Safety first

The author: Wolfgang Babel Managing Director, Krohne Analytics

Measuring pH is the most important analytical measurement when it comes to determining quality and controlling processes. Approximately 50 % of all measuring points in analytics are for pH. Every year, more than a million pH sensors are manufactured for process instrumentation. With so many measuring points, it pays to take a look at the factors influencing the reliability of pH measurements and consider what manufacturers of analytical measuring technology have done to reduce the risks.

A typical pH measuring point consists of a pH glass electrode with a PG 13.5 sensor head or a ¾” NPT electrode in a plastic housing. pH measurement is a very sensitive, high-impedance step that generates voltage in the range from -400 to +400 mV, depending on the measured value. These high-impedance signals must be partially fed to the converter via double shielded cables. The analogue voltage value is converted first to a digital value and then to pH using the Nernst equation. In many cases, temperature compensation is performed in parallel using temperature sensors integrated in the electrode as Pt 100, Pt 1000 or NTC. Pure analogue signals are supplied by the sensor and all intelligent signal processing must take place in the transmitter (converter). Over the years there have been many attempts to solve the pH-specific problem of high impedance using connectors (Sixplug, VP, Top68, etc.) and cable designs or by means of the circuitry in the transmitter but the decisive breakthrough has never been achieved.

Due to various influences such as medium, temperature, humidity, environmental conditions and pollution and depending on the application, pH instruments lose their accuracy over time and need to be calibrated, cleaned, regenerated and ultimately replaced. The calibration cycles for all electrochemical sensors vary from daily to once a year.

Analogue sensors meant that up until 2004 calibration, cleaning and regeneration had to be carried out directly at the measuring site. This often presented special challenges for maintenance personnel, for instance if the measuring points were installed on boilers several metres high. In addition, poor weather conditions frequently had an adverse effect on the accuracy of the pH calibration. Even today, some 70 % to 80 % of all pH measuring points are still calibrated this way and the risk of error should not be underestimated.

Owing to the necessary signal conversion, the sensor and transmitter became an inseparable pair – a combination that had to be available for each calibration. More than 250,000 two- or four-wire transmitters are produced annually and it is mainly the two-wire version that is sold to the process industry. The transmitter is still the most complex and failure-prone component in the entire measuring chain, consisting of an electrode, cable, transmitter and PLC. If we consider the potential errors in the measuring chain according to FMEA or SIL, the transmitter accounts for a large percentage. The reasons for this include faulty wire connections during installation, incorrect inputs in the complex operating unit and defective electronics. These errors can only be rectified with a great deal of maintenance and testing; approximately 100 to 300 minutes of mainte-nance per year and transmitter used to be recorded on average.

The birth of the digital pH sensor in 2006 did little to change these safety factors. The idea of inductive data and energy transfer between the sensor head and the data coupling solves the problem of high impedance. However, owing to the inductive data transmission principle, electronics, memory and processors are required in the sensor head as well as in the coupling of the connecting cable. And since there are now also electronic components in the cable, the overall risk of failure was not significantly reduced. At the same time, the system became proprietary, displeasing some users who did not want to be dependent on one manufacturer.

It was only in the years that followed that the revolutionary options created by digital sensor technology for offline calibration were recognised. They enabled the lifetime of the pH sensors to be extended several times over and the error risk during calibration reduced by calibrating in the laboratory.

Thanks to the digital technology, from 2006 onwards a growing number of customers were keen to omit the transmitter and integrate the entire electronics in the electrode’s sensor head. For economic and technological reasons this wish remained unfulfilled – until today.

Smartsens sensors are two-wire sensors featuring 4 to 20 mA Hart 7 communication – the fieldbus continues to dominate in 2013 due to its widespread installed base – and IECEx certification for Ex Zone 0.

With its IECEx II 1G ia IIC T4-T6 (Zones 0–2) certification class, the Smartsens pH sensor is approved for use in Zone 0. The IECEx certification is the most comprehensive of all and satisfies the criteria for Atex, Nepsi, FM and CSA.

The first step includes sensors for pH, ORP and conductivity and various other parameters. Especially designed for hazardous environments and hygienic operating conditions, Smartsens sensors are equally suited to other industries.

With their 4 to 20 mA Hart 7 communication, Smartsens sensors also support a truly open standard and can be connected directly to the master display using a standard cable. The sensors can be operated with any popular handheld device in the market today, onto which the Hart DDs can be downloaded. In addition to Pactware, Smartsens sensors run with common asset management systems such as Emerson (ASM), Siemens (PCS7), ABB, Smar, Invensys, Honeywell, Schneider, etc. The current range of Smartsens sensors can be utilised without any problems whenever the installed base is VP or Sixplug. Handling and installation have been kept as simple as possible; the connector prevents the wires from being connected incorrectly.

The sensors can also be calibrated offline. In this case, Krohne uses a special open standard: a Smartbridge Hart USB cable supplies the sensors with power directly from the PC during calibration, meaning this connection can be used to communicate with the PC in both directions. The user interface for offline calibration is Pactware, a frame application which complies with the FDT/DTM standard.

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