Measuring Thickness of Dielectric Materials using MTI Instruments Accumeasure™ Capacitance Instrumentation

Although the MTI Instruments Accumeasure™ capacitance based instrumentation is designed primarily to make noncontact measurements of position, displacement, vibration, or runout, it can also be used to make noncontact thickness measurements of dielectric1 materials. If the thickness is known or can be independently measured, the Accumeasure System can also be used to measure the dielectric constant of insulating materials.

The Accumeasure System uses a constant current signal source at either 16kHz (Accumeasure 1000 & 1500) or 100kHz (Accumeasure 5000) carrier frequency. The transducer (probe) is a passive element, in that it contains no active electronic circuitry. All of the active circuitry is contained within the Accumeasure electronic amplifier and supplied to the probe through low noise coaxial cable. The Accumeasure constant current amplifier circuitry supplies the control voltage required to keep the sensing current at a constant level over the rated displacement sensing range of the probe and amplifier combination. A high precision buffer amplifier is used to electrically drive the coaxial cable shield and the coaxial capacitance probe structure at the same amplitude and phase as the sensing signal. This effectively cancels all stray capacitances and permits the amplifier to respond only to the capacitance between the face of the sensor probe and the target surface.This results in Accumeasure amplifier having a linear response to either gap changes, or dielectric material thickness changes.

The Accumeasure equipment operates as a classic parallel plate capacitor system with the face of the probe being one of the plates, and the target or surface being measured as the other plate. A ground return path must be provided from the target back to the low side of the Accumeasure amplifier in order to complete the current path. In many practical situations or applications, the target is already at ground potential and the ground return path is automatically provided through the power line grounding and the grounded power cords, but it is usually best to attach a grounding wire directly from the target to the ground return connector on the Accumeasure amplifier. When measuring thickness of dielectric materials, a fixed gap is established between the probe face and the grounded return plate. The dielectric material to be measured is placed in the fixed gap, or placed in contact with the surface of the ground return plate. If the material to be measured is moving, it may be passed through the gap without contact. The MTI Instruments KD-CH-III-D, a precision calibration micrometer fixture is recommended for conducting these tests.

The Accumeasure System operates on the basic principle that Cp= K(A/D) where Cp= the capacitance formed between the face of the capacitance probe sensing element and the target surface; K =the dielectric constant of the air, plus other physical constants; A = the area of the sensing electrode at the face of the probe, and, D = the distance between the sensing electrode and the target surface. A more complete description of the Accumeasure operation and circuitry can be obtained by referring to the Users Manual for the Accumeasure amplifier in use.

MTI Instruments, Inc., 325 Washington Avenue Extension, Albany, NY 12205-5505. Tel: 518-218-2550; Fax: 518-218-2506.