Conductivity measurement is a reliable indicator of the concentration of most acid or base solutions. The desired chemical concentration is achieved using a two stage mixing procedure. During the first stage, the flow ratio control unit on the missing tank is set to provide (x) gallon per minute of the full strength solution and (y) gallons per minute of water. These values are adjusted to produce a concentration value which is slightly weaker than the desired value. This ratio control must include alarm capabilities to indicate "low flow" conditions for both the full strength solution and the water in order to prevent wasted chemicals or hazardous situations.
At the second stage, a conductivity sensor and analyzer function as a "trim control". This combination adds small amounts of full-strength solution to the mixing tank to produce the exact concentration desired.
For example, to produce a 4% caustic solution from a large bulk caustic supply at 50%, the flow ratio controller is adjusted to make a 3% solution and the conductivity information is used to add additional caustic to achieve the 4% concentration.
Conductivity is a very reliable index of the concentration for most acid and base (caustic) solutions. Figure 1 shows the correlation between conductivity and concentration for four common solutions.
For most solutions, there is a peak conductivity value. Before this peak value is reached, conductivity correlates positively with concentration; after the peak, it correlates negatively. So, if the concentration range passes through the peak for that chemical the conductivity value (except the peak value) represents two different concentration values. Therefore, it is mandatory that any application near the peak of a particular solution be carefully controlled.
In determining the proper loop components for a particular application, the material of construction will be of primary concern. A chemical resistance chart should be consulted (see Table 1), or an application data sheet completed and sent to the factory in order to insure an installation that will be suited for the intended application.
There are two basic sensor styles used for measuring Conductivity: Contacting and Inductive (Toroidal, Electrodeless).
With Inductive Conductivity (also called Toroidal or Electrodeless), the sensing elements (electrode coils) of an inductive sensor do not come in direct contact with the process. These two matched (identical) coils are encapsulated in PEEK (or Teflon) protecting them from the adverse effects of the process.
There is only one cell factor (constant) for the ISC40 Inductive Sensor. It covers nearly the entire conductivity measurement range ~ 50-2,000,000 µS/cm. Only on the low end (below 50 µS) does the accuracy of the sensor suffer.
Because the ISC40 Inductive sensor is virtually maintenance free; it is the first choice for any application. If the ISC40 cannot be used then it is recommended to use the 4-electrode design, model SC42 large bore sensor.
The model ISC40 sensors are designed for use with the EXA ISC analyzers. This combination exceeds all expectations for conductivity measurement in terms of reliability, accuracy, rangeability and price performance.
The accuracy is 0.5% of reading plus 0.5 microS/cm for any conductivity value, whether measured in rinse water or in concentrated acids. The materials of construction ensure a long life under harsh industrial conditions.
The erosion/abrasion resistant PEEK (Poly Ether Ether Ketone), which also features excellent chemical resistance in all solutions except fluoric acid or oxidizing concentrated acids. The ultimate material in terms of chemical resistance is the PFA (Teflon) for applications in hydrofluoric acid and oxidizing concentrated acids (nitric, sulfuric, oleum).
The ISC40G and ISC40s are available in PEEK (sensor type GG) for general use. In applications where sample is aggressive to Peek we offer the sensor in Teflon (Sensor type TG).
The ISC40 sensor is provided with a rugged Stainless Steel mounting thread, nut and gasket combination for ultimate flexibility in installation using bulk head installation technique. There is also a wide range of holders and options available for reliable in-line or off-line installation with double O-ring seals for long service life of the sensor. Additional models are available for use in Ball-Valve Insertion applications and in Sanitary Flange installations.
Both sensors have a large bore for optimal resistance to fouling processes and when properly installed, the flow will keep the sensor clean preventing measuring errors.
Inductive Conductivity technique for elimination of fouling and polarization errors.
Wide bore sensors for long term stability.
Installation flexibility by wide range of holders and by the use of universal bulkhead construction.
Wide rangeability in terms of conductivity (1 μS/cm to 2 S/cm) and temperature (-20 to 130ºC).
All applications where severe electrode fouling prevents the use of contacting electrodes.
All ranges except the (ultra) pure water applications.
All slurry applications where conventional systems suffer from plugging or erosion.
All applications where the 6 decade rangeability is necessary for accurate process control
Model PR10 Retractable "Hot-Tap" Holder
On-line measurements often present extra challenges, especially when routine maintenance is required. The PR10 is ideally suitable for applications where the sensors must be removed without interrupting or shutting down the process. Without any special tools the PR10 can be retracted safely from the process at pressures up to 5 bar (72 psi). The PR10 is a universal retractable assembly that can be used for all liquid measurements. The PR10 is designed to accept any commercially available pH/ORP or dissolved oxygen sensor that has a PG13.5 connection while still being backwards compatible with old Yokogawa electrodes.
For ease of use optional flush ports are available. In the retracted position the sensor can be kept moist, cleaned or even calibrated. This can all be done without process interruption or disassembly of the armature.
One model for pH/ORP, dissolved oxygen, conductivity and inductive conductivity sensors
All commercially available pH electrodes, such as PG13.5, SMART sensors, Vario Pin, YEF and DIN connectors can be installed in the PR10.
Integrated protection cage
A "scraper" is built-in to each assembly to effectively scrape or wipe off the
process from the PR10's shaft as it is being retracted from the process.
A safe "through the valve" insertion and retraction design
Simplified installation by optional ball valves with flanged or tapered connections
Optional flush port accessories makes it easy to keep sensors moist (pH) and clean, and also provides access for calibration if desired.
Calibration Standard Solutions
Yokogawa supplies Hamilton conducctivity standars whose stability of ±1% is guaranteed over a life cycle of 1 to 3 years, depending on the value.