Quartz crystal viscosity sensor principle and properties are studied
2011-3-25 source:lanjinshi.com

1 summary

Viscosity is liquid inside the performance of friction. Liquid viscosity measurement in petroleum, chemical, pharmaceutical and clinical medicine is widely used. The traditional viscometer have capillary viscometer, rotating viscometer, vibration ball type viscometer, etc. These measurement method is wide measuring range, high precision, but its weakness is or complicated structure or measurement complex. In this paper the viscosity of quartz crystal sensitivity principle, quartz crystal viscosity sensor structure, oscillating circuit and its characteristics are analyzed and studied.

2 quartz crystal viscosity sensor principle and structure

Sensor adopts type thickness shear vibration AT cutting model of quartz crystal, its vibration mode as shown in figure 1 below.

Figure 1 AT cutting type quartz crystal thickness shear vibration schemes

Its thickness direction and the Y axis parallel. Because of quartz crystal with piezoelectric effect, when Y direction after add alternating electric E2, be in namely XY plane strain produced inside, according to cut S6 elastic mechanics equations can be obtained quartz crystal oscillator frequency in the air.

Quartz crystal in liquid medium the oscillates, equivalent to two layer structure. Among them the first layer (0 acuities y acuities d) for quartz crystal layer, layer 2 (d acuities y acuities d + dy) for fluid layer. Convection, Newtonian fluid law intercalation, can be used to deduce the frequency offset caused by fluid Δ f:

Δ f = η ρ 1 k (1) 1/2

Type, k as and quartz crystal related structure constant η ρ 1, 1, respectively for measured liquid viscosity and density. Thus, quartz crystal in liquid the frequency change is liquid viscosity and density function. When the temperature is measured, and determine if liquid after its density, can then define liquid viscosity.

Sensor probe structure shown in figure 2.

Figure 2 sensor probe structure

In order to prevent fuses too long produces self-excited oscillation, quartz crystal and oscillating circuit are installed in the probe. Also in the probe with temperature sensor to monitor real-time sample temperature change, T/V switching circuit also sealed in the probe inside. By converting difference frequency signal and the temperature signal input enlarge mainframe.

Choose the fundamental frequency for 10MHz AT cutting type quartz crystal, wafer contour structure fig.03 shows. Chip center for circular area by silver electrode, according to energy localization effect, vibration energy concentrated in the middle area. Under the electrode To ensure quartz crystal oscillation stability, will meet with conductive adhesive electrode lead in chip edge chamfering place. Touch tablet fits adopted.therefore liquid way. Request as far as possible from center adhesive surfaces, electrode lead electrode area should as far as possible, in order to reduce lead to thin wafers produces stress.

Figure 3 chip structure

3 measuring system and oscillating circuit

Measurement system shown composed as shown in figure 4.

Figure 4 measurement system diagram

When will install a work crystals and temperature sensor probe after placing measured liquid density and viscosity influence of because of crystal oscillator frequency occurred frequency shift. To determine frequency shift, the system with another a reference crystals, it provides stable oscillation frequency.

The relative reference work crystals, crystal frequency offset by difference frequency circuit, and obtained the HSI measuring input 80C196 the size of its frequency. Measured liquid temperature via detecting circuit and A/D transformation input single-chip microcomputer system. Measured liquid density values by keyboard input.

Single-chip microcomputer software including frequency measurements subroutines, density measuring subroutines, density input subroutines, viscosity measurement subroutines. By measuring results LED display. System used by the two inverter composition of series resonant crystal oscillator. Oscillating circuit as shown in figure 5 shows.

Figure 5 oscillating circuit

Oscillating circuit working in quartz crystal series on the resonant frequency, at this moment, crystal equivalent impedance minimum, the positive feedback, easily the strongest since vibration. When crystal dipped into the liquid, due to the influence of liquid damper, oscillator output voltage amplitude decline. Therefore in an output terminal of the oscillating circuit with LC frequency selective increased level of high frequency amplifier circuit, in order to improve the oscillation signal amplitude. Enlarge after the signal with reference frequency by plastic difference frequency comparison.

4 characteristic analysis

Use figure 4 shows the measurement system of quartz crystal, in liquid medium experimental research work performance.

4.1 response time

Probe just into the water, and then biggest difference frequency decreased gradually 2min stability in 4100Hz, shown, as shown in figure 6. This shows that the sensor from begin to contact the liquid to stable oscillation need some time.

Figure 6 response time curves

4.2 probe insert depth influence

Table 1 listed in different liquid depths probe measurement of frequency. From the table to see, quartz crystal output frequency and in liquid in certain scope the depth is irrelevant.

Table 1 probe measured at different depth of frequency

4.3 linearity experiment

Will probe into different viscosity fluid samples, to measure the viscosity influence of crystal resonance to observe and record changes in frequency.

Each measuring a sample with acetone, after cleaning quartz crystal adsorption of liquid and dry, and then on to the next group of measurement.

Table 2 lists seven different samples of measurement result 10 times the average, a single measurement for discrete values 10Hz.

Table 2 sample measuring results average

Table 1 η ρ x = (1) 1/2, y = Δ f as poor frequency.

By the least square method to and get:

Δ f = η ρ 1 0.6985 (1) 1/2 + 3.403

Correlation coefficient = 0.9996.

Fitting and curve shown as shown in figure 7.

Figure 7 frequency - viscosity curve

The chart shows, Δ η ρ 1 with f (1) 1/2 have good linear relationship.

4.4 sensitivity

The regression equation of the sensor sensitivity S get for:

S = Δ η ρ 1 f / (1) 1/2 = η ρ 1 0.6985 / (1) 1/2

As liquid η ρ 1 reference coefficient (1) 1/2 each change a unit, for 698.5 difference frequency Hz, shows that changes the sensitivity of the sensor has higher.

4.5 temperature characteristic

Figure 8 says viscosity and temperature relations. That, along with the temperature by graph difference frequency increases, decreases. This is mainly composed of the influence of temperature on viscosity cause.

Figure 8 temperature - viscosity relation curves

To eliminate temperature influence, temperature compensation method is used in the software solution.

5 closing

Through the experimental analysis, quartz crystal of liquid viscosity, density have higher sensitivity. In most cases, measured liquid density difference, and viscosity dispersion big; and Therefore, the use of quartz crystal on viscosity is relatively appropriate measure properties. Quartz crystal viscosity sensor advantages are simple structure, high sensitivity, low cost, easy to miniaturization, its shortcoming is measuring scope slant narrow. How to expand its measurement range is need to address the issue.

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