2-Wire Transmitter/Analyzer FLXA21

The FLXA21 is 2-wire module-type liquid analyzer that can be connected to up to two detectors from pH /ORP meters, conductivity meters, inductive conductivity meters, and dissolved oxygen meters.

By connecting two detectors of the same type, a more reliable system can be constructed as a high-precision duplex system.

And the impedance, asymmetric potential, slope, etc. of the detector are constantly measured, and continuously diagnostics status such as the electrode is dirty, damaged, broken and low measurement liquid level.

This self-diagnosis function of the detector reduces maintenace costs and improve OPEX.

About OpreX

OpreX is the comprehensive brand for Yokogawa’s industrial automation (IA) and control business and stands for excellence in the related technology and solutions. It consists of categories and families under each category. This product belongs to the OpreX Analyzers family that is aligned under the OpreX Measurement category.


Features of FLXA21

High reliability with two detectors and reduced maintenance and instrumentation costs through digital communications

  • pH/ORP, conductivity, and dissolved-oxygen detectors can be connected to two of the same type
  • Backup system with two detectors for higher reliability measurements
  • Achieved reduced installation cost and space saving by measuring two different points
  • Two detectors enable continuous measurement without interruption during maintenance
  • Reducing maintenance/instrumentation costs through HART communications

Improvement of expandability by modular structure

  • Rich system construction by modular structure

Equipped with various advanced functions

  • Self-diagnosis function of the detector
    The impedance, asymmetric potential, slope, etc. of the detector are constantly measured
  • Health and maintenance timing forecasts
    Saves the latest five calibration results and predicts future maintenance and calibration timing
  • Built-in Quick Setup
    Screen instructions (guides) for easy minimal set-up

Touch screen for improved operability and enhanced operation screens

  • Reliable, friendly operation while interacting with the screen
  • Various display modes are available, and a high degree of freedom can be selected.

Sensors suited for a wide range of applications

  • Can be connected to various types of pH/ORP, conductivity, electromagnetic conductivity, and dissolved oxygen detectors

Selectable from 12 languages

  • Japanese, English, Chinese, Korean, German, French, Russian, Italian, Spanish, Czech, Polish, Portuguese

Compatible with KOSHA, ATEX and various other types of explosion-proof

  • Intrinsically safe type of ATEX, IECEx, FM, CSA and NEPSI, and nonincendive of FM and CSA.

Supports communication

  • HART, Profibus, FieldBus


Common specification

Area Classification General Purpose 
Class I Div II (without the use of IS Barrier)
Class I Div I (with the use of IS Barrier)
Enclosure Material Plastic (Polycarbonate)
Housing Rating IP66/ NEMA 4A/ CSA 3S
Power Supply 2-wire 24 VDC Loop Powered
Output Signal Bi-directional HART digital communication, superimposed on mA (4-20 mA) signal
Optional FF (FOUNDATION Fieldbus) and PF (Profibus) communication
Intrinsically safe type ATEX, IECEx, FM, CSA and NEPSI, and nonincendive of FM and CSA.

Features of pH and ORP measurement

Built-in calibration standard table, with temperature compensation function and stability check function, realizes high-precision pH measurement.
In addition to pH measurements, temperature and redox potential (ORP) can also be measured.
As a diagnostic function of the detector, impedance, asymmetric potential, slope, etc. are constantly measured to continuously diagnose the state of the detector such as dirt/damage of the electrode, disconnection, and lowering of the measuring liquid level. In addition, the latest five calibration results are stored to predict future maintenance and calibration timing.

pH sensors for FLXA21

pH and ORP measurement specification

Input Range pH: -2 to 16 pH (with option /K: 0 to 14 pH) 
ORP: -1500 to 1500 mV 
rH: 0 to 100 rH 
Temperature: Pt1000, Pt100, 6.8k, PTC10k, 3k Balco, PTC500: -30 to 140 ºC, NTC 8k55: -10 to 120 ºC
(The specifications are expressed with simulated inputs.)
pH Linearity: ±0.01 pH, Repeatability: ±0.01 pH, Accuracy: ±0.01 pH
ORP Linearity: ±1 mV, Repeatability: ±1 mV, Accuracy: ±1 mV
Temperature with Pt1000, PTC500, 6.8k, PTC10k, NTC 8k55, 3k Balco
Linearity: ±0.3 ºC, Repeatability: ±0.1 ºC, Accuracy: ±0.3 ºC
with Pt100
Linearity: ±0.4 ºC, Repeatability: ±0.1 ºC, Accuracy: ±0.4 ºC
(When combined with a pH detector)
Repeatability ±0.05 pH
Responsiveness 10 seconds (90% response time, temperature-equilibrium pH detector at 20 ºC and standard solution, well agitated)
Accuracy ± 0.1 pH (when using PH8EFP, PH8EHP)
± 0.15 pH (when using our standard detector other than the above)

Features of conductivity measurement

Built-in temperature compensation function and calibration standard table enable monitoring of the contamination of the cell for higher accuracy conductivity measurement.
Highly versatile, cell constants from 0.005 to 50 cm-1, two-way and four-way detectors are available.
It also supports five types of temperature detectors for accurate temperature compensation.
Performs pure water compensation in the analysis of demineralized water, steam, condensate, and boiler water. For monitoring of pure water in the pharmaceutical industry, the process liquid quality can be monitored according to the water quality standards of the USP item <645> (refer to item 645 of USP23 Article) guidance (USP).

Conductivity Measurement

Conductivity measurement specification

Input Specification Two or four electrodes measurement with square wave excitation, using max 60m (200ft) cable (WU40/WF10) and cell constants from 0.005 to 50.0 cm-1
Display range Conductivity min. 0.01 µS/cm, max. 2000 mS/cm (max 90% zero suppression)
Resistivity min. 0.001 kΩ x cm, max. 1000 MΩ x cm (max 90% zero suppression)
Temperature min. span 25 ºC, max. span 270 ºC
Performance (Accuracy)
(The specifications are expressed with simulated inputs.)
Conductivity 2 µS x K cm-1 to 200 mS x K cm-1: Accuracy: ±0.5%F.S.
1 µS x K cm-1 to 2 µS x K cm-1: Accuracy: ±1%F.S.
Resistivity 0.005kΩ / K cm-1 to 0.5MΩ /K cm-1: Accuracy: ±0.5%F.S.
0.5MΩ / K cm-1 to 1MΩ /K cm-1: Accuracy: ±1%F.S.
Temperature with Pt1000, Pb36, Ni100:  Accuracy: ±0.3 ºC
with Pt100, NTC 8k55:  Accuracy: ±0.4 ºC
Temperature compensation NaCl table: ±1 %
Matrix: ±3 %
Temperature 90 % (< 2 decades) in 7 seconds

Features of electromagnetic conductivity measurement

For non-contact measurements based on the principle of electromagnetic induction measurement, it can be used for conductivity measurements in a wide range of applications, from acid and alkaline solutions to salt-containing solutions, as level monitoring in the chemical industry, such as the food and chemical industries, the plating and surface treatment industries, and the paper and pulp industries. It also has a matrix compensation and output linearization function to accurately analyze solutions of strong acid and strong alkali. Only one sensor can be connected.

Electromagnetic Conductivity Measurement

Electromagnetic conductivity measurement specification

Input Specification Compatible with the Yokogawa inductive conductivity ISC40 series with integrated temperature sensor: NTC30k or Pt1000.
Input Range Conductivity 0 to 2000 mS/cm at 25 ºC reference temperature.
Temperature -20 to 140 ºC
Cable length max. 60 meters total length of fixed sensor cable + WF10(J) extension cable.
Influence of cable can be adjusted by doing an AIR CAL with the cable connected to a dry cell.
(The specifications are expressed with simulated inputs.)
Conductivity Linearity: ±(0.4 %F.S. + 0.3 µS/cm)
Repeatability: ±(0.4 %F.S. + 0.3 µS/cm)
Temperature ±0.3 ºC
Step response 90 % (< 2 decades) in 8 seconds

Features of dissolved oxygen measurement

This process analyzer achieves high functionality, high reliability, and reduced maintenance. Suitable for on-site installation and can be used under severe environmental conditions.
Not only galvanic sensors but also polarographic detectors can be used.
There are mg/L, ppm and % saturation units as the units for displaying dissolved oxygen concentration. In addition, for the best measurement accuracy, corrections are made for variations in atmospheric pressure, salinity in water, and temperature that differ depending on the altitude, etc.

Electromagnetic Conductivity Measurement

Dissolved oxygen measurement specification

Input Specification The FLXA202 accepts output from membrane covered Dissolved Oxygen sensors. These sensors can be Galvanic type, where the sensor generates its own driving voltage or Polarographic type, where the sensor uses external driving voltage from the converter.
The input range is 0 to 50 µA for Galvanic sensors and 0 to 1 µA for Polarographic sensors.
For temperature compensation, the FLXA202 accepts Pt1000 (DO30 sensor) and NTC22k elements (OXYFERM and OXYGOLD sensors).
Measurement Range DO Sensors Dissolved Oxygen: 0 to 50 mg/l (ppm)
Temperature: -20 to 150 ºC
DO30G Measurement range: 0 to 20 mg/l (ppm)
Temperature: 0 to 40 ºC
(Accuracy) (Performance in ppm mode)
Linearity ±0.05 ppm or ±0.8% F.S., whichever is greater
Repeatability ±0.05 ppm or ±0.8% F.S., whichever is greater
Accuracy ±0.05 ppm or ±0.8% F.S., whichever is greater
Temperature Linearity: ±0.3 ºC
Repeatability: ±0.1 ºC
Accuracy: ±0.3 ºC
  • Extension Terminal Junction Box BA10/WTB10

    Between measuring plant and control rooms, especially when the distance between these places is greater than the length of the standard appropriate electrode cables, the connecting equipment can be an expedient method for connecting sensor cables to a measuring instrument.

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Notas de Aplicación
Descripción General:

One of the primary applications for high purity water is for boiler feed water. The measurement of pure water pH can be one of the quickest indicators of process contamination in the production or distribution of pure water. Effective chemical treatment of the feed water is vital in maintaining the useful operating life and minimizing maintenance costs of the boiler.

Industry:Power, Pharmaceutical, Common

Descripción General:

Having too much oxygen in the process is not a problem for the biological system; however the cost for generating the oxygen is one of the largest expenses. By obtaining a good representative average of the dissolved oxygen present in the basin could save the plant large amounts of money. For this reason multiple measurements points are sometimes put into place.

Descripción General:

Wet scrubbers are used in utilities, paper mills, and chemical plants to remove sulfur dioxide (SO2) and other pollutants from gas streams. Undesirable pollutants are removed by contacting the gases with an aqueous solution or slurry containing a sorbent. The most common sorbents are lime, Ca(OH)2, and limestone, CaCO3

Notas de Aplicación
Descripción General:

Caustic soda and hydrochloric acid, produced in electrolyzer plants, are fundamental materials used in varieties of industries; chemicals, pharmaceuticals, petrol-chemicals, pulp and papers, etc. Profit is the result of the effective production with minimized running / maintenance cost. Proper control of the process brings you stabilized quality of products with the vast operational profit.

Descripción General:

The control of the world's water resource is arguably one of the most important issues. Water demand from industry and domestic users is set to rise throughout the industrialized world. Yokogawa has been applying minimized maintenance measurement systems.

Descripción General:

The proliferation of microorganisms and the resultant formation of slime is a problem which commonly occurs in aqueous systems. Problematic slime producing microbes may include bacteria, fungi and/or algae. Slime deposits typically occur in many industrial aqueous systems including cooling water systems, pulp and paper mill systems, petroleum operations, clay and pigment slurries, recreational water systems, air washer systems, decorative fountains, food, beverage, and industrial process pasteurizers, sweetwater systems, gas scrubber systems, latex systems, industrial lubricants, cutting fluids, etc.

Industry:Refining, Food and beverage, Power, Oil and Gas, Pulp and Paper, Chemical

Notas de Aplicación
Descripción General:

For control of batch neutralization, a pH measurement coupled with a timer-controlled chemical feed scheme provides very satisfactory results.

This system can be adapted for either acid waste or alkaline waste neutralization.

Notas de Aplicación
Descripción General:

There are a number of suppliers of oil and fat products used for edible purposes. These products include, but are not limited to olive oil, peanut oil, soybean oil, sunflower oil, lard, shortening, butter, and margarine. The raw materials for these products include animal by-products, fleshy fruits (palm and olive), and oilseeds. 

Industry:Food and Beverage

Descripción General:

For safe and efficient operation, the quality of feedwater, steam, and condensate in boiler plants must be controlled and maintained in the most favorable conditions.

Descripción General:

Process liquid analyzers such as pH meters, conductivity meters, ORP meters, and density meters play an important role at electrolysis plants in the control of concentrations of various process solutions. This requires both precision and stability under harsh conditions that include highly corrosive substances, high temperatures, and many impurities.

Descripción General:

Control of sodium chloride (NaCl) concentration at a salt dissolver where solid salt is dissolved in water, is highly important because of the electrolysis efficiency. A conventional way of measuring the concentration of supersaturated NaCl solution had been performed by using non-contact type sensors (e.g., γ-ray density meter) since NaCl, impurities, and precipitates are in the solution.

Descripción General:

In a semiconductor plant, a variety of chemicals are used in various manufacturing processes. The chemicals used for specific purposes are produced by diluting raw liquid with demineralized water using in diluting equipment, and the control of the concentration at this point is performed by conductivity measurement. 

Descripción General:

In the manufacturing process of Pharmaceutical, Chemical and Food & Beverage industries, the cleaning and sterilization of tanks and piping are done with various cleaning solutions, fresh or hot water and steam after manufacturing products. Clean-In-Place (CIP) is the system designed for automatic cleaning and disinfecting.

Notas de Aplicación
Descripción General:

Wastes have been considered to be a serious worldwide environmental problem in recent years. Because of increasing pollution, these wastes should be treated. However, industrial wastes can contain a number of valuable organic components. Recovery of these components is important economically. Using conventional distillation techniques, the separation of acetic acid and water is both impractical and uneconomical, because it often requires large number of trays and a high reflux ratio. In practice special techniques are used depending on the concentration of acetic acid. 

Descripción General:

Cyanide-bearing wastewater from mining and electroplating facilities and certain types of chemical plants is toxic and must be treated by oxidation with chlorine or chloride to bring the cyanide concentration within regulatory limits.

Industry:Electrical and Electronics

Notas de Aplicación
Descripción General:

Continuous technology improvement is ongoing in the pulp & paper industry to obtain the best possible performance. Problems at the wet end (stock preparation) can rarely be corrected downstream. That is why monitoring and controlling pH in pulp stock is critical to the paper making process. Essentially, at every stage in the manufacture of paper, correct pH values play a vital role.

Descripción General:

Reverse osmosis (RO) is a separation process that uses pressure to force a solution through a membrane that retains the solute on one side and allows the pure solvent to pass to the other side. More formally, it is the process of forcing a solvent from a region of high solute concentration through a membrane to a region of low solute concentration by applying a pressure in excess of the osmotic pressure.

Descripción General:

Continuous technology improvement is ongoing in the pulp & paper industry to obtain the best possible performance. The improved plant performance translates to the higher quality improvement and lower cost, and simultaneously environmental friendly plant operation.

Descripción General:

Wastewater from electroplating facilities and certain types of chemical plants contains toxic forms of hexavalent chromium such as chromate and dichromate. The hexavalent chromium in this wastewater must be reduced before the water can be discharged. This requires a two-step process: hexavalent chromium (CR6) is reduced to trivalent chromium (CR3); and CR3 is precipitated as chromium hydroxide.

Industry:Electrical and Electronics

Descripción General:

Removal of free oil and grease from a wastewater stream reduces the potential for equipment problems to occur further downstream. There are three forms of oil encountered in wastewater treatment at a refinery. 

Notas de Aplicación
Descripción General:

Sour Water is the wastewater that is produced from atmospheric and vacuum crude columns at refineries. Hydrogen sulfide and ammonia are typical components in sour water that need to be removed before the water can be reused elsewhere in the plant. Removal of these components is done by sending the sour water from the process to a stripping tower where heat, in the form of steam, is applied.

Descripción General:

Power plant boiler houses designed to burn coal or high sulfur oil are required by Federal and State pollution regulations to "scrub" (remove) sulfur dioxide from flue gasses to meet emission limits. SO2 in flue gasses is known to be harmful to the environment, as it is one contributor to the formation of acid rain. pH control is critical for the proper functioning of the scrubber system.

Descripción General:

Seawater leak detection is the post-condensation water quality management processes. Damage to the ion exchange resin, which deionizes the supplied water, is also monitored during this process, and both of these applications are executed by a conductivity analyzer. (AN10D01P01-01E)

Descripción General:

In the past, the boiler feed tank systems in sugar factories had to be checked several times a day to make sure there were no sugar solution leaks. This was a very laborious process and, as continuous monitoring was not possible, monitoring results were not reliable. When a leak occurred, recovery operations were very costly and time-consuming. (AN10D01K01-02E)

Notas de Aplicación
Notas de Aplicación
Descripción General:

In flue gas desulfurization systems that use magnesium hydroxide (Mg(OH)2) slurry, the consumption of the desulfurization agent (Mg(OH)2) is controlled by using online pH analyzers. A great concern in the pH measurement is heavy staining of the pH electrodes by the Mg(OH)2 slurry. To ensure accurate measurement, frequent cleaning of the electrodes with an acid is required, adding to both maintenance workload and cost.

Industry:Chemical, Power

Descripción General:

To defray energy costs, many industrial plants have their own boilers to generate steam in order to produce a portion of their energy needs. In addition to generating power, the steam may also be used directly in plant processes or indirectly via heat exchangers or steam jacketed vessels.

Notas de Aplicación
Notas de Aplicación
Notas de Aplicación
Descripción General:

Fish perform all their bodily functions in water. Because fish are totally dependent upon water to breathe, feed and grow, excrete wastes, maintain a salt balance, and reproduce, understanding the physical and chemical qualities of water is critical to successful aquaculture. To a great extent water determines the success or failure of an aquaculture operation.

Descripción General:

In stable dispersion of fine particles is the pre-requisite for the selective flocculation technique involving separation of ultra-fine valuable particles from the gangue. Among mineral processing techniques selective flocculation technique is known to have outstanding potential of capturing the particles of particular mineral in slurry of mixed mineral system by selective adsorption of water soluble polymers known as flocculants.

Reporte Técnico de Yokogawa
Descripción General:

Yokogawa has been offering the EXA200 series two- wire liquid analyzer for processes since 1990. This analyzer has been used in various fields including the quality control of raw materials in process plants in the electric power and petrochemical industries, reaction management of products, quality control in waste water facilities, and quality monitoring of river water and tap water.


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