Accurate measurement of pH in process environments

This article highlights certain recent advances which have helped to ensure that today's pH measuring system is effective in meeting current manufacturing needs.

Accurate measurement of pH in a Process environment is often a crucial factor affecting the yield and quality of products used in the manufacture of chemicals, pharmaceuticals, foodstuffs and beverages as well as acting as an important monitor for water treatment and effluent control.

Although the pH sensor has been around a long time, designing for use in the exacting conditions of the modern production process continues to tax the ingenuity of the R and D group bringing together as it does many disciplines including electrochemistry, glass and materials technology, connector design, metallurgy, polymer chemistry, etc.

Commercial pressures in manufacturing industry together with advances in production technology have placed increased pressure on manufacturers of pH measuring systems who have responded with both major developments and iterative improvements to sensor design, housing technology and transmitter function.

This article highlights certain recent advances, which have helped to ensure that today's pH system is effective in meeting current manufacturing needs.

While the technology to incorporate a Pt 100 or Pt 1000 sensor inside the body of a pH electrode has been available for some time the sensors on the market did not have a suitable IP 67 plug and socket.

Fixed wire versions were available but for industrial use this meant that sensor replacement required recabling back to the transmitter, often a time-consuming and costly retrunking job.

Now with the development by Mettler Toledo Process of the VP6 universal 6-pin connector combined pH and temperature probes with detachable cable connection at the sensor end are a reality.

The reference electrode and junction design is critical to the accurate long-term reliability of a pH sensor in an industrial process.

Modern designs use silver/silver chloride reference elements, which means that the reference electrolyte typically contains a concentration of silver ions.

If used in media containing sulphur compounds, e.g effluents, fermentation media, chemical process materials etc the sulphur combines with silver at the diaphragm progressively depositing silver sulphide in the pores of the diaphragm causing blockage and consequent gross signal drift.

Colloquially known as the 'black death'.

To prevent this damaging effect Mettler Toledo chemists have designed a silver ion free system by fitting an ingenious patented silver ion trap to the silver/silver chloride reference element.

Fitted today to both liquid and gel filled pH sensors the silver ion trap technology eliminates risk of silver sulphide contamination for the life of the sensor ensuring drift free measurement even in the most heavily contaminated media.

Sensor lifetime in process is a vital consideration for both commercial and process reasons.

Indeed in some heavily aggressive media pH sensor lifetime in the process can be measured in hours rather than days.

In long term or continuous processes, e.g., fermentations, effluent treatment and food processing, pH sensors may be required to remain in the medium for weeks on end.

The answer in these circumstances is to design a retractable housing which allows the sensor to be isolated from the process for regular calibration, maintenance or even replacement while the process continues.

Critical design areas where recent improvements have enhanced the performance of today's retractable have focused on three areas: Positive position indication.

In fully automated systems it is essential that a feedback signal is given from the retractable housing to the controlling PLC.

The latest versions of Mettler Toledo retractables housings are available with the option of pneumatic or inductive end-position indicators, which provide closed loop operation.

Internal seals.

The retractable member containing the pH electrode 'lives' in the process when the sensor is in the inserted position.

It is essential that the member seals are not exposed to the process medium, especially where the medium is chemically aggressive, otherwise the O-rings can become deformed, swollen or hardened leading to mechanical blockage and malfunction next time an attempt is made to retract the member.

The latest Mettler Toledo Process designs include a patented O ring positioning which minimises seal exposure and prevents potentially catastrophic jamming on retraction.

Operational safety.

Another important aspect of housing design is to ensure that process medium is prevented from escaping from the line or vessel during retraction and reinsertion.

The engineering design of the 'lock-chamber' on the latest Mettler Toledo range of housings incorporates a patented seal system to ensure maximum safety and protection at all stages of the process.

To complete the picture transmitter design has developed in several important directions.

Smart diagnostics provide both early warning of sensor failure and data on calibration periods essential to ensure that the measured value remains correct throughout the production batch.

HART protocol ensures that the transmitter fits into the process control system in the modern plant and Ex versions are becoming more widely available for applications of pH measurement in hazardous areas.

Cost is always an important consideration when choosing a transmitter standard for process measurement and latest electronics designs provide fully featured transmitters at highly attractive prices.

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