Sensors

Sensortech manufactures a diverse array of radio frequency sensors including open-frame planar, sealed flange-mount planar, tray sampling sensors, pipeline sensors and piston purge sensors. The following briefly describes a range of Sensortech RF products. For detailed information, refer to application reports and brochures.

Open Frame Planar Sensors
The most common and simplest sensor is the open-frame planar style shown in figure 7. This sensor may be located between conveying rollers in a board line such that board passes over the sensor with a constant air gap, usually 6mm. The open frame design allows debris to fall through the sensor, minimizing build-up. Sensor electronics are usually attached to sensor in order to minimize cable capacitance and maximize sensitivity. Sensor electronics may be separated in high temperature locations or where vibration is excessive. Maximum sensor cable length is 5 meter. Sensors are available in any length, but standard lengths range from 12-inch to 48-inch in 12-inch increments. Non-standard lengths are slightly higher priced. The open-frame planar sensor may be used in almost all types of board plant. Gypsum plants utilize multiple sensors including high temperature sensors located in kiln zones and sensors located after the dryer.

In a sealed sensor, an insulating material supports the center electrode, at the same time forming a flush sealed surface. Choice of insulator material depends on many factors including sanitary requirements, wear resistance and operating temperature. Materials include Teflon (PTFE) for food-grade applications. Teflon withstands high temperature (260°C) and is non-stick, but is quite easily worn down by abrasive materials. Ultra-high molecular weight polyethylene (UHMWPE) is also food-grade and much harder wearing than Teflon, but is limited to relatively cool applications (<60°C). High alumina ceramic is another useful material. It is extremely hard wearing, second only to diamond, but is brittle and not accepted in food applications. The most important consideration for any insulating material is that it must be dimensionally stable and absolutely non-hygroscopic.

Sealed sensors are available in various sizes and include rectangular and circular bodies. The most common application is located in the side of a hopper or bin such that the inside surface is flush with the inner wall of the vessel. In hopper/bin applications, it is important to maintain a fairly constant height of material above the sensor, particularly if the material is compressible since density changes as well as moisture content will influence the dielectric value. Another thing to consider with bin applications is that the flow of product is maintained over the sensor at all times. A static build-up of product on the sensor will obviously impair accuracy.

Sleds and Other Planar Applications

Planar sensors may be used to measure powdered products on belt conveyors. A scraper is located above the conveyor to level the product. A planar sensor is attached to the down-stream side of the scraper approximately 6mm above the leveled product. In some cases the scraper may be adjusted up or down to regulate flow. In this case the sensor moves with the scraper always maintaining a 6mm product/sensor gap. Often, the nature of the product does not allow easy leveling. In these cases, sometimes a sled sensor may ride on the product, essentially maintaining a constant sensor/product gap. The sled hangs from dual arms on each side forming a parallelogram action to maintain the sensor parallel to the conveyor. Using the scraper method or the sled, it is important to maintain a depth of product greater than the penetration of the RF field, particularly for metal conveyors.

Pipeline & Cylindrical Sensors

 Pipeline sensors measure moisture content of fluids and pastes. Typical applications include caramel, molasses, molten candy and fudges. The pipeline may be adapted to measure solids in a sampling mechanism such as that shown in application report #17. The regular pipeline sensor is 50mm diameter (75mm available) with a slight flat side incorporating a button sensor, fig. 11(a). This type of sensor is good as long as product flows uniformly throughout the section of pipe. If side-wall build-up occurs it is a problem. Most confectionary products flow through the pipes at very elevated temperatures, typically 120°C - 140°C. At the beginning of a production run, the pipe may be cold and a layer of product immediately forms on the inside of the pipe. This layer may never melt off throughout an 8 hour production run. Running hot water through the system prior to production is helpful. Some systems use steam jacketed piping. If this is not possible, a coaxial sensor should be considered.

Figure 11(b) shows the end view of a coaxial sensor. The electric field is radial from the center electrode to the outer pipe wall. Thus the electric field must penetrate moving product, even if a build-up occurs. Figure 11(c) shows a heater jacket surrounding the pipe. In this case the pipe is electrically heated to just below the normal operating temperature to ensure product does not tend to solidify. The main disadvantage of a coaxial sensor is that it normally needs removing from the process for cleaning and some users do not like having an obstruction in the pipeline.

Sampling Sensors

In cases where it is not possible to maintain a reasonably constant flow of product over the sensor, a sampling sensor may be considered.

The ST-2200 processor, when used in timed sampling mode, provides a digital output to activate a solenoid valve. This mode provides two programmable functions, fill time and purge time. Fill time is a do-nothing period of time to allow the sampling chamber to fill with product. Fill time is followed by a fixed 1-second measurement time which is then followed by the purge time during which the digital output is set high for the programmed duration. The instrument display is updated following the measurement interval and held until the next measurement.

Three types of sampling sensor are shown in figure 10. The down-chute sensor is a sealed planar sensor with low side walls forming a sensor tray. The tray is inserted into a vertical gravity-fed chute through a rectangular hole. The vertical flange bolts to the chute wall. Product free falling down the chute fills the tray to overflowing. A measurement is taken and then an air-purge nozzle empties the tray. Many low-density powdered products are conveyed by blowing or sucking through pipelines. This is known as pneumatic conveying and is widely used in the grain processing industry. By simply cutting a hole in the pipeline, the sensor may be saddle mounted with clamps. The angled sensor captures a small proportion of product. When full, a measurement is taken and a solenoid valve is activated to evacuate the sample chamber with compressed air.

The piston-purged sensor resembles a large syringe. A 150mm diameter fiberglass lined chamber forms a cylindrical sensor. A piston retracts allowing the chamber to fill with product. After measurement, the piston advances, returning the material to the product flow. Moisture Profiling System

A special type of gypsum system is the IMPS-4400 moisture profiling system. This consists of multiple sensors located in an array across the take-off line (exiting dryer). Each sensor measures a 50mm strip along the machine direction of the board. Sophisticated graphics software produces multiple graphical presentations of board moisture (see IMPS-4000 PowerPoint presentation). At the present time IMPS systems, while widely used in the gypsum board industry, are not generally used in wood-based board products. Density variations in these products tend to significantly influence the measurement.

The ST-2200 processor, when used in timed sampling mode, provides a digital output to activate a solenoid valve. This mode provides two programmable functions, fill time and purge time. Fill time is a do-nothing period of time to allow the sampling chamber to fill with product. Fill time is followed by a fixed 1-second measurement time which is then followed by the purge time during which the digital output is set high for the programmed duration. The instrument display is updated following the measurement interval and held until the next measurement.

Three types of sampling sensor are shown in figure 10. The down-chute sensor is a sealed planar sensor with low side walls forming a sensor tray. The tray is inserted into a vertical gravity-fed chute through a rectangular hole. The vertical flange bolts to the chute wall. Product free falling down the chute fills the tray to overflowing. A measurement is taken and then an air-purge nozzle empties the tray. Many low-density powdered products are conveyed by blowing or sucking through pipelines. This is known as pneumatic conveying and is widely used in the grain processing industry. By simply cutting a hole in the pipeline, the sensor may be saddle mounted with clamps. The angled sensor captures a small proportion of product. When full, a measurement is taken and a solenoid valve is activated to evacuate the sample chamber with compressed air.

The piston-purged sensor resembles a large syringe. A 150mm diameter fiberglass lined chamber forms a cylindrical sensor. A piston retracts allowing the chamber to fill with product. After measurement, the piston advances, returning the material to the product flow. Moisture Profiling System

A special type of gypsum system is the IMPS-4400 moisture profiling system. This consists of multiple sensors located in an array across the take-off line (exiting dryer). Each sensor measures a 50mm strip along the machine direction of the board. Sophisticated graphics software produces multiple graphical presentations of board moisture (see IMPS-4000 PowerPoint presentation). At the present time IMPS systems, while widely used in the gypsum board industry, are not generally used in wood-based board products. Density variations in these products tend to significantly influence the measurement.