New Smart Pressure Transducers Provide Networkability, Flexibility

By Dale Beardsley

One of the most important applications in the municipal water/wastewater industry is the determination of liquid levels in tanks, as well as in wastewater wet wells, reservoirs, and lakes. A variety of technologies have been employed for level measurement in water/wastewater applications, including radar, ultrasound, and resistance-tape.

One of the fastest-growing methods for level determination, however, is hydrostatic tank gauging (HTG). Hydrostatic level determination is based on using pressure measurements as a function of depth. In a typical set-up, three pressure transducers are used: one at or near the bottom of the tank, another at some level (at least one meter) above the first, and a third above the liquid level. Comparison of the pressure readouts from the two submerged transducers is used to calculate the liquid's specific gravity. The liquid level can be determined by comparing readouts from the transducer above the liquid (which measures atmospheric pressure or the vapor pressure in a closed tank) and the transducer at the bottom.

Series 510 smart KPSI™ hydrostatic level transducer with a Modbus-RTU framing format.

The growth of hydrostatic-based measurement in water/wastewater applications has been fueled by both its relatively low cost and its ease of installation. There has also been rapid growth of real-time networking for the same facilities. Along with their counterparts in many other industries, municipal water/wastewater managers have realized the benefits provided by a real-time network that links information from sensors with monitoring and control capabilities.

The implementation of real-time networks into municipal water/wastewater systems has been facilitated by the availability of fully integrated tank control units that feature networkable submersible transducers. These units provide managers with the capability to assess current status of all the elements of a municipal water/wastewater system, and to manage the system, depending on changing needs, on a real-time basis.

"Smart" Pressure Transducers

The rapid adoption of real-time networks into municipal water/wastewater management has in turn created a need for more sophisticated pressure instrumentation capable of supporting such networks. A new generation of "smart" pressure transducers is now reaching the market, ready to be incorporated into new or existing networks.

Smart Pressure Transducers: An Illustration

A good example of one of the new generation of smart pressure transducers for water/wastewater is the KPSItrademark Series 510, produced by Pressure Systems, Inc. The Series 510 transducer is designed specifically for remote environmental monitoring, including not only tank gauging applications, but also well monitoring and dewatering, level monitoring, and ground and surface water monitoring.

Typical HTG system employing an integrated tank control unit.

The Series 510 transducer incorporates a microprocessor that uses the Modbus protocol (RS-485 command/response). The Modbus protocol is widely used in the tank gauging market for both water/wastewater and petrochemical applications. The transducers communicate using the Modbus-ASCII or RTU framing formats, operating at a 1200, 2400, or 4800 baud rate. The inclusion of Modbus protocol enables the transducers to communicate with any Modbus-based PLCs and/or control networks. This enables online data acquisition and programming, managed from the tank control unit or centralized management facility.

The Series 510 has been designed to minimize power needs: its current draw is only 10 mA, and it can accommodate unregulated 12 vDC power, allowing a supply voltage of 6 to 28 vDC. This enables it to be battery-powered; moreover, with such a low power requirement, several sensors can be linked to a single recorder through one cable. The Series 510 can support operation of up to 24 devices through a single IS barrier on a 1,000 foot cable.

The microprocessor in the transducer, in addition to managing the network interface, implements sophisticated compensation algorithms to correct the transducer's output for temperature, hysteresis, non-repeatability, and non-linearity. It also manages an optional sleep mode, enabling the transducer to conserve power using lowest power mode. With the built-in compensation algorithms, the Series 510 provides reproducibility and full-scale accuracy to ± 0.10% TEB (total error band).

The transducers are compliant with Class 1, Div. 1, and are designed for maintenance-free operation. The compact transducer (12 oz.) has an outer package of welded 316 stainless steel or titanium, and it incorporates a unique cable seal system that prevents accidental cuts to the cable jacket and eliminates errors resulting from cable elongation.

The Series 510 transducer is also available with optional lightning/surge protection, featuring a barrier to protect the power source and a separate barrier located inside the unit itself. A solid-state section intercepts the leading edge of the surge within nanoseconds. A gas discharge tube then "crowbars" up to 20,000-ampere currents to ground. The tube remains in the crowbar state until the surge has passed, then automatically resets the line to normal operation without the need to reset a circuit breaker. This protection feature is strongly recommended, especially for transducers installed in elevated storage tanks, or those located in regions that are hilly or prone to thunderstorms.

About the Author

Dale Beardsley is Sales Manager for Pressure Systems Inc.