Expanded wet-sensor versatility

FLEXIBILITY and rapid reconfiguration are the name of the game for process skid builders. Be it food and beverage or pharmaceutical applications, these OEMs often need to quickly swap a pH probe with a resistivity or conductivity probe; network those measurements and others to one control screen using protocols such as Modbus, DeviceNet and Profibus without the need for intermediate analyzer boxes; and be operational faster than you can say “plug-and-play.”

Datastick is based on a modular concept with interchangeable sensor heads in a variety of parameters. It consists of a universal sensor module (the “stick”) and interchangeable communications adaptors for digital communications protocols and fieldbuses. The original sensor parameters included pH, ORP, resistivity, conductivity, and toroidal conductivity. The four new parameters are dissolved oxygen (DO), ozone, raw-water turbidity and drinking-water turbidity. Each sensor head is digital, pre-calibrated, and knows its serial number, sensor type, calibration data, and other diagnostics for validation purposes. The sensor heads are completely interchangeable without recalibration.

Sensor and communications power requirements are low, according to the manufacturer, typically 120 mW from a 10-30 VDC Class II power source.

“Fieldbus and communications protocols available right out of the stick include Modbus, Profibus, DeviceNet, RS-485, RS-232 and USB 2.0 serial communications,” says Bruce Bathurst, president of Aquasensors. “HART and Foundation fieldbus will be available soon.” He adds wireless sensor operation is available by taking the Modbus output into one of the commercially available wireless gateways, and then onto the control system. “This makes the system well-designed for remote monitoring sites,” claims Bathurst.

Local display and analog output are provided by a Modbus module, AV38, with display, programming, up to two current loops, and two Form C relays. Local display isn’t required for operating the all-digital system.

“The DO and ozone sensors are similar because they’re both based on Clark Cell technology, and they’re supplied in a variety of body materials, two types of sensor-tip protection, standard or custom-filling solution, and either an FEP Teflon membrane or a custom solution,” says Bathurst. “Both the DO and ozone cells also measure temperature, and are air calibrated at the factory.” The company claims accuracy of ±0.04 ppm with a resolution of 0.01 ppm, and a step-response time of 90% in 90 sec over a 0-40 ppm range at 25 °C, and states the sensor’s performance is unaffected by cable length.

The turbidity product is provided in two sensor types, raw water and drinking-water turbidity. The raw-water sensor looks like all the other Datastick products, and like most other parametric sensors. It has a range of 0-4,000 Nephelometric Turbidity Units (NTU) with accuracy of 0.2 NTU and resolution of 0.1 NTU, and with a step-response time of 90% in 15 sec. Like its drinking-water counterpart, the raw-water turbidity sensor uses quartz or sapphire windows, with a multi-color source, having detectors located 90º and 180° to the source.

“Turbidity and temperature are pre-calibrated at the factory, and the sensor retains calibration and diagnostics for validation in food, beverage, and pharmaceutical applications,” says Bathurst.

The drinking water turbidity sensor has a flow-through cell into which the sensor inserts, and operates over a range of 0-100 NTU with resolution of 0.001 NTU, according to Aquasensors. Accuracy is ±2% of reading, or ±0.015 NTU, whichever is greater, and ±5% of reading above 40 NTU. Step response time is 90% in 120 sec. Similar to its raw-water counterpart, turbidity and temperature are pre-calibrated, and the sensor provides validation information. The drinking-water sensor has specific power requirements—24 VDC at 8 W—that are higher than other Datasticks, with local display and flow-through cell.

All Datastick sensor assemblies are CE-marked for emissions/immunity and safety, and are UL-rated Class I, Div. 2.