Differential pressure transducers, where the difference in pressure between two points is measured, are one option for liquid level measurement in sealed tanks, for both hazardous and non-hazardous applications. Since the measurement is based on the physical force of a liquid on a diaphragm, there is less chance for error in various applications. Differential pressure transducers have the advantage over top-mounted transmitters in sealed tanks in applications where the fluid is reflective, has foam, or is clear.
Using linear variable differential transformer (LVDT) technology, differential pressure transducers can operate in extremely cold ambient conditions (-50 deg C) without harm or damage to the device. An LVDT is a common type of electromechanical transducer that can convert the rectilinear motion of an object to which it is coupled mechanically into a corresponding electrical signal. LVDT linear position sensors that are readily available can measure movements as small as a few millionths of an inch.
When designed with extremely high forward proof pressure (1,500 psi) and burst pressure (5,000 psi), differential pressure transducers can monitor liquid or gas level in storage tanks without fear of rupture. A wide range of explosion-proof and non-incendiary approvals are available for Class I, II, and III environments.
Using the latest technologies in electronics, differential pressure transducers can achieve low current consumption. This allows remote telemetry units (RTUs) and machine-to-machine (M2M) and SCADA (supervisory control and data acquisition) system integrators to apply additional transducers on the same solar circuit or minimize battery maintenance and replacement.
Today’s advanced differential pressure transducers use a digital ASIC (application-specific integrated circuit) in order to compensate for and minimize inaccuracies over pressure and temperature. Through custom programming and measuring the performance of the transmitter using highly accurate pressurization systems in temperature chambers, the devices become even more accurate. Most important, the differential pressure transducer does not employ any fluid-filled cavities or O-rings.
Typical differential pressure transducer technologies use an oil-filled capsule with a corrugated diaphragm to measure differential pressure of liquids and gases. Otherwise, the diaphragm can be mounted to a fitting using an O-ring to seal. In cases where freezing or high overload conditions occur, these technologies have the potential failure mode of diaphragm rupture and leakage of the fluid fill into the process media.
New advances eliminate the need for fluid-filled cavities because there is no need to transfer the pressure through a fluid-filled cavity. The LVDT measures the difference in pressure between P1 and P2 through the movement of the diaphragm and thus the LVDT connecting rod. Today, a thick metal housing and diaphragm can be implemented to withstand high proof and burst pressures.
The high or upstream process connection is installed at the bottom of the tank with the low or downstream side installed with quarter-inch tubing (minimum) connected to the top of the tank. The larger the diameter of the pipe, the lower the potential of a capillary effect from moisture buildup.
Certain applications use pressure or vacuum pressure above the liquid; in these cases the pressurized line can be connected directly to the low side of differential pressure transducers. In applications where liquid from the tank or condensation might build up in the low line, a drip leg and drain valve are recommended to maintain accurate level measurement.
The drip leg allows incidental condensation to accumulate and re-evaporate without affecting the transducer readings. The drain valve allows for quick maintenance and draining of the line without sensor removal, saving time and money in the long run. By understanding the maximum height of the liquid and its specific gravity, measuring liquid level becomes simple and cost effective.
Greg Montrose is marketing manager at American Sensor Technologies, a manufacturer of pressure sensors and related instrumentation through standard and custom product designs, based in Mount Olive, N.J. American Sensor Technologies makes sensors, transducers, and transmitters for measurement of hydraulic pressure, hydrogen pressure, and tank level, among other applications. The company’s AST5300 series of differential pressure transducers employ LVDT technology, which enables measurement of movements as small as a few millionths of an inch. For more information, visit www.astsensors.com.