Sensors: Make Sure They Suit Your Application

Posted by Ray Sullivan on Tue, May 14, 2013 @ 11:14 AM

At times we see sensor specifications in plant upgrade or new treatment plant projects that don't appear to suit the particular application in the most cost effective manner.

Usually we see a pH sensor specified for potable water treatment with characteristics that far exceed the requirements of the process fluid. The sensor will work satisfactorily, but it costs much more than it should.

For one project a non-contacting torroidal conductivity sensor was specified for a non-corrosive surface water application with low conductivity, about 500uS.

There were several problems with this:

  1. As conductivity goes down, the uncertainty of the measurement for inductive type torroidal conductivity sensors goes up significantly.

  2. Inductive sensors are primarily a good choice for cases where there are very corrosive acids, bases or electrolytes that would otherwise quickly degrade the metal electrodes on a contacting conductivity sensor.

  3. Inductive sensors are an excellent choice to measure VERY high conductivity samples.

  4. Inductive sensors are a good choice to avoid fouling, bubble formation in the measuring cell, or the accumulation of debris.

None of these conditions applied to the application.

Providing more than is expected or paid for is a good thing for customers. Paying more for something that's not needed is not so good. Paying double for decreased accuracy and unneeded features, as in this case, just can't happen for municipalities with tight or shrinking budgets and for all of us who live in those municipalities.

For this project we proposed a contacting conductivity sensor with a cell constant and range appropriate for the application. The sensor design we offered, shown below, features a CPVC insulator for typical water treatment plant chemical resistance, 316 SS electrodes, double O-rings,and provides little if any potential for debris build up.

Foxcroft conductivity sensor resists fouling

 

Reducing sensor cost isn't achieved by reducing price or quality, it's achieved by selecting a sensor with appropriate characteristics. The benefit my be seen through better process control, less frequent cleaning, less frequent calibration, longer sensor life, or a lower purchase price.

For some applications, like fluoride resistant pH measurement, you may initially pay more than you would for a "competitive" sensor. But, you can actually lower your cost with our HF and acid resistant pH sensor by obtaining accurate measurements, replacing the sensor less frequently, and by surviving infrequent process upsets that wipe out most sensors when the pH drops dramatically.

Our sensor mentioned above does not contain antimony, extra thick or coated standard pH glass. Rather, it features a specially engineered pH glass, actually a glass like material, for fluoride resistance and a reference junction engineered to function in acid. It not only resists deposits on the glass surface, it can survive acid service and cleaning. Since this is now a "standard" sensor, you get a high performance sensor without a "custom engineered" price.

In many applications you may lower operating cost by simply matching sensor components to suit your process instead of using a "one size fits all" approach that may actually be over or under engineered for your particular type of water.

A customer monitoring wastewater for low pH influent with a lowest cost general purpose probe can dramatically increase sensor life with a sulfide resistant submersible pH sensor.

It's important to note that not all sensors are designed, or, "created" equal. Many of the sensors used in our instruments were designed specifically, and rigorously field tested, to overcome longstanding difficulties in process measurement while providing accuracy and repeatability. Some of these sensors have no design or functional equivalent anywhere in the market.

The take away from this is to check that the sensor being offered to you is actually suitable and optimal for your process. Among many questions you should consider:

  • Is it accurate in the range required?

  • Does the sensor body material's chemical compatibility, pressure and thermal resistance match the process fluid?

  • Can the sensor withstand the process fluid, and perhaps more importantly, the chemicals and physical force needed to clean it? Even though your wastewater may have a mid-range pH, your pH sensor may need to withstand acid cleaning of scale buildup. In some cases we recommend using a 15% HCL solution because 5% dilute HCL solution is insufficient.

  • Does the reference junction and or ion selective membrane have the design and chemistry suitable for the application?

  • Can a sensor with an external preamplifer provide equal performance than a more costly sensor with an integral preamplifier? With this approach you buy the preamplifier one time instead of with every replacement sensor.

Before making your next sensor purchase, don't automatically settle for what is offered, you may be able to get better process performance while helping your staff's workload and budget.

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Tags: sensor, pH sensor, conductivity sensor, ISE electrode