In this sixth essay of my series on selecting sensors, we get to the real meat and potatoes of the job—writing a workable specification. If you've been following each step of the process outlined in the series, you realize you can't just pick a product out of a catalog.
It may seem as if it's been a long and tedious task to reach this point, but you are now better prepared to write a spec that will get you the measurement you need. Whether the price tag is high or low, in the end you will sustain the lowest cost because you won't have to start over as a result of a poorly selected sensor that fails to meet measurement requirements.
Traceable Calibration
In my original article, "A Twelve-Step Sensor Selection Checklist", I put traceable calibration last in the list of steps to follow. It really belongs first. To paraphrase the book that has become my measurement bible, Traceable Temperatures (ISBN: 0 471 49291 4), traceability is not something you sort out after a measurement is made or a device purchased. It is something you plan for at the outset of any significant measurement task.
If you cannot link the measurement to a national standard or a repeatable reference definition, you are sunk before you begin. Be sure to insist in the specification that your sensor have a calibration certificate that states the calibration uncertainties in the range of intended use (plus a bit extra) and the traceability links. The certificate should also state the method used and include the serial number and calibration certificate identification numbers of the reference device. When in doubt, check typical certificate recommendations from NCSL International, the National Institute of Standards and Technology, or those vendors that have their calibration capabilities certified by the National Voluntary Laboratory Accreditation Program or the American Association for Laboratory Accreditation.
Other Things to Remember
Be sure to include in the specification all the special items and spares needed for protection, data interfaces, power supplies, air or water filters, and all the key elements relating to the proper installation and long-term support of the sensor. Don't forget the special tools and training required for those charged with maintaining the sensor system.
Items sometimes overlooked by a new staff member are the corporate safety and power requirements for any system installed in an operation. Be sure the equipment you specify meets these demands. You may have to contact the corporate engineering and/or safety group to find the related documents. ISA has contributed greatly to safety standards and sells many of them on their Web site.
The chemical industry also publishes a huge amount of safety information, some of which you may need to know about in detail, depending on the product you are measuring and the potential hazards your sensing system may introduce into a facility. Check out Interactive Learning Paradigms' 100 free Web resources on material safety data sheets.
If your sensor is to be installed in a location where flammable liquids and/or gases are present or could be present, you should review your preliminary specs with the local safety officer. You may need special isolation devices for your sensor or its support items, but you should have identified them already. They belong in your spec, too.
Additionally, if your sensor contains radioactive materials or potentially hazardous chemicals, the safety and/or nuclear safety officer will want to know about it. He or she can advise you about posting proper warnings and notices, special labeling, and action plans for proper disposal of any portion of sensitive items removed from the installation. Radioactive items require special handling and are required by law to be inventoried at several levels. The Nuclear Regulatory Commission maintains control over medical and industrial radioisotopes.
An engineer I knew actually found a radioactive thickness gauge in a trash bin at a facility where we worked. It was part of an obsolete instrument that was to be scrapped. No one had told the janitor about proper disposal procedures. Aside from the possible hazard to third parties, the company could have been subject to significant fines from state and federal nuclear regulatory agencies.
In Closing
When you estimate the total cost of a sensor implementation, especially in an industrial environment, you'll often find that the actual sensor is one of the lowest priced items in the total specification. One installation of a simple video camera in the pouring shed of a steel mill required lengths of cable so extensive that an additional video repeater amplifier was necessary to support the camera. Cable, repeater, and installation labor costs far exceeded the cost of the camera.
Remember: It never hurts to be too detailed, but it always hurts to miss something important and find out after the fact.
Good luck, and great spec writing!