The mechanical design of the plane-concave 1.1 MHz resonator is shown in Figure 4.5. In this work, however, a Teflon cell boundary was used instead of a steel boundary. For a more detailed treatment of the 1.1 MHz resonator, see [37].

4.2.3 General measurement procedure with a resonator

The overall measurement procedure consists of several individual measurements. Before a measurement, the apparatus must be prepared for the respective substances. Every measurement series is preceded by a measurement with water. The properties of water are very well known and reflect the operating condition of the resonator. The measured values constitute, in effect, a fingerprint of the particular resonator. If the water measurement is satisfactory, measurements of the solution under investigation can begin.

Since the measurements depend on temperature, the temperature sensor and its electronics must first be calibrated. Temperature stability within the measurement volume is also extremely important. It is achieved by thermostatically controlled enclosures, additional temperature control of the measuring room, and an extended waiting period after setting a new temperature. Temperature stability is checked by a so-called drift measurement, in which the position of the main resonance is monitored.

Every measurement of the substance under investigation is accompanied by a reference measurement, usually carried out after the principal measurement. The reference solution must be selected carefully and must have certain properties in common with the principal solution. Such a reference measurement is important for determining the instrumental properties of the resonator discussed in Section 4.2.1. Accordingly, the reference liquid must have values of sound velocity and density similar to those of the principal solution.

4.2.4 Errors in resonator measurements

• Strong sensitivity to proper filling: incomplete wetting of the transducers or air bubbles in the substance have a very strong effect on the resonances.
• Because resonator measurements are always accompanied by reference measurements, it is difficult to reproduce exactly the conditions of the second measurement, particularly the temperature.
• Errors associated with fitting the resonances when several subsidiary modes are present can have a strong influence on the final results.

The total error lies in the range of 5–10% (cf. Table 4.2).