The Ask Control Engineering blog covers all aspects of automation, including motors, drives, sensors, motion control, machine control and embedded systems. Control Engineering answers questions from readers of Control Engineering's print and online magazines, newsletters and other publications.

Is it water or orange juice?

November 12, 2010

Dear Control Engineering: I am working in the process to automate a pasteurizer. I am looking for a sensor technology to detect the change from water to orange juice, and from orange juice to water in the output process. It has a color sensor, but it doesn't detect the change all that well. Do you know a better sensor? Thank you. –Erick

When looking at a question like this, we have to think about how water is different from orange juice, and then which of those differences is the easiest to detect or quantify. Apparently color, the most obvious choice, isn’t good enough, so what are some of the other possibilities? Orange juice is more acidic and has more sugar in it, so those might be more practical to detect. I asked Jim Gray, application manager at Rosemount Analytical Instruments for some suggestions. He says, “I think conductivity using a toroidal sensor would be the way to go. That would be the easiest from a maintenance standpoint. The conductivity of OJ should be greater than the conductivity of water, although I don’t have any number handy. The problem with pH would likely be the sensor drying out when the process is down. A Coriolis flowmeter would provide a continuous inferential measurement of concentration but is much more expensive. Conductivity should be sufficient for a status indication.”

A toroidal conductivity sensor.A toroidal conductivity sensor is a doughnut-shaped probe that inserts into the process liquid stream. The toroid itself has two coils. One is powered and creates a magnetic field. The second is a receiver. The current induced in the receiver is in proportion to the conductivity of the surrounding liquid. The probe can be encased in an appropriate grade of plastic to withstand the process liquid. Units are available that are designed for sanitary applications in food processing.

--Peter Welander, pwelander(at)