Description

Did you know that glass thickness can be measured with a capacitance probe?

    You will need some equipment:

  1. Digital Accumeasure D200
  2. ASP-2500-CTA Dielectric probe (The capacitance probe has a sensing area of 11.25 mm)
  3. Range extension = 3X, 10Hz filter

chart - dielectric probes


Introduction: 

An industrial glass fabricator needed a way to determine the thickness of glass, before cutting it to shape and coating it. The glass sample was marked with indices to find the measuring points.

glass thickness area

The sample center point -5- thickness was checked with a Heidenhain contact gauge accurate to 0.1 µm. This thickness (2.0042 mm) was used to set the initial calibration of the D200 amplifier.  The glass sample was placed on a grounded metal surface and the probe was clamped at 2.8 mm gap.

2 mm glass thickness
While the probe height was set to 2.8 mm with no sample present, the probe is capable up to 7.5 mm gap (2500 µm range times 3X yields 7.5 mm). A lower gap equals a better resolution with less noise and extended range increases accuracy and provides some freedom of movement.
Next the sample was slid from point 1 to point 9 (diagonal scan); the plot of the scan is shown below. The lower plate is a ground reference, the glass is the dielectric and the upper plate is the probe.
non-contact glass thickness

Data shown below were taken at all 9 points; readings are in millimeter (mm)

9 point glass measurement
1) 2.0043 2) 2.0045 3) 2.0050
4) 2.0041 5) 2.0042 6) 2.0050
7) 2.0038 8) 2.0042 9) 2.0040

A slight tilt is noted in the sample (between 3 and 7 seconds) as the glass is slid from point 1 to point 9

Solution: 
The data was stable and correlated with the contact gauge measurement. The resolution at 10Hz is about 0.02 -0.03 µm. The manufacturer was able to set up an inline thickness tester to ensure the glass met requirements. By using multiple probes and a fixture, the process was streamlined.

We can walk you through the process. Get in touch with one of our engineers now.

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