handbook of modern sensors

296 7 Position, Displacement, and Level
adjusted until the bridge output voltage is a minimum and the energy transmitted is
a maximum.
7.8.2 Thin-Film Sensors
Sensors for measuring the thickness of a film range from mechanical gauges, to
optical, to electromagnetic and capacitive. Optical methods are limited to transparent
or semitransparent films. The planar electrodes that mimic a parallel-plate capacitor
produce high output; however, to be accurate, they and the sampled film must be
nearly perfectly parallel, which often is not practical, especially for the surface where
the film is positioned on a curvature. Thus, different types of electrode have been
proposed.
An example of a simple capacitive sensor that can measure thickness of liquid
film is presented in [15]. The liquid film thickness was measured via the capacitance
between two small-wire probes protruding into the liquid (Fig. 7.46C). The liquid
acted as a dielectric between two plates of a capacitor, with the plates being two
small-wire probes. If the liquid has a different dielectric constant than air, a change in
liquid level results in a change in the probe’s capacitance. The capacitance changes
were measured by incorporating the probe into a frequency-modulation circuit. A
fixed frequency was the input to the circuit, and the output frequency depended on
the probe’s capacitance.
Another type of an electrode is spherical and was proposed for a dry dielectric
film [16]. The capacitance is measured between the metal sphere (a stainless-steel
ball having a diameter between 3 and 4 mm) and a conductive base (Fig. 7.48A).
To minimize a fringing effect, the ball is surrounded by a driven shield that helps in
directing the electric field only toward the base electrode through the film.
7.8.3 Liquid-Level Sensors
There are many ways to detect levels of liquids. They include the use of the resistive
(see Fig. 7.1B), optical (see Fig. 7.28), magnetic (see Fig. 7.24), and capacitive (see
Fig. 3.8 of Chapter 3) sensors. The choice of a particular sensor depends on many
factors, but probably the defining factor is the type of a liquid. One of the most
challenging is liquid gases, especially liquid helium, which has a low density and
low dielectric constant, not mentioning its storage in the enclosed Dewar bottles at a
cryogenic temperature. Is such difficult cases, a transmission-line sensor may be quite
efficient. The sensor operates on a principle that is similar to the one that was described
for ablation sensing (Fig. 7.47). For detecting the liquid levels, the transmission-line
sensor may be constructed as shown in Fig. 7.49.
The probe resembles a capacitive-level sensor shown in Fig. 3.8 of Chapter 3;
however, its operation does not rely on the liquid dielectric constant, as is the case
in Fig. 3.8. The probe looks like a long tube with an inner electrode surrounded by
the outer cylindrical electrode. The probe is immersed into liquid, which may freely
fill the space between the electrodes. The electrodes are fed with a high-frequency