Sensor Selection-A Question of Perspective

E-mail Ray Peacock

This is the fourth essay in a series expanding on an article I wrote for Sensors titled "A Twelve-Step Sensor Selection Checklist". In October, I discussed the need to consider factors that influence measurement conditions. This month, I'll talk about other considerations and best practices for making a list of sensor candidates.

Don't Forget the Big Picture
Before you can get a clear idea of the sensor you need for your application, you must see the broad perspective. Remember, the sensor is part of a larger, multifaceted system. So, you'll have to keep a number of things in mind.

For example, make sure the sensors you select for your list include 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 meters, 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 meters 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, and that cost us time and money.

Make sure you also 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 have cleanliness requirements and often need 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?

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.

It's a Team Effort
In putting together your list of sensor candidates, you may prefer a sensor system from Company A because you like their products. At the same time, the plant maintenance staff may want another sensor because they have a well-oiled working relationship with its manufacturer. It's been my experience that if maintenance shows the least preference for Company B's products, you'll probably wind up buying its product.

Also, let the maintenance staff review where the sensor will be located, mounted, and supplied with the necessary coolant or gas purging. They will know of any special considerations that need to be included in the specification.

Get feedback from the people responsible for installation. You may need extra-long or special cables or possibly a wireless transmitter. You want to determine if the power in the operations area needs regulation or filtering. If you don't know, the installation folks will.

Well, that's it for now. See you next time!


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