Leveling the Playing Field
Considerations when specifying a level tranducer
- By Dale Beardsley
- May 01, 2002
Looking for a level transducer and unsure about what specifications are important? Trying to figure out exactly which type of level instrument to invest in that will not only save money, but also offer performance, reliability and longevity? This article explains which standard features, accessories and options to consider when looking for the most reliable and cost-effective level transducer on the market.
Level transducers are installed and used to measure water levels in a multitude of applications, including, but not limited to wastewater wet wells, elevated storage tanks and monitoring leachate, reservoirs and lakes miles from civilization. Knowing the basics about transducers and exactly what they do is extremely helpful when working them into a specific application. Also, because so many applications are remote, choosing a transducer that is reliable and won't require much maintenance is also important. Some guidelines to follow when specifying the most appropriate transducer for a unique application include rugged packaging, surge/lightning protection, aneroid bellows, signal conditioning, a traceable calibration card and custom configuration.
First and foremost, the transducer must able to handle harsh environments. Because the instrument will undoubtedly be exposed to severe conditions throughout its lifetime, rugged packaging helps to prolong the life of the transducer by keeping it safe from the elements. Also, be certain that the transducer has fully encapsulated electronics to better protect the unit from temperature extremes and moisture incursion.
For on-board ships, chemical/petroleum or clean water treatment applications, choose a submersible or above-ground transducer that can withstand the rigors of salt (or brackish) water, a corrosive environment or chlorinated water. Corrosion of or dents to the transducer's diaphragm are major causes of premature failure; buying transducers with all-welded housings made of 316 stainless steel will help prevent this. Although stainless steel addresses most application needs, for really corrosive or tough environments, a transducer with titanium-wetted parts may be considered.
Whether the application requires a submersible or above ground transducer, it is strongly recommended that a unit with optional surge/lightning protection be considered. This is the only way to effectively guard a transducer against fast rising voltage transients or severe current surges associated with lightning strikes and power surges. Buying a transducer that features this option is especially important for applications that are prone to such problems, such as transducers mounted on an elevated storage tank or located in hilly regions.
One example is the optional surge/lightning protection from Pressure Systems Inc. for units with 4-20 milliampere (mA) or 0-5 volts direct current (VDC) outputs. A barrier is provided to protect the power source; another barrier is located inside the unit itself. A solid state section intercepts the leading edge of the surge within nanoseconds. Then, a gas discharge tube "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. Be sure to choose a manufacturer that backs up their surge protection option with a lifetime warranty.
To provide moisture protection for submersible transducers, consider using an Aneroid Bellows in conjunction with vented gage units as an alternative to using desiccant-filled vent filters.
Here's how the vent filter works: as the desiccant absorbs moisture, the blue granules slowly change color until they eventually turn pink. Once that happens, the filtering process stops and moisture can then enter the vent tube. This creates a moisture travel path to the sensor; in time, this renders the transducer useless.
An Aneroid Bellows, on the other hand, can be used to prevent moisture from entering and condensing the vent tube. Extremely sensitive to atmospheric pressure, the maintenance-free Aneroid Bellows provides immediate response to pressure changes, ensuring reliable and accurate liquid level measurement. This field-installable device connects to the cable vent tube via a flexible extension provided with the bellows, without the need to perform the periodic maintenance of replacing desiccant required by a vent tube. Keep in mind, however, that an Aneroid Bellows works best with pressure ranges in excess of two pounds per square inch (psi) and with accuracies of ±0.25 to one percent.
When an output from the level transducer of 4-20 mA or 0-5 VDC is required (most level monitoring and control systems in existence are configured to accept this output), the sensor can be combined with a signal conditioning circuit. This circuit typically (1) regulates the incoming voltage and generates stable voltage(s) upon which the circuit operates, (2) provides precision sensor excitation, (3) contains an instrumentation amplifier which conditions the millivolt output of the sensor to the desired format and (4) includes null and gain adjustments for the purpose of setting the zero pressure and full scale outputs to the required setpoints. The process through which the sensor and conditioning circuit are calibrated requires a complete pressure and temperature test system, capable of generating precision test pressures to within ±0.02 percent of full scale and holding test temperatures to within ±0.1 degrees Celsius.
Traceable Calibration Card
It's best to purchase a transducer that comes with a traceable calibration card to allow the user to verify the unit's accuracy. The card specifies input/output conditions, specifies actual data reflecting the unit's static accuracy and thermal characteristics and is National Institute of Standards and Technology (NIST) traceable. Typically, the difference between the room temperature best fit straight line (BFSL) output from the actual output is calculated at each measurand (pressure point) and records the difference as a percent of full scale. The results are then printed on the calibration card.
Look for a company that offers custom configurations, where any pressure range is considered standard and a variety of analog outputs are offered.
Additional factors to consider when specifying a level transducer: Has it been 100 percent computer-tested, calibrated and serialized? Is it fully temperature compensated? Is there a choice of excitation and output? What type of cable is offered? Is it datalogger compatible? Does the device offer high static accuracy and repeatability? Do you really have time to build in four to six weeks lead-time into your project/installation, or should you consider specifying a transducer from a manufacturer that offers products in days, not weeks?
Specifying the correct transducer for your application is extremely important. Extensive research before making such a purchase is recommended. By following the basic guidelines listed above, finding the most reliable and cost-effective level transducer on the market should be a little easier.
This article originally appeared in the 05/01/2002 issue of Environmental Protection.