INFICON XTM/2 Thin Film Deposition Monitor - Manual (Page 51)

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XTM/2 Operating Manual
2.7.3 Biological
The measurement of biological specimens is subject to many of the same
problems as covered in the measurement of liquids.
2.7.4 Measurement of Liquids
The measurement of the mass of a liquid on the face of a crystal is a technique
that is subject to very large errors. The two primary problems with liquids are
that they are not infinitely rigid structures and do not necessarily form in uniform
layers. Because liquids do not oscillate as a rigid solid, not all of the mass
participates in the resonance. Consequently, not all of the liquid is detected. In
some ways, the crystal is more appropriately called a viscosity sensor. The
second problem is that liquids tend to form spheres on the face of the crystal
after only very modest accumulations of a few monolayers. This aggravates the
problem caused by non-infinite rigidity. Another aspect of the problem is that the
liquid spheres form at random locations across the crystal. Because monitor
crystals have differential radial mass sensitivity an uncontrollable measurement
problem exists. Spheres formed at the center of the crystal contribute more than
spheres formed near the edge of the sensor’s aperture.
2.7.5 Use as a Frequency Counter
The ability to measure a crystal’s frequency between 6.000000 and 5.000000
MHz may be accessed by setting the "displayed units" configuration switches 9
and 10 (refer to
section 2.5.2 on page 2-15 ). The displayed frequency is
averaged according to the setting of the recorder and display averaging
configuration switches. In addition, by pressing the LIFE key, even finer
frequency resolution is displayed (x.xx or x.x Hz), suppressing the most
significant data. Data to 0.1 Hz is always available through the computer
interface for the latest 0.25 sec. measurement.
The recorder output and programmed limits of this instrument behave as if the
"displayed units" configuration switches were set to 00 or "kЕ".