Gauge Your Level Instrumentation Options

The pharmaceutical industry is not unlike other industries in its quest for better performing and more reliable instrumentation. And in the specific case of continuous level measurement, drugmakers increasingly are choosing fully solid-state, non-mechanical devices that deliver higher accuracy, need less frequent calibration and are installed in such a way that an instrument failure won't compromise process uptime or derail a batch that's in process.

At Genentech's South San Francisco, Calif., manufacturing site, hundreds of tanks and dozens of fermentation trains produce oncology drugs, vascular medicines and other specialty biotherapeutics. Metrologist Steve Ward has led the charge toward standardizing on two relatively recently developed technologies for measuring liquid level,magnetostrictive and guided-wave (probe-based) radar.

In both cases, high-accuracy, long-term stability and,perhaps most importantly,the ability to remove the probes from the top of the tank for calibration or replacement were deciding factors.

"Our processes run anywhere from 10 to 30 days," Ward says. "If we lose a batch in process, it's a lot of money down the drain."

An added bonus Ward has seen is in the area of calibration. Genentech devised an off-line calibration bench equipped with laser and target for verifying the accuracy of its radar and magnetostrictive probes.

"We can pull the probes and calibrate them, even while the process is running," Ward explains. In the past, the load cells used to measure tank contents had to be calibrated by filling the tanks with purified water run through a Coriolis mass flowmeter for reference.

"The procedure was time-consuming and purified water is very expensive. Plus, once it's been used, it has to be treated," Ward says. "Now we've eliminated routine water calibrations. We still go back to verify,but now it's every three or four years rather than every six to 12 months.

"We've tried almost everything," he concludes, "and we've settled on these two technologies because of increasing demands for higher accuracy and mounting pressure not to shut down."

At other pharmaceutical sites, however, level measurement is deemed so sufficiently problematic that pressure-based systems and load cells are relied on to infer rather than directly measure liquid level.

Abbott Bioresearch, Worcester, Mass., is ramping up production of the company's newly approved human anticlonal antibody Humira, prior to spinning out production to other manufacturing sites. Mark Edlinger, metrology manager, prefers a combination of load cells and pressure-based instrumentation to his other level instrumentation options. He has even used a top-mount, probe-style device with a pressure sensor mounted at the bottom of the probe to overcome the potential liabilities of bottom-mount pressure transmitters in some applications. Similar to other top-mount probes, it can be readily pulled and calibrated on a spool piece.

"I'm a strong proponent of pressure," he says, citing application-specific issues with capacitance probes and non-contact radar technologies. "Our biggest challenges are foaming and agitation."

Call it an axiom of applied instrumentation technology: The number of options available to measure a given process variable is directly proportional to the difficulty of that measurement. And while measuring liquid level in a tank is conceptually simple, the types of surfaces and interfaces encountered in industrial processes vary so widely that over the years more than 20 fundamentally different types of instruments have been developed and applied.

For the most part, when a continuous process level measurement is needed, the choice today falls to five major classes of level measurement devices: capacitance/radio frequency (RF) admittance, radar, ultrasonic, magnetostrictive and nuclear. Two approaches that don't strictly measure level at all,pressure transmitters and load cells,are also often used. If a point-level switch is needed rather than a continuous measurement, this opens the options up to another handful of alternative technologies. (See sidebar, "When a Level Switch Will Do.")

And while there are few absolute application rules, there are some basic "cost of entry" parameters that any level instrument used in a sanitary process must meet,excepting, of course, load cells and nuclear gauges that to their credit do not contact the process directly.

"They have to be made of process compatible materials. They have to be designed and manufactured such that there are no surface blemishes or crevices where bacteria can harbor. And they also need to be easy to clean, easy to sanitize, easy to inspect," explains Boyce Carsella, product manager, new technologies, for instrumentation supplier Magnetrol (Downers Grove, Ill.).

"In the end, though, sanitary is a user-defined parameter," Carsella adds. "It all comes down to what the user needs."

"The main difference between sanitary and general process instrumentation is the connection. It has to be suitable for clean-in-place (CIP) or easily taken apart," adds Jonathon Rowe, marketing manager, pressure and level, Invensys Foxboro (Foxboro, Mass.). In North America, Tri-Clamp connectors are used extensively with a variety of gasket materials, depending on the application. Fill fluid, in the case of remotely sealed pressure transmitters, must also be process compatible.