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Analytical sensors with the transmitter in the sensor head


Today, some 70% to 80% of pH mea- suring points are still calibrated in this way and the risk of error involved is not to be underestimated.


Transmitter


Krohne documents the journey it has taken in the development of SMARTSENS, a line of digital analytical sensors with integrated transmitter technology. The sensors feature the transmitter in the sensor head, eliminating one of the greatest sources of error in the measuring chain Calibration


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n April 2013 Krohne introduced SMARTSENS, a line of digital ana- lytical sensors with integrated transmitter technology providing a direct 4/20mA and HART 7 output. This removes the need for the external transmitter, effectively eliminating one of the greatest sources of error in the measuring chain. Measuring pH value is the most important analytical mea- surement when it comes to determin- ing quality and controlling processes. Approximately 50% of all measuring


Due to various influences such as medium, temperature, humidity, environmental conditions and pollu- tion, and depending on the applica- tion, pH measurements 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. Because of the analogue sensors, cali- bration, cleaning and regeneration


The SMARTSENS sensors support an open standard with 4...20 mA Hart 7 communication and can be directly connected to the process control system


Due to the necessary signal conver- sion, the sensor and transmitter became an inseparable pair: this com- bination must be available for each calibration. More than 250,000 2- and 4-wire transmitters are produced annually, with mainly the 2-wire transmitters sold to the process indus- try. The transmitter is still the most complex and failure-prone element in the entire measuring chain consisting of electrode, cable, transmitter and PLC. If you consider the possible error in the measuring chain according to FMEA or SIL, the transmitter repre- sents one of the main sources of error based on percentage. Reasons for this include faulty wire connections during installation, faulty entries in the complex Operating Unit and faulty electronics. These errors can only be rectified with a great deal of mainte- nance and testing costs, on average approx. 100 to 300 minutes of mainte- nance per year per transmitter were recorded.


The digital pH sensor


Little about these safety factors changed with the birth of the digital pH sensor in 2006. The idea of induc- tive data and energy transfer between sensor head and data coupling solves the problem of high resistance. However, because of the inductive data transmission, electronics, memory and processors are required


‘Thanks to digital technology, starting in 2006 many customers had the increasing desire to omit the transmitter and integrate the entire electronics in the sensor head of the electrode. For economic and technological reasons this remained a wish - until today. Now, Krohne SMARTSENS analytical sensors feature the transmitter in the sensor head, considerably reducing possible error sources in the measuring chain’


points in analytical measurement are pH. Every year more than 1 million pH sensors are manufactured for process instrumentation. With so many mea- suring points, it pays to take a look at the factors influencing the safety of a pH measurement and what actions manufacturers of analytical measuring technology have taken to reduce risks.


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had to take place on site at the mea- suring site up until 2004. This often presented special challenges for maintenance personnel as sometimes the measuring points were installed at locations several metres high. In addition, frequently poor weather conditions contributed greatly to the inaccuracy of the pH calibration.


in the sensor head and in the coupling of the connecting cable. And since there are now electronics in the con- necting cable as well, the overall risk of failure has not been significantly reduced. At the same time, the system becomes proprietary which displeases some users who do not want to be dependent on one manufacturer.


JULY/AUGUST 2013 Process & Control


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