Cartiere del Garda ( www.gardacartiere.it) specializes in high-quality coated papers. According to Instrument Manager Fulvio Prati, for many years a Micro Motion Coriolis flowmeter from Austin, Texas automation supplier Emerson Process Management ( www.emersonprocess.com) reliably and accurately monitored the concentration of the paper coating mixture. However, because the recirculation system on the feeding tank generated entrained air, it was necessary to add expensive additives to the coating mixture to prevent quality problems.

Removing the additives would reduce production costs, but would cause up to 20 percent air to be entrained in the coating mixture. Since the existing Coriolis flowmeter was not designed to provide the same degree of density measurement accuracy under these conditions, Cartiere del Garda sought an alternative. What they chose was an on-line, solids concentration measurement of the coating solution, made directly at the film coater machine feed.

A minimum accuracy of +/-1 percent of solids was required to guarantee the high quality of the paper being produced and to avoid waste caused by out-of-specification batches, said Prati. A Micro Motion 7845 straight tube, liquid density meter now provides highly accurate density measurements under entrained gas conditions.

The 7845 delivers accuracy of better than +/- 0.5 percent with very high stability and repeatability, says Prati. A 4-20 mA signal to the control system, related to line density, allows density to be converted into percentage solids using a formula embedded in the control system, he explains.