Transportability of measurements

In noncontact temperature measurement, accuracy trumps repeatability.

By Ralph G. Rudolph

PlantServices.com

On many production lines, critical product properties must be measured and controlled carefully to achieve the desired product quality. For instance, when producing steel sheet that the auto industry stamps into fenders, doors and other panels, the mill must control many variables. These range from the initial chemistry of the molten steel to the way the final cold-rolled sheet is processed. Failing to control some variable can mean the stamped door panel can’t meet shape specifications. If it’s too thick, has variable thickness or lacks the correct strength or hardness, it won’t perform properly in the stamping die. It’s all about springback -- the tendency of bent or shaped metal to resist mechanical deformation.

There’s a lot more to it, though. Even the steel’s microscopic surface roughness must be controlled because the roughness profile determines how much lubricant the part retains during stamping, which relates to tearing defects and paintability issues.

For any value-added product -- meaning specifications are so tightly controlled that customers pay extra if the product is guaranteed to perform as advertised -- hundreds of measurements often must be made and controlled during processing. Such value-added products are where manufacturers make most of their profit.

Two places at once
But, most plant professionals are unaware of the problem of transportability. It’s a phenomenon that often prevents achieving the guaranteed properties in more than one place at a time. I’ll take an example from the steel industry.

A large steel mill developed high-strength steel that must be subjected to, among other things, a precise time-temperature-thickness reduction cycle to achieve the desired properties. Months of metallurgical studies determined the limits of temperature and other variables that achieve those properties.

But these studies were based on the specific instrumentation that was used to measure the variables. Another steel plant tried to produce the identical product using the “same” measured variables for its production and was surprised to find that its product didn’t meet specs. Plant management found this troubling. From a bottom line vantage point, which is what it’s all about, a single coil of off-spec steel might represent a $50,000 loss. Produce more than a few off-spec coils and people begin to holler. That’s the sound of real money talking.

My contract with the plant involved an investigation that revealed different instruments and settings were being used at the two plants to measure the same critical temperatures. This is important because the noncontact radiation thermometers (pyrometers) being used might be different types (single color versus ratio), might use a different wavelength, might be adjusted (correctly or otherwise) at a different emissivity, might be sighted at a different angle, might exhibit different atmospheric effects, or might have different reflection effects. Any of these factors can alter the measured temperature.

For example, the first plant might determine the desired temperature range to be 1,250°F to 1,350°F, but the second plant might measure it as 1,350°F to 1,450°F. And the difference isn’t necessarily linear. The process criteria developed at the first plant weren’t transportable to the second plant without modification.

This isn’t meant to imply that the measurements at either plant were correct. The second plant might need to run its own metallurgical tests for months to determine what settings it must use for its particular instrumentation and settings to achieve the same results as the first plant. The delay means lost time and money.

A big difference
The plant that developed the process knew the results it achieved using its equipment and settings were repeatable. But repeatable isn’t the same as accurate. For a process to be transportable, every critical measurement must be accurate as well as repeatable.

And that isn’t easy to achieve with different instruments, setups and settings. Think industry-wide rather than locally, which often goes against the grain. Don’t forget the human factor -- a facility’s management doesn’t wish to think they’ve been doing things incorrectly for years.

Transportability isn’t limited to different facilities using different equipment or settings. It can happen on the same operating line after new equipment is installed. For example, several years ago, we consulted for one facility where the personnel complained that the noncontact radiation thermometers I recommended didn’t produce the same readings as the old system. The plant had made the upgrade intelligently, installing the thermal systems in parallel and logging both sets of readings over a period of months. The indicated differences were large enough to justify modifying their operating variables, which they hesitated to do.

The plant engineers failed to grasp the point of the exercise. I had recommended new instrumentation because the readings would be more accurate and more repeatable. This would allow the plant to produce more on-spec product. To this day, plant personnel continue to translate the readings from the new instrumentation into terms of the old instrumentation and they achieved no quality improvement.

Unintended consequences
Changing instruments isn’t always the problem. In another case, for example, I received a panic call from the hot-rolling division of a steel mill. They had been making on-spec product for years, and suddenly most of the product was off-spec. I gathered up my own instrumentation, flew to the plant and started logging my independent data to compare it with what the plant’s computer was logging. Each of the plant’s temperature readings was consistently less than mine.