Conductivity sensor for ultra-pure water

Process analytics

Conductivity sensor for ultra-pure water

In order to determine very low ion concentrations in ultra-pure water, a sensor is required that can reliably measure even the slightest conductivity. On top of that, international pharmaceutical guidelines and rules have to be met. The Conducell UPW Arc sensor – explicitly developed to meet these requirements – conforms to USP 645, EP and JP.

High quality ultra-pure water is a basic requirement to ensure permanently high product quality in the biotech, pharmaceutical and cosmetic industries. Absolutely pure water is indispensable for the production of medicines and hypodermic fluids as well as in medical research facilities. It goes without saying that meeting the strictest possible standards and quality requirements is a key priority. One of the most important criteria for the quality of pure water is its conductivity. Conductivity depends on the concentration of ions in the fluid. Ultra-pure water has a very low conductivity level. Only one in a billion water molecules (H2O) dissociates into H+ and OH- ions. The intrinsic conductivity of water is at the lower limit of the conductivity scale (0.055 µS/cm at 25 °C) and plays a central role when ultra-pure water has to be analysed in the pharmaceutical industry. Foreign matter such as salts, organic impurities or germs must be avoided as far as possible. The less foreign matter or impurities present in ultra-pure water, the lower the conductivity and therefore the higher the quality. The conductivity of ultra-pure water in the loop depends on the customer’s requirements and usually varies between 0.2 and 1 µS/cm. The total organic carbon (TOC), number of germs and endotoxin concentration are other relevant characteristics of ultra-pure water. Only conductivity and TOC can be measured online and allow the quality of ultra-pure water to be assessed in real time.

In order to determine extremely low ion concentrations in ultra-pure water, a sensor is required that can reliably measure even the slightest conductivity. On top of that, international pharmaceutical guidelines and rules have to be met. The Conducell UPW Arc sensor was explicitly developed by Hamilton Bonaduz to measure very low conductivities and meet the requirements of pharmacopoeias such as USP (United States), EP (Europe) and JP (Japan). To achieve optimal results, the sensor must be installed in an appropriate position without precipitates or air bubbles. The flow direction must also be considered.

The Conducell UPW Arc is based on in-line conductivity measurements with a two-pole sensor. This principle depends on the electrical resistance measurement: the conductivity cell consists of two electrodes with an alternating voltage applied between them. The more ions present in the solution, the more intense the electrical current and, according to Ohm’s law, the higher the conductivity between the poles. The ultra-pure water quality can be monitored for conformity with USP 645 and an alarm issued in case of non-compliance. Another advantage of the Conducell UPW Arc sensor is the direct signal output via the analogue 4…20 mA or digital Modbus interface. Using the optional Arc Wi sensor adapter, a Zigbee signal can also be transmitted either wirelessly to the Arc View handheld device or via Bluetooth 4.0 and an app to a smartphone or tablet. No matter which option customers choose, the functionality is the same and compatible with the complementary GMP Compliance Package.

Precise conductivity measurements require a calibrated sensor and meter, both of which have to be tested and verified regularly. One important step in this procedure is the determination of the cell constant according to traceable conductivity standards. The cell constant obtained in this way must not deviate more than 2 % from the constant specified in the Declaration of Quality. If the sensor is installed in a pipe or tank, it can be verified by carrying out a simultaneous measurement with a traceable reference sensor in its proximity. In this case, the sensor cannot be removed for verification purposes. USP 645 additionally requires the meter to be verified using certified, traceable, high precision resistors in order to detect mutually compensating errors. One of the sensor’s advantages here is the fact that it consists of two components: the Arc module with the integrated micro transmitter and the conductivity cell. According to USP 645, they can be separated from each other. This feature allows the conductivity cell to remain installed in the ultra-pure water tank or the pipe during the verification, which takes place using the optional UPW Simulator. This consists of a traceable, high precision resistor and is connected to the Arc module. The certified value measured by the resistor can be read off on the process control system or the optional Arc View handheld device and compared with the specified value. Temperature compensation can be deactivated, as required by USP 645.

The Conducell UPW Arc sensor is available with two different, widely used connections: Pg13.5 and TC 1.5″. Both versions guarantee excellent and optimally accurate measurements and are suitable for both hot and cold water systems. The measuring range is from 0.01 to 1500 µS/cm, in other words from very cold ultra-pure water to 0.01 molar potassium chloride solution – one of the most important conductivity standards (ASTM D1125) at 25 °C. Thanks to the Arc module, the sensors provide diagnostic information, histories, calibration data and alarm monitoring information on the ultra-pure water. Besides high-quality components, the sensor also convinces with its hygienic design. All wetted parts are FDA-approved.

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Dr. Knut Georgy

Senior Market Segment Manager Process Analytics,

Hamilton Bonaduz