Emphasis on Repeatability
Repeatability—variation of measuring results in multiple measurements performed on the same workpiece using the same instrument—is one of the factors most critical to measurement quality.
Repeatability in coating thickness measurement is affected by multiple factors. Among them are transmission and conversion of the analog signal generated by the thickness measurement instrument’s sensor to a digital format that can be processed through the device’s microprocessor.
Enter the MiniTest 700 series coating thickness measurement instruments from Elektro-Physik, USA, Inc. (Arlington Heights, IL). The instruments use digital sensors to process thickness data at the point of measurement, thus providing what the company says is the industry’s highest level of repeatability.
The usual approach with coating thickness measurement devices includes generating an analog response from the sensor, then sending this signal to a host gage for conversion to a digital signal. This signal is then processed through a microprocessor to generate the calculations needed to display a value for the thickness of the coating that was just measured.
ElektroPhysik says transmission of the analog signal via “probe wires” to the host device can impact measurement accuracy by making the signal susceptible to environmental and other conditions. The company’s new instruments eliminate potential measurement errors by generating the coating thickness value at the point of measurement—in fact, values are actually produced in the sensor where it touches the coating. As a result, only the analyzed and processed digital coating thickness value is transmitted through the devices’ wires, not an analog signal.
According to the company, sensors for the new gages are manufactured using a proprietary process that involves up to 50 calibration points per unit. During manufacturing, each sensor is also encoded with individual temperature compensation to minimize the effects of common temperature-related conditions and changes during measurement—something Elektro-Physik, USA, Inc. says is not possible for analog sensors.
The lineup consists of three models. MiniTest 720 instruments feature dedicated built-in probes optimized for measurement of coatings on ferrous or nonferrous substrates. A model with a combination probe for measurements on both types of substrates is also available.
MiniTest 730 gages are supplied with external probes with internal substrate recognition capability that allows the instrument to automatically switch between magnetic induction and eddy current measurement techniques depending on the substrate material. Series 740 gages feature user-configurable probes that can be quickly converted from built-in to external operation by adding a probe cable.
MiniTest 720 and 730 models have on-board data storage for 10,000 readings in 10 memory fields. Series 740 models can store up to 100,000 readings in up to 100 memory fields. All the instruments have an ergonomic design for ease of use and liquid crystal displays that can be rotated up to 180º to facilitate viewing of measurement results regardless of operator position.
Other features for all models include a comprehensive statistical analysis package with batch statistics, user-selectable high and low limit settings with visual and audible alarms and an IrDA 1.0 data port. Various user-selectable measuring modes cover a range of common applications from spot check requirements to high-accuracy measurements with surface roughness compensation.
This important first step can help prepare the metal for subsequent surface finishing.
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