Capacitance gauges prove their mettle in flat-film extrusion.

While capacitance gauges have long been available for noncontact film-thickness measurement, they've suffered from a poor reputation for accuracy and stability--until now. Equipment supplier Solvetech Inc. of Claymont, Del., says it has overcome earlier limitations and even installed noncontacting capacitance gauges in several sophisticated flat-film operations.

"In some cases, customers who had previously rejected capacitance technology have become strong advocates," says Solvetech president Doug Lawrence. These users demanded faster resPonse times and fuller product coverage than nuclear gauges could provide at the same cost, Lawrence explains. What's more, these customers have taken capacitance gauges beyond their traditional thickness-measurement role by using the gauge data to diagnose the causes of film variability.

As performed by the high-tech film producers, this predictive-maintenance strategy ultimately pinpoints the sources of machine-induced gauge variability through an analysis of the gauge's high-frequency signal. "We use the information to look at the components of machine-direction variability caused by process equipment," explains Dick Hughbanks, research associate at a DuPont Co. flat-film facility in South Carolina.

According to Lawrence, capacitance gauges "excel as a process troubleshooting tool" because they provide more data points than nuclear gauges-in part because they don't need to scan across moving webs, but "see" an entire cross-section of the web surface at once. The noncontacting capacitance technology works by relating web thickness to the strength of an electric field that the gauge creates around the film.

While Solvetech makes a variety of gauge models, Lawrence says the company's multichannel unit offers the best coverage. "It sees every point across the web, so you can validate the whole thing," he says. "You get the same coverage as if you parked multiple beta-gauges across a line." DuPont, meanwhile, makes use of a special single-channel gauge tailored specifically to film-variability measurement. One channel is sufficient, assuming that the machine-generated variability stretches across the web (see schematic).

One key to Solvetech's gauging approach is a high-frequency signal, which offers response times short enough to see MD changes on webs moving as fast as 1000 ft/min, Lawrence says. The gauges have a frequency response of 100 Hz, which translates to 100 data points per second per channel. A 10-channel gauge would thus measure 1000 points/sec.

FILM STUDIES

To use the gauge as a troubleshooting tool, the data are fed into a spectral analyzer via an RS232 port for an on-line Fast Fourier Transform (FFT) analysis. Lawrence explains that the FFT can help users correlate even brief gauge variations to process conditions at a given point in time. For slowline speeds, Lawrence says, PC-based software can perform the FFT analysis off-line at a lower cost.

In a typical extrusion operation, the frequency analysis can isolate the effects of individual equipment variables--roll speed or tension for example. DuPont has also used the approach to correct problems relating to gear drives and vibration that would otherwise have been identified only through use of specialized equipment. "Frequency analysis has replaced other analysis techniques," Hughbanks says. And it has resulted in improvement in how the drives are specified, he adds.

To be sure, nuclear gauge manufacturers can also offer FFT analysis (see PT, Aug '93, p.46) . But Lawrence notes that the non-scanning capacitance gauges have a shorter response time, allowing them to pick up more fleeting defects. "It's tough to do variability testing with a beta gauge at production speeds because it doesn't see enough points on web."

"The difference in response times is key," agrees Hughbanks, pegging the nuclear gauges he uses at around 2-3 Hz and capacitance devices at a minimum of 50 Hz. "It fills in areas we were unable to look at before." Nuclear gauges, meanwhile, still see use at DuPont for looking at long-wavelength and TD variations.

FILM REPUTE

In going after markets normally served by nuclear gauges, Solvetech has had to battle a perception that capacitance gauges drift too easily because of environmental factors such as temperature and humidity. "Capacitance has had a bad name in terms of accuracy and stability," admits Lawrence. The company claims to have lessened the impact of these weaknesses with a host of proprietary improvements, including ones to the thermal insulation, gauge-head electronics, and calibration techniques.

Lawrence describes the variability studies as a "side benefit" of capacitance gauges. Solvetech's high-tech customers--including several stretch-film producers--started to use the gauge signal as a predictive maintenance tool only after buying the gauge for its full-coverage capabilities.

Because Solvetech's multichannel capacitance gauge is mounted on a C-frame, it does have some web-width limitations. The largest unit fits webs only up to its maximum throat depth of 36 in.--or 72 in. when using two units together.

As for accuracy, the company typically promises between 0.1% and 1.0% of film thickness. (CIRCLE 32)