus {1.05*(1300 mm) + (3 bar)}, or -265 mmWC. Therefore, the transmitter should be calibrated -1145 mmWC to -265 mmWC to measure liquid levels of 200 to 1000 millimeters above the lower nozzle.

Note that the static pressure in the vessel does not affect the calibration because it appears on both sides of the differential pressure transmitter where it effectively cancels out. Further analysis also will reveal that locating the differential pressure transmitter at different elevations does not affect the calibration.

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