Quadruple Sensor Lifetime with a Retractable Housing

By Mettler Toledo CE |

A housing is not just there to hold a sensor firmly in place. With the right housing you can clean, calibrate or exchange sensors without stopping or contaminating the process. You can also dramatically increase sensor lifetime.

Select the right sensor housings for your processes
The measurements from in-line analytical sensors are invaluable in the control of many pharma processes and for ensuring APIs meet regulatory requirements.

Selecting appropriate sensors for each application is therefore critical. But one aspect of in-line measurement often does not get sufficient attention – sensor housings.

The role of a sensor’s housing is not just to securely hold the sensor in the process pipe or vessel. Depending on the application the housing must be capable of tolerating high process pressures and temperatures, withstanding aggressive chemicals, preventing process contamination, allowing access to the sensor without process interruption, or even cleaning and calibration of the sensor while the process is running.

When choosing a sensor housing, here are some things you need to consider.

Is the batch run less than a day?
Generally, short batches (less than a day) are relatively low risk, i.e. the cost of losing the batch is not high, and the conditions in the process are not severe (no sensor exposure to high process temperatures, SIP or sticky / fouling media). For these a stationary housing will be adequate. Between batches the sensor can be removed from the housing for cleaning and calibration.

However, if the cost of the batch is high, and/or the medium could foul the sensor and interfere with measurement accuracy, a retractable housing would be more efficient.

Is the batch run more than a day?
Long batches (more than a day) tend to involve higher risk. The cost of losing the production batch is often significant, so batch integrity becomes very important. This means greater reliance on sensor performance. For example, measurement drift of a pH sensor due to extended exposure to high process temperatures could severely affect process control. Contaminants affecting measurement or the presence of sticky material may also result in inefficient process control. Sensor failure, and hence complete loss of the analytical measurement, during the process could result in batch loss.

When process conditions are likely to affect the quality of an in-line measurement, a retractable housing is the better option. Such housings allow access to the sensor for cleaning, calibration or replacement while the process is running. Further, the ability to periodically withdraw the sensor from the process has an additional benefit. If the process conditions are extreme and continuous measurement is not essential, the sensor can be withdrawn into the housing and reinserted for five minutes every half-hour, or by whatever schedule is appropriate. Such an approach can increase sensor lifetime fourfold or more.

Are SIP cycles performed?
In addition, retractable housings allow removal of the sensor from the pipe / vessel during SIP. An SIP process can last for 30 minutes or longer. Exposing a pH / ORP, dissolved oxygen or dissolved CO2 sensor to high temperature SIP conditions for such a period will not only cause sensor drift (necessitating calibration verification), it will also significantly reduce sensor lifetime.

When a sterilization is performed empty (the sensors are not in contact with a liquid during the sterilization phase), the lifetime of pH sensors is rapidly reduced. Performing sterilization in a liquid is generally less aggressive, but high temperature remains the most important parameter, with pressure variations, affecting sensor lifetime. When sterilization is carried out in an empty vessel, it is therefore preferable to use a retractable housing to sterilize sensors, particularly if experience has shown that the sterilization severely impacts sensor lifetime.

Even if sterilization is performed in the liquid phase, using a retractable housing presents several advantages, including less stressful conditions for the sensor.

Is the environment sterile?
In some situations it is advantageous to be able to sterilize the sensor within the sensor housing. If the housing allows, this means a failing sensor can be extracted from the process and replaced with a new sensor. The replacement sensor can be sterilized within the housing before being inserted into the process, therefore maintaining sterile conditions.

Is sensor cleaning performed regularly?
If the process medium has a high particle load or easily adheres to the sensor, regular cleaning may be required. This can be very time consuming for maintenance staff. An automatic sensor cleaning system combined with a retractable housing can alleviate the burden. Certain systems can also automatically calibrate pH sensors.

We offer a range of retractable housings to meet all requirements in pharmaceuticals production. Features include the Tri-Lock™ safety system that prevents the release of process media during sensor retraction / exchange; pneumatic control; and compatibility with EasyClean™ automated cleaning / calibration systems.

This white paper covers several process considerations that can help you to select the right housing for maximizing the life of your in-line analytical sensors.

Retractable Process Adaptions (Housings)
Process analytical solutions for enhanced process safety

•   Intelligent sensor locking system – If no sensor is installed, no action is possible, preventing unwanted operation.
•   Highly efficient cleaning chamber and large bore size design ensures particle/ dirt is flushed away.
•   The complete retractable housing conforms with ATEX (94 / 9 / EC) directive requirements.
•   Timesavings due to electrode/sensor maintenance and replacement without process downtimes.
•   Versions with manual or pneumatic operation, as well as automated cleaning and calibration.
•   Models for fully hygienic/sterile operations for process reliability.
•   Versions with integrated sensor detector for enhanced safety.
•   Process-independent maintenance, replacement and cleaning/(re-)calibration of electrodes/sensors without jeopardizing process safety, sterility/hygiene.

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