October 2015Maintenance, Test & Measurement, Calibration
When steel surfaces are being prepared by abrasive blasting for hot dip galvanising and other protective coating applications, an assessment of the profile height is important in the surface preparation quality control procedures. This ensures that any profile specification requirement is met, as the area of the surface and hence the coating coverage or the hot dip galvanising process is affected by the profile height. For many years, analog gauges have been used to measure the profile peak-to-valley height.
Surface profile measurement
The Elcometer 224 digital surface profile gauge uses the well-proven method described in ASTM D 4417 method B. A conical, tungsten-carbide tip with a 60° angle and a 50 μm radius tip, locates the bottom of the valleys of the profile and the distance to the reference flat surface, formed by the foot of the gauge sitting on the peaks, is measured electronically and displayed for the user.
To set up the gauge for use, the measuring tip is aligned to the foot by placing the gauge on a flat glass slide and adjusting the gauge to read zero.
The gauge calibration condition can be simply verified by the operator using a ceramic reference shim with a circular hole cut for the tip to pass through. These shims are nominally 500 μm thick and the actual measurement is confirmed by taking three thickness measurements round the circumference of the hole using the average of the three readings as the actual value of the thickness of the shim. This value is recorded on a label that is attached to the shim. If any thickness value has a difference of more than 5 μm, in respect of the other values, then the shim is rejected.
If the gauge does not read zero on the glass slide and 500 μm on the ceramic reference shim then the gauge is not in calibration and should be returned for repair and recalibration.
The gauge is available in two software versions, Basic and Top and each version can be supplied with an integrated measurement assembly or a separate measurement assembly on a cable for difficult to reach areas. The cable can be standard or, if required, armoured for enhanced protection.
The Basic version displays the running statistical values, but does not have memory for the individual readings. The Elcometer 224 Top version has a memory for up to 150 000 individual readings in up to 2500 batches and has Bluetooth wireless communication to allow the batches of readings to be uploaded to a suitable PC for analysis, reporting and storage.
All four versions of the gauge have a range for profile height measurement of up to 500 μm and can take readings at a rate of more than 60 per minute with an accuracy of 5% of the reading or 5 μm, whichever is the greater.
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