MTI AccuMeasure 9000, User Manual

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4.3 Conductive Target Thickness Measurements
The dual probe amplifier, Accumeasure System 9000-2 may be used to perform thickness
measurements. The probe amplifier summation circuit in the 9000-2 unit should be set to produce a
[Channel 1 + Channel 2] summation for the correct addition of the two probe signals (contact MTI if there
is a question about the exact configuration of the summation circuit). With the 9000-2 unit set up to
perform the thickness measurement, the rear panel located, Sum Signal Output (refer to Figures 3-5 and
3-6, on pg.13) produces an output that has an
increasingly positive magnitude as the target thickness
decreases.
MEASUREMENT PROCEDURE
1. Fixture the probes on opposite sides of the target with the centerline of the two probe sensing
faces directly in line with each other. Remove the target and set the air gap between the probe
sensing faces so it is equal to the mean thickness of the target plus the sensing range of probes.
6mm (0.236 inches) and probes with a range of 1.5mm (0.059 inch) are being used, the total gap
between the probe faces would be set at approximately 9mm (0.354 inch). Make sure the probe-
to-target air gap for Probe 1 and Probe 2 does not exceed the measuring range of the probes.
2. Connect the probes to the Accumeasure System 9000.
3. Place a target with known thickness midway between the probes. This test target should be the
same thickness as the object that will be gauged.
4. Ground the target.
5. Connect a digital voltmeter (DVM) or oscilloscope (whichever is used) to the Channel 1 output.
6. Adjust the probe gap until the output voltage is between 0 and +10V (+5 to +7 volts is ideal). Keep
in mind the maximum excursion of targets to be gaged. The output voltage (gap) should not
exceed the linear range of the probe.
7. Press the Channel 1 offset switch on the front panel until the blue “offset on” LED is lit. Adjust the
offset control to 0 volts.
8. Connect the DVM or oscilloscope to the Channel 2 output and repeat steps 6 and 7 for Channel 2
output.
9. Connect the DVM or oscilloscope to the sum signal output on the rear panel. It should read 0
volts, corresponding to the test target thickness. Thicker targets will read as positive numbers on
the meter display. Thinner targets will read as negative displacement values.
10. To determine thickness, multiply the voltage change by the probe sensitivity factor. For example,
1.27mm (50 mil) range probes have a sensitivity factor of 0.127um (5 mils) per mV. If the output
signal increases by +20mV, then the target is increasing in thickness by 2.54um (100 mils).
THICKNESS MEASUREMENTS NOTES
1) A very large guard signal in close proximity to an adjacent very low signal can easily cause a 1%
thickness error. Therefore, when possible the probe cables should not be routed in close
proximity to each other.
2) The fixtures that hold the probes can contribute significantly to poor readings. Use of low
coefficient of thermal expansion materials, isothermal designs, and capturing the probes as close
to their measurement ends as possible can help minimize drift effects.
3) The field from the probes can interact with adjacent and proximate conductors. For example, a
probe that is operating in a well can couple to the walls of the well and produces erroneous
effects. It is recommended that the well diameter be 50% greater than the probe outer diameter
to ensure linear operation at large gaps. It may be necessary to re-calibrate the probes if they are
operating in recessed conditions.
4) Avoid target tilt errors. Range-extended probes (“pushed probes”) are less susceptible to target tilt
error. However, noise increases with push. Target tilt warps the capacitance field, starting at the
point of the target’s closest approach. Using smaller probe sensing areas with large air gaps will
minimize the tilt effect.
5) For thickness measurement of non-conductive, dielectric materials contact MTII to obtain
application notes that describe the use of the Accumeasure
TM
System on non-conductive targets.