2-Wire Dual Channel Transmitter/Analyzer FLEXA
The model FLEXA® two-wire analyzer is used for continuous on-line measurements in industrial installations. It offers an option for single or dual sensor measurement, making it the most flexible 2-wire analyzer available. The model FLEXA® modular-designed series analyzer offers 4 parameter choices – pH/ORP (oxidation-reduction potential), contacting conductivity (SC), inductive conductivity (ISC) or dissolved oxygen (DO) – with the respective sensor module.
For dual sensor measurement, the sensor modules must be the same parameter – pH/ORP and pH/ORP, SC and SC, and DO and DO. Dual sensor measurement offers additional functionalities including a variety of calculated data from the two measuring parameters, as well as, the option to program the analyzer as a redundant system. In the redundant system the main output parameter is automatically switched over to the second sensor output in case of the main sensor’s failure. ISC is only available as a single measurement.
Contacting Conductivity Sensors SC42/SC4A
Why measure Conductivity?
It can be used as a fairly inexpensive and low maintenance indicator of water quality. Good quality water with little contaminants will have a low conductivity. There is far less maintenance than pH and minimal calibration. Conductivity is used in Boiler feedwater, RO, and Demineralizer applications.
Since conductivity, up to a point, increases with an increase in the concentration of the ions in the process, we can use it to correlate to a % concentration value of solutions such as acids or bases. Again, since conductivity is non-specific, % concentration can only be measured on ONE chemical at a time and not a mixture of contributing chemicals.
There are two basic sensor styles used for measuring Conductivity: Contacting and Inductive (Toroidal, Electrodeless).
When Contacting Sensors are used, the conductivity is measured by applying an alternating electrical current to the sensor electrodes (that together make up the cell constant) immersed in a solution and measuring the resulting voltage. The solution acts as the electrical conductor between the sensor electrodes.
The accuracy of conductivity measurements can be influenced by the following factors:
- Contamination: Deposits or coating on the electrode surface of a 2-pole cell have a similar effect to polarization errors, i.e. the conductivity reading is lower than usual. These effects can also be prevented with a 4-pole conductivity cell.
- Cable resistance: The cable resistance can induce error in the measurement and must be taken into account.
- Temperature: Conductivity measurements are temperature dependent, if the temperature increases, conductivity increases.
Differential pH and ORP Sensors
The cation differential pH and ORP sensors were designed for difficult applications where conventional sensors are ineffective. These include measurements such as brine solutions to applications as diverse as electrolysis processes and cheese manufacturing.
The problems experienced in these applications most often relate to the reference cell and are the result of either:
- high diffusion potentials at the reference junction
- high temperatures (often encountered in applications such as in chlorinated brine processes)
- and high ground-loop currents.
Yokogawa’s cation differential sensors feature a sealed glass reference and do not have a liquid junction nor any electrolyte to come in contact with the process. Therefore, the sensors do not suffer from diffusion potentials or fouling, which ensures measurement stability while being virtually maintenance free. There are different versions of differential pH sensors available and the significance of the advantages they have to offer over a traditional electrode is appreciated only after experience of using a conventional reference electrode, both in theory and practice, in these difficult applications.
Dissolved Oxygen Sensors for Pharmaceutical Applications
Yokogawa is working with HAMILTON to offer best-in-class dissolved oxygen sensors for pharmaceutical applications. Since their introduction in 1995, HAMILTON oxygen sensors have become world leaders because of their signal performance and low maintenance requirements. The rugged design of the …