The main benefits of using ultrasonic technology, as it exists today, over radar technology in the measurement of open channel flow monitoring (OCM) are in three areas:
The initial purchase of the level instrumentation is only one cost to consider. Some of the other real costs may not be necessarily obvious at first glance. You also want to consider the instrumentation that you’re purchasing. For example, radar units that can be purchased at prices competitive to ultrasonic instrumentation will most likely be loop-powered units. These radar transmitters have rod-style antennas with large beam angles. Are these bad instruments? Not, at all! In fact, these radar units can be very beneficial to your process but they may be prohibited for outdoor and uncovered use by the FCC. If this is the case, you would not be able to use your radar unit in this application; you would need to go with an ultrasonic unit or a higher priced radar unit that did comply with FCC guidelines.
If you choose to go this route, keep in mind that while the loop-powered configuration is often desirable in other applications, it’s important to consider what comes with an ultrasonic controller:
For the radar unit, which will probably not have a controller, you will need to have your PLC expert program the PLC for the correct head versus flow calculations based on the primary device being used. The programmer will even set alarms, engage the logging functions, set up a totalizer and supply an HMI screen showing the different process variables. Quick work for an experienced PLC programmer but it’s much easier for an instrument technician to program the ultrasonic controller.
Do you want to change the totalizer resolution? Do you need to reset the max flow, alarm set points, mounting configuration of the transducer and/or move the entire level system to a new OCM application? Or perhaps want to check the calibration and, if necessary, perform the annual state-mandated calibration procedure? Quick work for an experienced PLC programmer but it’s quicker for an instrument technician to program the ultrasonic controller.
The ultrasonic system also allows for potential savings in the event that “acts of nature” decides to wreak havoc at your location. The ultrasonic controller would most likely be located remotely (up to 1200ft away from some systems) from the transducer, which would be mounted over the flow stream. Now, we are talking about a $400 – $500 replacement cost for the transducer versus a $1,000 – $1,500 cost to replace the entire radar unit.
It is true that the cost of radar instrumentation has been coming down, but that doesn’t necessarily favor their use in OCM applications. The radar gauges that can be purchased in the range of $1,000 to $1,500 are most likely going to be low frequency, e.g. 6 GHz range, which results in stated accuracies of +/- 0.4” with some stating that the accuracy would actually be +/-1.57” when the material level is within 24”.
For many applications this would be just fine and the attributes that make radar level measurement desirable would make it economically advantageous as well.
BUT NOT IN OCM APPLICATIONS! Imagine what an inaccuracy of 0.4” could amount to over the course of a month when you report to the state entities, or charge other municipalities for treating their waste.
Let’s face it…a signal travelling at 1,129 ft/sec and one travelling at 186,000 miles/sec will require different processing, and the radar processing is going to be more involved and, therefore, require longer processing times. Radar units that use pulse Time-of-Flight (TOF) processing are faster than those that use the Frequency Modulated Continuous Wave (FMCW) processing technology. But, even TOF technology is less responsive in radar than that same processing technology is in ultrasonic level measurement.
In a rapidly changing flow environment, this responsiveness can be crucial to accuracy for obvious reasons. The larger the flow rates expected, the larger that inaccuracies caused from a lack of responsiveness are also to be expected. These inaccuracies will be imbedded in the totalizer outputs, and may not be obvious until someone is called upon to justify those totals.
In summary, I believe there are opportunities for radar level measurement in the environmental industry, but I don’t recommend their use in OCM applications.