Picking the Right Sensor

Ray Peacock

Before you can select an appropriate sensor for an application, you must consider the big picture. After all, the sensor is part of a larger, multifaceted system. For the sensor to work well, you must define its specifications within this context. Therefore, you'll have to keep a number of things in mind.

Installations and Interfaces
Make sure the sensor you select includes installation and interfacing components that meet the needs of the application. A few years back, I was involved in a project in which we had to run signal and power cables 30 m, from a shielded instrumentation room to the measuring device. The cable was installed in an extremely hostile environment, where the ambient temperature ranged from nearly 0°C to over 100°C. To operate in this environment, the cable required a unique construction (as did the sensor), multiple conductors, and a special connector. The cable supplied with the device was only 10 m in length, so we had to order (and wait for) a longer replacement after the original equipment had already been delivered.

No one gave serious thought to installation before the actual work began. That cost us time and money.

Calibration and Maintenance
Also, be sure you include calibration and maintenance requirements in your specification. If the sensor or its support equipment needs water- or air-cooling, purging, or specialty gases for calibration or operation, you must add the necessary long-term support items to the specification. For instance, cooling and purging air can require dewatering and filtering to ensure supply quality. That calls for an alarm to remind you to change or backwash the filter and periodically inspect the system to ensure it is operational.

If reliable operation during long production runs is an essential factor in a sensor choice, seriously consider how you can maintain measurement integrity in the event of sensor failure. Two options are redundant sensor installation and/or readily available spare parts. With the first approach, installing two sensors at the same location allows the control computer to compare the readings of the parallel devices, accept the higher reading, and send a maintenance alarm when the difference between the two goes beyond a preset threshold or if both fail. If you choose the second approach, you must determine what kind of sensor best supports a quick change if failure occurs. Can it be done without shutting down production?

In the End
All these factors bear not only on the sensor selection but also on the necessary accessories and spares. These considerations need to be factored in before a final specification is drafted. They can significantly affect the order size, budget, and installation, not to mention the possible discounts your purchasing department can negotiate with the prospective suppliers.

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