Fieldbus pH/ ORP Transmitter/ Analyzer

What is Fieldbus?

Fieldbus Foundation defines "Fieldbus is a digital, two-way, multi-drop communication link among intelligent measurement and control devices." It is one of several local area networks dedicated for industrial automation. The Internet is having a major impact on information flows. Yesterday's hierarchical systems the field signals used in process instrumentation have been standardized so that control systems and field devices from a variety of suppliers can be interconnected using standard 4 to 20 mA analog signals. A "standardized" digital communication for industrial automation is changing the production system.

The FOUNDATION™ fieldbus standard developed by the Fieldbus Foundation™ constitutes the next level of standardization and it is designed to meet modern needs.  In addition to having interconnectivity equivalent to that available using 4 to 20 mA analog signals in a conventional field network, FOUNDATION™ fieldbus allows multiple devices to be connected to a single FOUNDATION™ fieldbus, permits the interactive communication of various types of information, and enables the distribution in the field of intelligent functions including self-diagnostics and control functionality.

When FOUNDATION™ fieldbus was first proposed, more attention was given to its ability to convey multiple field signals over a single cable and the benefit this would have in reducing wiring costs. But now the focus is on its ability to transmit various types of information in addition to field signals and to distribute intelligence to distributed field devices. These features enable remote monitoring, real-time self-diagnostics, and proactive maintenance of field devices, as well as plant resource management using field communication. This will enable the costs of operating instrumentation systems to be greatly reduced.

Flexibility, reliability, low maintenance and low installation costs are among the benefits of theFLXA21 pH/ORP analyzer. Designed to meet the exacting requirements of measuring pH and ORP in the modern industrial environment, it contains many features to ensure peak precision whatever the application.

This 2-wire (loop powered) analyzer is housed in a robust IP66/NEMA 4A field mountable case. The need for expensive cabling is minimized. Sensor diagnostics is enhanced by an improved impedance check, and the addition of a logbook feature. Calibration history is available in the display, and full logbook data is accessible through the HART bi-directional communication link, and is used to store important configuration, calibration and diagnostic data. Prediction of sensor failure is possible by reference to the logbook.

Microprocessor-aided calibration uses internal buffer tables and stability checking to ensure maximum accuracy with minimum effort. Process temperature compensation enhances accuracy in applications where the influence of temperature is seen in process pH changes. pH and ORP or rH measurements can be made simultaneously when an appropriate sensor combination is used.

Yokogawa has implemented three leading process Fieldbus technologies in their FLXA21 series: HART, Foundation Fieldbus H1 and Profibus PA. All transmitters are provided with the important and widely used Device Description files that provide information on parameters and other data in each device, as well as the ability to include algorithmic relationships such as calibrations.

  • Universal pH/ORP.
  • Possible to input high impedance reference electrode.
  • On-line sensor checking.
  • Process temperature compensation.
  • Differential input amplifier with equipotential screening.
  • Freely configurable ITP, slope and asymmetry.
  • Easy to use EXA control panel.
  • Password protection for all levels of software.
  • HART®, PROFIBUS-PA, FOUNDATION Fieldbus H1 communications.
Input ranges pH: -2 to 16 pH
ORP: -1500 to 1500 mV
rH: 0 to 55 rH
Temperature: -30 to 140℃
Output ranges pH: Min. 1 pH, Max. 20 pH
ORP: Min. span 100 mV, Max. span 3000 mV
rH: Min. 2 rH, Max. 55 rH
Temperature: Min. 25℃ , Max. 200℃
Temperature Compensation Range: -30 to 140℃ (for 8k55 sensor -10 to 120℃ ).
Sensor types: Pt100, Pt1000, 3 kΩ RTC, 5.1 kΩ PTC, 8.55 kΩ NTC, 350 Ω PTC, 6.8 kΩ PTC, 10 kΩ PTC. Automatic or manual compensation to Nernst equation. Process compensation by configurable coefficient. Adjustable ITP (Iso-thermal point of intersection).
Calibration Semi-automatic, using tables in transmitter for pH 4, 7 & 9 buffer solutions, or using user-defined tables, with automatic check of measurement stability.
Manual, using standard sample, by correcting reading to value of standard.
Calibration by slope and asymmetry potential setting. (IEC60746-2)
Communication FOUNDATION Fieldbus H1
Ambient temperature -20 to 55℃
Humidity 10 to 90 % RH (Non-condensing)
Display Black/White 213 x 160 pixels LCD with touch screen
Power supply Nominal 24 volt DC loop powered system.
Output Signal Digital communication signal based on FOUNDATION Fieldbus protocol
Function Block Three AI blocks
Enclosure Material Plastic (Polycarbonate)




±0.01 pH
±0.01 pH
±0.01 pH




±1 mV
±1 mV
±1 mV


The lifetime of a pH sensor has a significant impact on the overall annual costs of a pH measuring loop. Optimizing four key factors will decrease these costs and optimize process control and overall plant efficiency.

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