November 2012Maintenance, 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, analogue gauges have been used to measure the profile peak-to-valley height. The operator has been required to assess groups of readings to determine the typical value for the area being measured. The Digital Surface Profile Gauge, with capability to display running statistical values for the profile height or display charts, makes the measurement and the decision-making with regard to surface profile both easier and quicker.
Surface profile measurement
The Elcometer 224 digital surface profile gauge uses the 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 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 m on the ceramic reference shim then the gauge is not in calibration and should be returned for repair and re-calibration.
The gauge is available in two software versions, Basic and Top, each version can be supplied with an integrated measurement assembly or a separate assembly on a cable for difficult to reach areas. The cable can be standard or, if required, armoured for enhanced protection.
All four versions of the gauge have a range for profile height measurement of up to 500 μm (20 mil) and can take readings at a rate of more than 60 readings per minute with an accuracy of ±5% of the reading or ±5 μm (0,2 mil), whichever is the greater.
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