Efficient automation is built on reliable measurements that fall within acceptable error tolerances. Whether your approach is called statistical process control or Six Sigma, the very root of measurement quality depends on selecting the right sensor for the job. To do this, you'll have to pick a sensor that meets your measurement span requirements.
Measurement Span
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 handle on your application's operating parameters.