A Blueprint for Measurement

Originally published online on Jun 19, 2007 – Author: G. Raymond Peacock, Temperatures.com Inc. – Publication: “Sensors”

I’m a measurement guy. I believe making competent, reliable measurement within acceptable error tolerances is at the root of efficient automation.

This approach is known by various names, including statistical process control and Six Sigma. Both are prominent in automation’s lexicon, and for good reason.

With the Defense Department and the automotive, aerospace, and pharmaceutical industries demanding strict adherence to this philosophy, even the casual engineer has to sit up and take notice.

But measurement isn’t as simple as you may think.

Measurement Span Requirements
There’s a lot to do before making measurements, not the least of which is selecting a suitable sensor for the application.

To help you with this task, I’d like to revisit an article I wrote for Sensors titled “A Twelve-Step Sensor Selection Checklist”. The discipline embodied in the checklist is based on the premise that selection of the sensor requires up-front planning to ensure you do the job right the first time.

In the first of the 12 steps, you establish measurement span, or range, requirements. If you are replacing a sensor, the job should be a little easier, but that doesn’t mean you should replace the old sensor with the same type of device.

The technology and measurement requirements may have changed, or there may be ways to improve the process with a more capable sensor.

So now is the time to ask some questions:

* What process/product requirements affect the overall measurement range or span? Are they as claimed by the process staff? Are they the same as before (if a replacement)?
* Are there less costly, more capable sensor types than the one originally specified for the task? Will their ranges cover your needs?
* Can you minimize inventory requirements in multi-unit installations by using a single model with a wider span that would work in more than one location?
* Should you consider using two units in each location in a critical process? This approach guarantees that the failure of one sensor will not shut down the line, and if one starts to drift, you can alarm on significant reading differences.
* What role does sensor response time and product speed play? What does that mean for your application—possibly different span options? For example, in cases of IR radiation thermometers (pyrometers), you have to consider tradeoffs among several additional instrument parameters that affect measurement range (e.g., response time, operating waveband, and measurement spot size at a given distance).
* What are the physical size, shape, and weight options? Will the available space(s) accommodate typical devices?

Everyone knows the span/range requirements until they get down to the nitty-gritty. Then things get fuzzy. To avoid problems, get a complete handle on the actual application requirements.

More to Come
The remaining steps of the checklist call for:

* Setting accuracy and uncertainty goals
* Identifying factors that influence measurement conditions
* Making a list of sensor candidates
* Reviewing error analyses and tradeoffs
* Finalizing the measurement specifications (insist on traceable calibration certification; it’s the only way you can reliably trace a measurement back to an approved reference)
* Solicit bids
* Select both device and vendor
* Inspect
* Install
* Commission
* Verify Online

I plan to examine each of these steps over the next year and to expand on the original content of the article. In my next essay, I’ll examine accuracy and uncertainty in depth.

Bye for now. See you next month.

This article and the balance of the series are online, as of December 2008, at www.Sensormag.com. They are reprinted here with links to the originals as a series and with the kind permission of the publisher of SensorsMag.com, Questex Media Group, Inc. It has been modified slightly in format to make it more readable on these Web pages.

By The Way (BTW), there’s a short but sweet little article with links to some other, related ones on selecting a temperature sensors at my original web site “About Temperature Sensors”, just CLICK HERE to see it.