Sensors as the direct interface to the process must be exactly matched to the chemistry of the measured medium and to the prevailing pressure and temperature conditions. These essential requirements for error-free, low-maintenance, and reliable recording of process parameters are perfectly met by the new, upgraded range of Knick sensors. This serves a wide range of different applications; in conjunction with suitable fittings and analyzers by Knick, top performance can be achieved, even in hazardous locations.
pH Sensors with Application-Specific Properties
Even today, sensors with the classical glass membrane are the most reliable and most accurate pH sensors, and for this reason they are of course also the standard at Knick. As pH sensors have a temperature-specific characteristic (Nernst), the temperature must be recorded at the same time as the pH value for compensation purposes.
Most pH sensors from Knick are fitted with integrated temperature detectors. Here, the clear technical benefits call for a Pt 1000 temperature detector rather than a Pt 100. The necessary multi-pole connection for these sensors is provided by a Variopin (VP) connector cap.
So-called “highly alkali-resistant glass“ has proved successful as a pH-sensitive glass with universal properties for the majority of applications, in particular for processes with high pH values or for CIP cleaning. As pH glass has a very high impedance, and the resistance increases further as the temperature decreases, Knick provides a special pH glass with low impedance for measurements at very low temperatures. Apart from the composition and form (mechanical stability) of the pH-sensitive glass membrane, the individual process conditions place particular requirements on the reference systems. Examples of these are chemical influences, wide pressure and temperature ranges, polluted, particle-laden media, hygiene conditions, and sterilization capability. Different variants, mainly of the reference systems and thus ultimately of the pH sensors too, are also based on these varying requirements.
In media with pronounced impurities and high pressures, reference electrodes with open junctions and solid polymer electrolyte (e.g. SE 533) have proved successful.
If the measured medium has very low conductivity, it is preferable to use sensors with internal pressure electrolyte pads or refillable, pressurizable liquid electrolytes in combination with one or more ceramic junctions (e.g. SE 552 or SE 551). Only special fine-pored ceramics can be cleaned hygienically.
Pressurized or refillable sensors also offer clear advantages in heavily polluted media and media that contaminate the reference system through the cleaning effect of the continuous electrolyte outflow.
Sensors with large-area PTFE ring junctions also have very long service lives in these difficult media. Protection against sensor contamination is achieved here by an additional, pressure-compensated electrolyte bridge (e.g. SE 553).
Some operators of food and cosmetic production processes need to eliminate any possibility of broken glass to safeguard their products. For these applications, Knick has glass-free pH sensors available, which manage without a glass membrane as a pH-sensitive element, and so avoid any possibility of accidental breakage. Instead of the glass membrane, these sensors have an ISFET (ion-sensitive field-effect transistor) with a pH-sensitive coating, embedded in a robust plastic (PEEK) shaft.
Most of the pH sensors offered by Knick are available not only in the standard length of 120 mm but also in 225 mm length for use in the Unical® 9000 and Uniclean® 900 automatic cleaning and calibration systems. Retractable fittings with ideal functionality for this purpose are Ceramat® and SensoGate®.
Conductivity Sensors for the Whole Measuring Range
The conductivity of aqueous media covers a range of more than eight decades, starting with ultrapure water with 0.055 µS/cm and going as far as fully dissociated acids or bases with more than 1,000 mS/cm. The applications for conductivity measurement vary accordingly: from checking the purity of feed water in power plants to measuring highly concentrated electrolyte solutions and concentrated sulfuric acid. Measuring conditions are often difficult, as the processes take place in wide pressure and temperature ranges and the measured media may also be heavily polluted.
Ideally, these very different requirements can only be fulfilled by different sensors which are adapted to the application-specific conditions by means of special properties. Knick offers the whole range of sensor technologies for this: sensors that measure with two or four electrodes, or electrodeless on the inductive principle. All sensors are equipped with a temperature detector for automatic temperature compensation of the conductivity level. Various process connections, such as threads, flanges, clamps, screw fittings, etc. are available.
In media that have low conductivity levels and are neither heavily polluted nor contain coating-forming components (oil, lime, etc.), highly accurate measurements can be carried out with 2-electrode sensors that have a coaxial electrode geometry. Various models are available for this. Alongside the particularly robust SE 604 sensor with its high temperature and pressure stability for monitoring ultrapure water e.g. for feed water in power stations, the product line also includes a sensor (SE 620) that is particularly suitable for hygienic processes on account of its electropolished surface with very low surface roughness (<0.8 µm).
With the PortaSim® Cond conductivity simulators specially developed for these models, the connected analyzers, including the sensor cable, can be validated according to the USP <645>standard.
A sensor with special graphite as electrode material (SE 630) allows the measurement of low to medium conductivity, even at moderate pollution levels and in corrosive media. For very high pressures and temperatures, Knick offers a special sensor model (SE 600) which, through the selection of special materials (PTFE / platinum), is also resistant to extreme chemical and corrosive influences. Thanks to its four-electrode design, this sensor also has a considerably wider measuring range than sensors with two electrodes.
Electrodeless (toroidal) sensors by Knick are also distinguished by their especially wide measuring range. They are practically impervious to pollution, and the measurement is unaffected even by non-conducting coatings. The sensors have a smooth, easy-to-clean design.
As the entire process-wetted sensor body consists of only one material, a maximum of chemical resistance is achieved through the use of high-quality high-tech plastics such as PEEK or PFA.
Versions made of PP also allow these properties to be used economically for less demanding requirements in process technology, such as process, industry and wastewater. In conjunction with Knick transmitters, the conductivity sensors can also be run in 2-wire devices and hazardous-area applications.
Low-Maintenance Oxygen Sensors
Sensors by Knick for measuring dissolved oxygen are characterized by a high level of process safety. The robust design using durable materials and the modular structure of the sensors lead to a high level of reliability and low maintenance. As a module, the steel-mesh-reinforced, PTFE-coated membrane can be replaced quickly and easily, as can the electrode system with its complete inner body.
The sensors offer a high resolution of 6 ppb. With a version for trace measurement, a value of 1 ppb can be achieved, while its insensitivity to high CO2 partial pressures also makes it well suited to the beverage industry.
The versions in a hygienic stainless steel design with an extremely smooth surface are EHEDG-certified and 3A-compliant. They are sterilizable, autoclavable, and CIP-resistant.
A low-priced version with a plastic body is available for simple applications. All DO sensors can also be used in conjunction with Knick transmitters in 2-wire devices and in hazardous locations.