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.
These same techniques can be used to determine the calibrations for interface level measurements. Note that these techniques involve applying hydraulics to the installation and application. Nowhere do we use terms such as elevation, suppression and span. The use of these terms can easily confuse and mislead the practitioner.
What if the liquid density changes during operation? What if the change is due to changes in the composition of the liquid? What if the change is due to temperature changes? What if the vessel is filled with a different liquid that has a different specific gravity? These are important questions that should be asked (and answered) when considering the use of differential pressure level measurement instruments. Repeating, differential pressure measurement does not measure liquid level—it infers liquid level—so specific gravity changes can affect the performance of the level measurement. In practice, the specific gravity of many liquids is known and relatively stable, so that differential pressure techniques are commonly applied to many liquid level measurement applications.
The differential pressure transmitter should be operated within its published specifications to maintain accuracy. The span of a transmitter is the difference between the 100% and 0% calibration values. Differential pressure transmitters have specified minimum and maximum spans. For example, a given differential pressure transmitter may be calibrated with spans between (say) 400 mmWC and 4000 mmWC. In addition, the transmitter zero may also be raised or lowered by up to, for example, 4000 mmWC. Calibrations that do not meet the transmitter specifications are potentially subject to significant error. The calibrations in the examples were 0 to 1100, 770 to 1650, and -1145 to -265 mmWC, respectively. Each has a span greater than 400 mmWC and less than 4000 mmWC. In addition, their zeros are not raised or lowered by more than 4000 mmWC. Therefore, all of these calibrations are within the transmitter specifications.
However, the calibrated span specified for another transmitter model of the same manufacture may be between 100 mmWC and 1000 mmWC, and allow the zero to be changed by 1000 mmWC. This transmitter would not be applicable to the first and third examples where the span is 1100 mmWC, and the zero is lowered by 1145 mmWC, respectively. However, it could be used in the second example where the span is 880 mmWC, and the zero is raised by 770 mmWC. Using this lower range transmitter (1000 mmWC) will usually be more accurate because of the smaller absolute errors associated with other specifications such as temperature, pressure and ambient temperature affects. Therefore, all being equal, it's generally desirable to use the lower range transmitter to reduce measurement error.
The maximum flow rate of flowmeters is often specified to be significantly higher than the design flow rate to allow for transients and increased plant throughput over time. In level measurement, the vessel size is fixed, so using a higher range differential pressure transmitter provides no similar benefit and typically results in additional measurement error that can be avoided by using a lower range transmitter.
Using the available information properly is another potential problem. Some years ago, distributed control system inputs were incorrectly configured to correspond to the maximum transmitter spans. Aside from using incorrect values, the levels should have been expressed in percent. Using absolute level measurement units such as inches, feet, millimeters or meters increases the potential for error because operators must remember the height of each vessel to put the level measurement in context with the vessel. This can easily become overwhelming and cause operator errors because plants often have hundreds of vessels. For example, a vessel operating at 2.8 meters does not readily indicate a problem to the operator even though the vessel overflows at 3.0 meters. On the other hand, the operator can easily determine that a vessel operating at 93% level might warrant attention and that a vessel operating at 97% may need immediate attention.
Differential pressure measurement is a workhorse of industrial level measurement that's been used for decades and will continue to be used for decades to come.
David W. Spitzer is a principal in Spitzer and Boyes, LLC and a regular Control contributor.