FI108261B - Method for detection of hygroscopic material such as salt in a SAW dew point pickup - Google Patents

Description

108261

The invention relates to a method for detecting a hygroscopic material such as salt in a SAW dew point sensor.

The relative humidity of the gases can be measured by a variety of techniques, such as measuring the change in the dielectric constant or conductivity of the moisture-absorbing material. The relative humidity can also be determined by measuring the gas density or dew point.

Direct measurement of the gas dew point can be accomplished, for example, by measuring a change in the reflection coefficient of a cooled mirror surface, or measuring a change in the specific frequency of a cooled quartz crystal to detect mass accumulating thereon. Of course, dew point measurement involves: i::: also measuring the active surface temperature of the dew point sensor.

* * The use of a surface wave transducer to detect the dew point is recognized by • · *. *. · 25 mm. U.S. Patent 4,378,168; and [K.A.

• · · V · Vetelino et al., Sensors and Actuators B35-36 (1996) 91-98]: The .V surface wave sensor (SAW sensor) is commonly used to measure the dew point. The use of a surface wave sensor is achieved. . ·. 30 additional advantages over optical measurement: • Surface wave sensor • · ·. ···. several independent electrical parameters (transmission, frequency, reflection; ··· coefficient, impedance) can be measured simultaneously, which can be analyzed to provide information on the amount of water on the sensor surface, its state, and the fouling of the sensor. The change in the attenuation or frequency of the surface wave sensor is typically proportional to the change in mass on the surface of the sensor. In optical measuring instruments, for example, the dew drop size or ice surface quality can strongly influence the behavior of the reflection or scattering coefficient as a function of water or ice mass. Further, the sensitivity of the surface-5 wave sensor is better than what can be achieved by the optical method. [Vetelino et al.]

Particularly in the industrial environment there is a problem with the accumulation of hygroscopic impurities, such as salts, on the surface of the sensor 10, where they alter the measured dew point. This occurs because under the same conditions the partial pressure of the water vapor on the surface of saline water is significantly lower than on the surface of pure water. A method 15 for detecting impurities on the surface of a capacitive dew point sensor is described in U.S. Patent 4,626,774.

The object of the present invention is to overcome the drawbacks of the above-described technique and to provide a completely new type of method for detecting a hygroscopic material such as salt. , 20 sex in a SAW dew point sensor.

* '·' The invention is based on the measurement of a sensor impedance-dependent quantity such as sensor reflected power, reflected field strength, or direct sensor capacitance or sensor impedance, and its value is compared to a normal situation. · · * ♦ 1 1 to the corresponding value.

More particularly, the process according to the invention is characterized by what is set forth in the characterizing part of claim 1 • 30.

• · · • · ·

The invention provides considerable advantages.

With the sensor structure of the invention, the accumulation of hygroscopic material on the sensor surface can be effectively detected and if the hygroscopic material is detected, the sensor can be cleaned to restore the original measurement accuracy.

The invention will now be further explored by means of the embodiments shown in the accompanying figures.

Figure 1 shows a principle connection of a measuring device according to the invention.

Figure 2 shows the behavior of a measurement signal without hygroscopic material as measured by an RF circuit analyzer.

Figure 3 illustrates the behavior of a measurement signal when hygroscopic material is present on the sensor surface as measured by an RF-15 circuit analyzer.

In this document, the SAW dew point sensor is defined as a sensor whose acoustic surface wave is capable of propagating over a range exposed to a measurable quantity. Typically, sensor 20 as a whole is comprised of piezoelectric material; includes transmitting electrode structures for generating a surface wave and receiving electrode structures for receiving a surface wave.

Figure 1 shows a typical oscillator coupling of a SAW dewpoint sensor, where a surface wave traverses a quartz substrate from one IDT antenna to another. The connection comprises an operational amplifier 1, a sensor element 2, and adapter coils 3, which are used to adjust the sensor impedance to the desired one.

The sensor 2 itself comprises pairs of finger electrodes 5 and 6, which are • · *. at one end connected to earth potential and at other ends *. The pair of finger electrodes 5 acts as an acoustic wave generator and the finger electrode pair 6 acts as a receiver for the wave K. Finger electrodes 5 and 6 are also called IDT-35 antennas. Frost or dew 4 formed on the surface of the surface wave transducer 2, between pairs of electrodes 5 and 6, 108261 suppresses the surface wave, thereby measuring the amount of dew or frost by measuring the amplitude passing through the transducer. The dew formed on the surface of the surface wave sensor reduces the surface wave propagation speed, whereby the wavelength being 5 also determines the frequency of the transmitted signal determined by the antenna patterns on the surface of the sensor [Vetelino et al.].

For example, the salt accumulating on the surface of the sensor 2 in an industrial environment forms a water conductive liquid film on the surface of the sensor 10. Since the transmitter 5 of the sensor and the receiver antenna 6 are typically insulated with an oxide layer to prevent short circuits, the salt layer changes the capacitance between the antenna halves without shorting the antennas to galvanic earth. The capacitance formed by the salt layer and the antenna halves is typically lossy. The value of the capacitance changes as the amount of water on the sensor surface increases. The change in capacitance and its loss typically results in a change in the fit between the sensor and the power supply, which can be measured preferably by the amplitude of the reflected power. For example, when a dry sensor of the type shown in Fig. 1 is fed through a coaxial cable (50 ohms), the coils 3 ': and 4 provide the frequency response of Fig. 2 with a maximum. At 77.4 MHz. When the sensor in question dips, · at a frequency of about 67 MHz, a depth of typically -30 dB is detected · · 25 minimum reflected power as shown in Fig. 3, which occurs only when the surface of the sensor contains hygroscopic material such as salt. Advantageously, by measuring the reflected power or the: Y: reflected field strength or the sensor capacitance or the sensor impedance of the sensor 2, it is possible to detect the presence of salt on the sensor ... 30, which results in a measured dew point value. reliable and sensor cleaning is required.

Typically, the measurement is implemented in practice by generating an acoustic surface wave for the SAW dewpoint sensor and measuring the RF signal reflection at at least one frequency not in the SAW dewpoint sensor passband, and 5108261 deduces the change afloat.

5 Salt also causes water to accumulate instead of droplets as a water film, which alters the sensor's acoustic impedance and thereby contributes to RF power matching.

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Claims (6)

108261 A method for detecting a hygroscopic material such as salt in a SAW sensor, which method generates an acoustic surface wave and measures its change, such as attenuation, in a SAW sensor, characterized in that 10 - determines the impedance of the sensor (2) when water is present. and comparing it to the impedance value of the pure sensor. Method according to claim 1, characterized in that the impedance is measured by measuring the reflection coefficient of the sensor (2) at at least two different frequencies. Method according to claim 1 or 2, characterized in that the reflection coefficient of the sensor (2); ; is measured by swiping through a predetermined frequency range. The method according to any one of the preceding claims, characterized in that the change in the impedance of the surface wave sensor • · * * ·] * 25 is measured in the frequency range 10 - 500 MHz and the value in the same frequency range is compared to the reference situation the water that accumulates is pure. The method according to any one of the preceding claims,; 30 characterized by measuring transducer transmission, «*», which is measured when there is no water or ice on the surface of the transducer to provide additional information on the amount of dirt on the transducer. A method according to any one of the preceding claims, characterized by generating an acoustic 108261 surface wave on the SAW sensor and measuring the reflection of the RF signal at at least one frequency not in the passband of the SAW sensor, and inferring from the to the measured reference value 5, the presence of hygroscopic impurity on the sensor surface. 10 4 I 4 • • • • • • • • • • • • • • • 1 1 · · 1 * · * 8 108261