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.

    See More



Measurement of pH in high purity water presents many difficulties. Yokogawa's pH instrument's design success comes from addressing the unique challenges of the application.

Application Note

Conductivity measurement can be used as a reliable indicator of the real-time brine concentration. Using an online process analyzer removes the need for timely grab sample analysis. 


Dairy wastewater is a mixture of organic compounds, suspended solids, fats, and bacteria that can produce a film or coating on equipment in the process. 


SMART digital sensors and advanced data management software enhance reliability and process safety.


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.

Application Note
Application Note

The level of dissolved oxygen is critical to the quality and consistency of any brew. Too little dissolved oxygen results in the fermentation step of the process will lead to: off flavors (e.g. poor removal of diacetyl and acetaldehyde), poor yeast crop in terms of quantity and vitality, and low ester and alcohol production. 


Yokogawa’s Process Liquid Analyzers series help pigment producers achieve a more reliable and accurate analysis of pH during industrial bioprocesses, improving product quality with less total operation costs.


Ion Exchange is a method for the exchange of ions between two electrolytes or between an electrolyte solution and a complex molecule. In most cases the term is used to denote the processes of purification, separation, and decontamination of aqueous and other ion-containing solutions with solid polymeric or mineralic ion exchangers.

Application Note

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.


The kraft process, also known as kraft pulping or the sulfate process, is a technology for conversion of wood into wood pulp that consists of almost pure cellulose fibers. Today, the kraft process is used in approximately 80% of paper production.


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.


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.


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.


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. 


Clean-in-place (CIP) is the system designed for automatic cleaning and disinfecting in the food & beverage, pharmaceutical, and chemical industries. Tanks and piping are cleaned and sterilized with various cleaning solutions, fresh or hot water, or steam after manufacturing products is completed.

Application Note

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. 


The detectors used to perform dissolved oxygen (DO) measurement in oxidation ditch type sewage treatment plants tend to become dirty quickly and require frequent cleaning. It has been d ifficult to remedy this problem inexpensively. One solution that significantly reduces the sensor maintenance workload is the use of float holders.


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


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.


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.


Many Ethanol plants running today are using a combination style pH electrode with a non-flowing reference to measure pH in the Mash Slurry transfer line from the Mash slurry mix tank to cook. The Mash is being pumped out of the Mash Slurry tank is at approximately 180 °F and 40 to 60 psig.


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


Removal of free oil and grease from a wastewater stream reduces the potential for equipment problems downstream. Three forms of oil are encountered in wastewater treatment at a refinery: Free Oil, Emulsified Oil, Dissolved Oil.


Yokogawa’s conductivity transmitters and converters possess USP functions that make this seemingly complex and troublesome requirement pain-free and automatic.


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.


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.

Application Note
Application Note

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


To defray energy costs, many industrial plants have their own boilers to generate steam 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. 

Application Note
Application Note
Application Note

Fish are totally dependent upon water to breathe, feed, grow, maintain a salt balance, and reproduce. Water quality determines the success or failure of an aquaculture operation.


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.


Heat exchangers are devices that provide the flow of thermal energy between two or more fluids at different temperatures. Heat exchangers are used in a wide variety of applications. These include power production; process, chemical and food industries; electronics; environmental engineering; waste heat recovery; manufacturing industry; and air-conditioning, refrigeration, and space applications. Yokogawa offers a means to control heat exchanger leakage and prevent costly maintenance, repair and downtime with an easy-to-use, virtually maintenance-free condensate conductivity measurement method.

Why is pH Calibration important? How to clean pH glass? Which pH buffers should I use?  How fresh should pH buffer solution be?  How long between calibrations?
Edition 1

Why is pH Calibration important? How to clean pH glass? Which pH buffers should I use?  How fresh should pH buffer solution be?  How long between calibrations?

Without using the BA11 active junction box the maximum distance is 100 meters. When using the BA11 it is possible to extend a further 100 meters. Making it a total of 200M. ...
To prepare the sensor for installation follow the below steps: Carefully remove the watering cap. Rinse the sensor with water. Check the interior of the pH glass membrane for air bubbles. Allow any bubbles to rise to the top by shaking the sensor ge...
The contacting and inductive sensors can both be measured using the FLXA202 analyzer but they require a different input module.  The input module required varies based on the model number of the FLXA21/202.   Look at the data...
All Yokogawa analyzers use an open architecture with adjustable temperature compensation, adjustable isopotential point, and adjustable slope. They are compatible with all direct pH, ORP, and conductivity sensors. There are two exceptions to th...
To test the conductivity input do the following:  Write down the cell constant and change it to 1.00 (this is not mandatory, it makes the math easier)  Place a jumper wire between 13 to 14 and another one on 15 to 16 (not necessary on t...
First, check the grounding. A bad ground can cause this failure.  Next: cycle the power to the unit. Shut the power off for 30 seconds then restart.  If that doesn’t clear the error reset the analyzer to factory defaults. Be ...
Yes, the input module can be changed in the Converter.    See the FAQ: Which Input Module do I order for my FLXA21/202 analyzer? ...
In a dual input FLXA202 or FLXA21 analyzer you can use a HART Splitter to convert the HART® dynamic variables into current outputs or contact outputs. The model number is "1W-KFD2-HLC-EX1.D" and the description is "P+F HART Splitte...
Yokogawa's pH and conductivity sensors include a temperature sensor to allow for temperature compensation. The pH and conductivity sensors have a relatively large temperature mass and respond slowly to the changes in the process temperature. This...
Yes, the SENCOM Module in these sensors allows a Modbus Master to connect using Modbus RTU protocol and access the following data: pH Temperature compensated pH ORP pH compensated ORP rH Temperature Junction resistance value Sensor details ...


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