TW201715185A - Real-time detection method during freezing process capable of detecting the state of materials in real time without terminating the freezing process - Google Patents

Abstract

Provided is a real-time detection method during freezing process, which includes a preparing step, a freezing step and a detection step. The preparing step is provided to prepare an ultrasound sensor and a freezing device. The ultrasound sensor includes a main body defined therein an accommodation space and a piezoelectric unit formed at the bottom of the main body. The freezing device includes a freezing member for providing low temperature. The freezing step is provided to arrange the ultrasound sensor on the freezing member, place materials in the accommodation space of the main body, and set the temperature of the freezing member to be lower than an end freezing temperature of the materials. In the materials freezing process, the detection step is provided to apply electrical pulse signal to the piezoelectric unit for exciting the piezoelectric unit to generate ultrasound signal, receive a first signal of the ultrasound signal reflected from an interface between the materials and the bottom, and measure the amplitude of the first signal.

Description

Instant detection method for freezing process

The present invention relates to a method for detecting a freezing process, and more particularly to an instant detecting method for a freezing process.

Freeze lyophilization technology removes moisture from food or medicines to extend food preservation, improve drug stability, and maintain nutritional value.

The process of the freeze vacuum drying technique is divided into a freezing stage and a vacuum drying stage in order of occurrence. In terms of the freezing phase, this phase has many important parameters such as initial freezing point, end freezing point, and freezing rate, all of which affect the final The quality of the material. Therefore, if the initial freezing time point and the complete freezing time point can be accurately measured to know the freezing rate and the freezing time, thereby adjusting and optimizing the entire freezing process for different materials, not only can the quality of the final material be improved, but also The refrigeration unit can be turned off at the correct time to save energy consumption.

One of the common ways to monitor the freezing process is to measure the temperature change of the material throughout the freezing process through a temperature sensor. The relevant parameters of the freezing stage, however, the temperature sensor must be in direct contact with the material or buried in the material, so when the material is frozen, the temperature sensor is not easily removed from the material. Removing the temperature sensor can also cause unnecessary contamination of the material; other common methods are through differential scanning calorimetry (DSC), thermo-mechanical analysis (TMA), and dynamics. Dynamic mechanical analysis (DMA) and other instruments measure the characteristics of the material to estimate the relevant parameters of the freezing stage. Although the measurement does not directly contact the material in this way, the above equipment is not only expensive but also uses the equipment to measure. When the characteristics of the material are required, the freezing process is stopped and the material is taken out for measurement, and the state of the material cannot be detected immediately during the freezing process.

Accordingly, it is an object of the present invention to provide an instant detection method for a freezing process.

Thus, the instant detection method of the freezing process of the present invention comprises a preparation step, a freezing step, and a detecting step.

The preparation step is to prepare an ultrasonic sensor, and a freezing device. The ultrasonic sensor includes a body defining an accommodating space, and a bottom portion formed on the body and located in the accommodating space. In addition to the piezoelectric unit, the freezing device includes a cavity, and a freezing member located in the cavity for providing a low temperature.

The freezing step is that the bottom of the body of the ultrasonic sensor is disposed on the freezing member, and a material is placed in the accommodating space of the body, and the temperature of the freezing member is set to be lower than Complete knot of the material The frozen temperature is used to freeze the material.

The detecting step is to apply an electrical pulse signal to the piezoelectric unit during the freezing process of the material, so that the piezoelectric unit is excited to generate an ultrasonic signal transmitted in the direction of the material, and receive the ultrasonic signal. A first signal reflected by the interface between the material and the bottom, and measuring the amplitude of the first signal to instantly detect the state of the material during the freezing process.

The invention has the effect of applying an electrical pulse signal to the piezoelectric unit of the ultrasonic sensor to excite an ultrasonic signal transmitted in the direction of the material, thereby measuring and analyzing the reflected first signal. The amplitude during the freezing process to instantly detect and judge the state of the material during the freezing process.

2‧‧‧Ultrasonic Sensor

20‧‧‧Preparation steps

21‧‧‧ body

211‧‧‧ bottom

212‧‧‧ Around the wall

213‧‧‧ accommodating space

22‧‧‧Piezo unit

221‧‧‧Piezoelectric layer

222‧‧‧ Conductive layer

3‧‧‧Freezer

30‧‧‧Freezing step

31‧‧‧ cavity

32‧‧‧Frozen parts

4‧‧‧Materials

40‧‧‧Test steps

5‧‧‧Supersonic Signal Transmitter

6‧‧‧Digital Oscilloscope

△P _CUT ‧‧‧cooling period

P ₁ ‧ ‧ eutectic time point

△P _FUT ‧‧‧freezing period

P ₂ ‧‧‧complete freezing time

L ⁿ ‧‧‧first signal

L _w ‧‧‧second signal

Other features and effects of the present invention will be apparent from the following description of the drawings. FIG. 1 is a flow diagram illustrating a detection flow chart 2 of the instant detection method of the freezing process of the present invention. An ultrasonic sensor according to an embodiment of the instant detection method of the freezing process of the present invention is shown in FIG. 3; FIG. 3 is a schematic diagram of a bottom type, which assists in explaining a bottom of the ultrasonic sensor of FIG. 2; FIG. The schematic diagram illustrates the connection relationship between a freezing device, an ultrasonic signal transmitting receiver, and a digital oscilloscope of the present invention; FIG. 5 is a temperature versus time diagram illustrating one of the instant detection methods of the freezing process of the present invention. Comparative example of the temperature profile during the freezing process; 6 is a graph showing amplitude versus time, illustrating a first example of a method for detecting an instant of the freezing process of the present invention, and a graph of a first signal and a second signal during the freezing process; FIG. 7 is an amplitude vs. time. The relationship diagram illustrates the specific example 1, the specific example 2, and a specific example 3 of the first signal in the freezing process; and FIG. 8 is a plot of amplitude vs. time, illustrating the specific example 1 , a specific example 4, and a specific example 5 of the first signal in the freezing process.

<Detailed Description of the Invention>

Referring to FIGS. 1 through 3, an embodiment of the instant detection method of the freezing process of the present invention comprises a preparation step 20, a freezing step 30, and a detecting step 40.

First, the preparation step 20 is performed to prepare an ultrasonic sensor 2 and a freezing device 3. The ultrasonic sensor 2 includes a body 21 and a piezoelectric unit 22 disposed on the body 21. The freezing device 3 includes a cavity 31, and a freezing member 32 located in the cavity 31 for providing a low temperature.

Specifically, the body 21 has a bottom portion 211 and a surrounding wall 212 extending upward from the periphery of the bottom portion 211. The bottom portion 211 and the surrounding wall 212 define an accommodation space 213. The piezoelectric unit 22 is disposed on the bottom portion 211 outside the accommodating space 213 and has a piezoelectric layer 221 connected to the surface of the bottom portion 211, and a piezoelectric layer 221 is formed on the piezoelectric layer Conductive layer 222 on 221 .

It should be noted that, in this embodiment, the surrounding wall 212 of the body 21 is further protruded downward from the periphery of the bottom portion 211 to surround the piezoelectric unit 22, so that the cross-sectional side view of the body 21 is presented as The H-shaped aspect shown in FIG. 2, but the aspect of the body 21 is not limited thereto, as long as the piezoelectric unit 22 can be prevented from directly interfering with foreign matter and causing interference. In addition, the piezoelectric layer 221 is intended to receive a mechanical signal to generate a mechanical energy. Therefore, as long as a piezoelectric material can be used as the material of the piezoelectric layer 211, the embodiment of the present invention is applicable to the embodiment of the present invention. The material of the piezoelectric layer 221 may be a piezoelectric single crystal, a piezoelectric polycrystal, a piezoelectric polymer, a piezoelectric composite material, or the like, but is not limited thereto; the conductive layer 222 is used for external electrical connection, and therefore, It is a material that can be electrically conductive, and is not particularly limited.

Referring to FIG. 4, after the preparation step 20 is completed, the freezing step 30 is performed, and the bottom portion 211 of the body 21 of the ultrasonic sensor 2 is disposed on the freezing member 32, so that the piezoelectric unit is 22 is located between the bottom portion 211 and the freezing member 32, and then a material 4 is placed in the accommodating space 213 of the body 21, and the temperature of the freezing member 32 is set to be lower than the full freezing temperature of the material 4. , the material 4 is frozen.

Specifically, the freezing member 32 is composed of a shelf disposed in the cavity 31 of the freezing device 3 and a cooling unit (not shown), and the panel is cooled by the cooling unit. The body 21 is disposed on the slab to -20 ° C to -40 ° C, and the material 4 placed on the body 21 is frozen through the freezing member 32.

The detecting step 40 is performed in the freezing step 30, and during the freezing process of the material 4, an electrical pulse signal is applied to the piezoelectric unit 22, so that the piezoelectric unit 22 is excited to generate a direction to the material 4. And receiving a first signal L ⁿ reflected by the ultrasonic signal on the interface between the material 4 and the bottom 211, and measuring the amplitude of the first signal L ⁿ to instantly detect the material in the frozen The state of the process. The detecting step 40 is to apply the electrical pulse signal to the piezoelectric unit 22 by an ultrasonic pulser/receiver 5 to generate the ultrasonic signal and receive the first signal L ⁿ . The amplitude of the first signal L ⁿ is displayed by a digital oscilloscope 6.

In detail, the detecting step 40 uses the piezoelectric unit 22 of the ultrasonic sensor 2 as an upper electrode to electrically connect a transmitting line of the ultrasonic signal transmitting receiver 5 to the piezoelectric unit. a conductive layer 222 for continuously applying the electrical pulse signal to the piezoelectric layer 221 of the piezoelectric unit 22 during the freezing process to generate the ultrasonic signal transmitted to the material 4; and then sensing the ultrasonic wave The bottom portion 211 of the body 21 of the device 2 serves as a lower electrode, and a receiving line of the ultrasonic signal transmitting and receiving device 5 is connected to the bottom portion 211 for receiving the ultrasonic signal from the material 4 and the bottom portion 211. The interface reflects the first signal L ⁿ . After receiving the first signal L ⁿ , the ultrasonic signal transmitting receiver 5 transmits the first signal L ⁿ to the digital oscilloscope 6 connected to the ultrasonic signal transmitting receiver 5 to display the first signal. The amplitude of L ⁿ during the freezing process.

It should be noted that, since the electrical pulse signal is continuously applied to the piezoelectric unit 22 during the freezing process, the ultrasonic signal transmitting receiver 5 receives a plurality of the first signals L ⁿ (n =1, 2, 3...). The selection of the first signal L ⁿ is not particularly limited. When different materials 4 are used, the first signal L ⁿ which is easier to observe may be selected as appropriate.

In addition, since the material 4 is placed in the accommodating space 213, in addition to forming an interface with the bottom portion 211 of the body 21, another interface is formed with the air, and therefore, the ultrasonic signal is further composed of the material 4. The interface between the air and the air is reflected to obtain a second signal L _w . That is to say, when the ultrasonic signal is transmitted to the material 4, the first signal L ⁿ and the second signal L _w reflected by the two interfaces are respectively obtained.

In more detail, the material 4 undergoes different stages during the freezing process, that is, the freezing process includes a cooling period ΔP _CUT , which occurs after the cooling period ΔP _CUT a frozen period ΔP _FUT , a eutectic time point P ₁ occurring between the cooling period ΔP _CUT and the freezing period ΔP _FUT , and a ΔP _FUT occurring after the freezing period The complete freezing time point P ₂ . When the material 4 passes through different periods of the freezing process, the reflected first signal L ⁿ and the second signal L _w may have different results. Therefore, the present invention utilizes the reflection of the ultrasonic signal, and then The reflected ultrasonic signal is converted into a time-varying amplitude intensity by the digital oscilloscope 6, so that the material 4 enters the correct time point of different periods by different amplitude intensities.

In the present embodiment, at the time of complete freezing point P ₂ to generate the first signal L ⁿ will be greater than the amplitude of the amplitude of the first signal L ⁿ generated in the freezing of △ P _FUT; to the total The amplitude of the first signal L ⁿ generated by the crystal time point P ₁ is greater than the amplitude of the first signal L ⁿ generated during the freezing period ΔP _FUT . The average amplitude of the second signal in the L _w △ P _CUT cooling period is greater than the second signal generated in the freezing of △ P _FUT generated average amplitude L _w, the detailed experimental results show that the correlation capacity.

In order to more clearly explain the instant detection method of the freezing process of the present invention, five specific examples and one comparative example will be described below. These specific examples 1 to 5 are carried out in accordance with the above embodiment in accordance with the following scheme.

<Specific example 1>

A specific example 1 of the instant detection method of the freezing process of the present invention is to prepare a stainless steel bottle having a height of 35 mm as the body 21, and coating a surface of the bottom portion 211 of the body 21 with a layer of lead zirconate titanate (Pb(ZrTi)O ₃ , PZT) As the piezoelectric layer 221, a silver paste is applied to a part of the surface of the piezoelectric layer 211 as the conductive layer 222, thereby constituting the ultrasonic sensor 2.

In this specific example 1, the refrigeration unit 3 is an air-cooled slab type freeze dryer (TYFD-50005, Tai Yiaeh, New Taipei City, Taiwan). When the ultrasonic sensor 2 is prepared, the ultrasonic sensor 2 is placed on the freezing member 32 of the freezing device 3, and at the same time the ultrasonic signal transmitting receiver (5072PR, Olympus, Tokyo, The transmitting line and the receiving line of Japan 5 are respectively connected to the conductive layer 222 of the piezoelectric unit 22 and the bottom 211 of the body 21, and then the temperature of the freezing member 32 is set at -20 ° C, and the cavity 31 is maintained. The pressure was 1 atmosphere (101.3 kPa), and 25 mm of water was filled in the accommodating space 213 as the material 4, and the freezing process was monitored for 40 minutes.

During the entire freezing process, the ultrasonic signal transmitting receiver 5 continuously applies a pulse voltage of -360 V to the piezoelectric unit 22 and a pulse repetition rate of 100 Hz to 5000 Hz. The pulse signal is received by the ultrasonic signal transmitting receiver 5 at the bottom 211. The first signal L ⁿ and the second signal L _{w are} received.

It should be noted that before the ultrasonic sensor 2 is loaded with the material 4, the electrical pulse signal can be applied to the body 21 to know that the ultrasonic signal is purely on the bottom of the cylinder. The result of the first signal L ⁿ reflected back by the interface between the air. In addition, the specific example 1 is an example in which the first signal L ^{4 with} n=4, which is easier to observe, is selected as an example.

<Specific examples 2 to 3>

The specific conditions of the specific examples 2 to 3 of the instant detection method of the freezing process of the present invention are substantially the same as those of the specific example 1, except that the temperatures of the freezing members 32 of the specific examples 2 to 3 are respectively set at -30 ° C and -40 ° C.

<Specific examples 4 to 5>

The specific conditions of the specific examples 4 to 5 of the method for detecting the freezing process of the present invention are substantially the same as those of the specific example 2, except that the specific examples 4 to 5 are filled with water in the accommodating space 213. The heights are 5mm and 15mm respectively.

<Comparative example>

The implementation condition of a comparative example of the instant detection method of the freezing process of the present invention is substantially the same as that of the specific example 2, except that the comparative example does not apply the electrical pulse signal to the piezoelectric unit 22, but only Measuring the object through a thermocouple (not shown) temperature sensor Material 4 changes in temperature during the freezing process.

<Data Analysis>

Referring to Figure 5, Figure 5 shows the temperature change of the material 4 of the comparative example during the freezing process. It can be seen from the temperature change of FIG. 5 that after the material 4 is filled in the accommodating space 213, the cooling period ΔP _CT is first passed for about 3 minutes, and the temperature is gradually decreased from 25 ° C to 0 ° C; 8 minutes of freezing period ΔP _FT , at this time, the temperature is maintained at 0 ° C without significant change, mainly because the material 4 releases its latent heat and gradually crystallizes to form a solid state; while the time is gradually increased, the temperature of the material 4 is It is reduced to the same level as the preset freezing temperature to ensure complete freezing.

However, it can be seen from the results of FIG. 5 that if only the temperature parameter of the material 4 changes with time during the freezing process, it is impossible to know exactly at which point in time the material 4 is completely frozen, but it is necessary to Allowing the material 4 to continue to lower the temperature to ensure complete freezing is not only time consuming but also causes energy and cost wear.

Therefore, the present invention further detects the state of the material 4 in the freezing process by an ultrasonic pulse-echo technique, and the result is shown in Fig. 6.

Referring to FIG. 6 and referring to FIG. 4, FIG. 6 shows the experimental result of the specific example 2. When the material 4 is not installed in the accommodating space 213, the electrical pulse signal is applied to the piezoelectric unit 22. And selecting the first signal L ^{4 of} n=4 to display the amplitude of the amplitude in the digital oscilloscope 6, and simultaneously measuring the second signal L _w reflected by the interface between the material 4 and the air, in detail Figure 6 can be roughly divided into six stages:

(1) The amplitude of the first signal L ⁴ reflected by the body 21 not loaded with the material 4 is mainly generated by the interface of the bottom portion 211 of the body 21 and the air, and therefore, in the first 3 minutes. The measured amplitude of the first signal L ⁴ is generated for the same air, so there is not much difference; for the second signal L _w , since the material 4 is not filled, the The signal of the second signal L _w .

(2) When the material 4 is placed in the accommodating space 213, the amplitude of the first signal L ⁴ decreases, mainly because a part of the energy of the ultrasonic signal is absorbed by the material 4, so that it is reflected back. The amplitude of the first signal L ⁴ is small; and the second signal L _w generates a signal due to the addition of the material 4.

(3) When the material 4 starts to freeze, it will first pass the cooling period ΔP _CUT for about 3 minutes. This stage is mainly because the material 4 is cooled without any phase transition, therefore, the first signal L The amplitude of ^{4 does} not change significantly; in contrast, the second signal L _w , because the second signal L _w is reflected by the ultrasonic signal passing through the material 4 as a whole, the second signal L _w The amplitude fluctuations gradually decrease as the material 4 begins to freeze.

(4) After the cooling period ΔP _CUT , when the material 4 is at the eutectic time point P ₁ , the amplitude of the first signal L ⁴ drops significantly again, mainly because the material 4 begins to freeze, the super The acoustic signal changes the acoustic resistance of the material 4 that is frozen, so that the amplitude of the reflected first signal L ⁴ becomes smaller; the amplitude of the second signal L _w also drops significantly for the above reasons.

(5) During the freezing period ΔP _FUT , the amplitude of the first signal L ⁴ has a large fluctuation, mainly because the material 4 will be present in the material 4 during the icing process. Starting from the bottom, the ice crystals are gradually formed, and therefore, the large amplitude fluctuation mainly comes from the bubbles and ice crystals in the material 4; since the material 4 starts to freeze, the material 4 begins to produce an ice crystal structure, the super The energy transmitted by the sound signal into the material 4 is scattered by the ice crystal structure of the material 4, so that the amplitude of the second signal L _w gradually decreases to approach a relatively low point.

(6) After the freezing period ΔP _FUT , when the material 4 is at the full freezing time point P ₂ , since the bubble in the material 4 has been completely eliminated, the fully frozen state is exhibited, and the ultrasonic signal is The interface between the material 4 and the bottom 211 is not absorbed by the water or bubbles in the material 4, so that the amplitude of the reflected first signal L ⁴ will rebound significantly; L _w of the amplitude of the second signal due to the ultrasonic signal transmitted into the material is completely frozen state 4, it is completely frozen the scattering material 4, having a relatively low amplitude approaches.

It can be seen from the analysis of each stage mentioned above that the sound resistance of the material 4 is in a changing state during the freezing process. Therefore, the amplitude of the first signal L ⁴ is at a relatively low point, and when the material 4 is completely frozen. The sound resistance is in a stable state. At this time, the amplitude of the first signal L ⁴ may change significantly (see the complete freezing time point P _{2 in} FIG. 6 ), thereby showing that the freezing process of the present invention is instantaneous. The detection method can indeed know the amplitude of the first signal L ⁴ at the eutectic time point P ₁ and the complete freezing time point P ₂ , so as to know the exact time point when the material 4 reaches the complete freezing.

Referring to FIG. 7, FIG. 7 is a comparison diagram of the experimental results of the specific examples 1 to 3. The parameters changed in the specific examples 1 to 3 are the temperatures (-20 ° C, -30 ° C, -40 ° C) of the freezing member 32, As can be seen from the results of Fig. 7, the temperature of the freezing member 32 is adjusted to mainly affect the rate of the freezing process. When the temperature is lower, the full freezing time point P ₂ can be quickly reached.

For the purpose of clarity, the relevant experimental data of the specific examples 1 to 3 are summarized in Table 1 below; wherein the time difference of loading the material 4 into the ultrasonic sensor 2 is due to human operation, The time difference of the material 4 is compared with the time of the whole freezing process, and the freezing process time is much longer than the material 4 loading time difference. Therefore, the measurement result does not cause too much error due to the difference. Be added that, seen from Table 1, Example 1 and completely freezing point of the particular time of the particular P ₂ Example 2 of large difference, and 2 cases of complete freezing of the particular time point of the particular P ₂ Example 3 The difference is small, the main reason is that when the temperature of the freezing member 32 is set to be less than -30 ° C (the specific examples 2 to 3), the speed at which the material 4 reaches the complete freezing time is close to saturation. In other words, when the temperature of the freezing member 32 is below -30 ° C (this specific example 2 to 3), the time required for the material 4 to reach the full freezing time point P ₂ is very small.

Referring to FIG. 8, FIG. 8 is a comparison diagram of the experimental results of the specific example 1 and the specific examples 4 to 5, mainly for fixing the temperature of the freezing member 32 to -30 ° C, and adjusting the height of the material 4, as shown in FIG. As a result, it can be seen that the height of the material 4 itself affects the speed of reaching the full freezing time point P ₂ , and the lower the height of the material 4 , the faster the complete freezing time point P ₂ can be reached. For the purpose of clarity, the specific experimental data of the specific example 2 and the specific examples 4 to 5 are summarized in Table 2 below.

In order to more clearly understand the relationship between the height of the material 4 of the specific example 2 and the specific examples 4 to 5 and the cooling period ΔP _CUT time and the freezing period ΔP _FUT time, the data in FIG. 8 and Table 2 will be used. The following formula (1) and formula (2) are arranged, wherein the height of the material 4 is represented by h (mm): Δ P _CUT =0.98+0.15× h .............. .........................................(1)

△ P _FUT = 4.93 + 0.27 × h ......................................... ..............(2)

It can be seen that when the height of the material 4 is a known value, the time of the material 4 in the cooling period ΔP _{CUT and the} time of the freezing period ΔP _FUT can be obtained by the above formula (1) Equation (2) is further estimated and learned.

In summary, the instant detection method of the freezing process of the present invention applies the electrical pulse wave signal to the ultrasonic sensor 2 by the ultrasonic pulse echo technique during the freezing process, so that the piezoelectric layer 221 is excited. An ultrasonic signal transmitted in the direction of the material 4 is generated, and the ultrasonic signal is reflected back to the first signal L ⁿ by the interface between the bottom portion 211 of the body 21 and the material 4, and the first signal is analyzed by the digital oscilloscope 6 The amplitude of the signal L ⁿ during the freezing process, so as to instantly detect the state of the material 4 in the freezing process, to know the correct eutectic time point P ₁ and the complete freezing time point P ₂ , so that the present invention can be achieved. purpose.

The above is only the preferred embodiment of the present invention, and the scope of the present invention is not limited thereto, and the simple equivalent changes and modifications made by the scope of the patent application and the patent specification of the present invention are It is still within the scope of the invention patent.

20‧‧‧Preparation steps

30‧‧‧Freezing step

40‧‧‧Test steps

Claims (5)

An instant detection method for a freezing process, comprising: a preparation step of preparing an ultrasonic sensor, and a freezing device, the ultrasonic sensor comprising a body defining an accommodation space, and a body formed on the body a bottom portion of the piezoelectric unit outside the accommodating space, the freezing device includes a cavity, and a freezing member located in the cavity for providing a low temperature; a freezing step, the ultrasonic sensor The bottom of the body is disposed on the freezing member, and a material is placed in the accommodating space of the body, and the temperature of the freezing member is set to be lower than a temperature at which the material is completely frozen. The freezing member freezes the material; and a detecting step, during the freezing process of the material, applying an electrical pulse signal to the piezoelectric unit, causing the piezoelectric unit to generate an ultrasonic signal transmitted in the direction of the material And receiving a first signal reflected by the ultrasonic signal on the interface between the material and the bottom, and measuring the amplitude of the first signal to instantly detect the state of the material during the freezing process. The method for prompt detection of a freezing process according to claim 1, wherein the freezing process comprises a cooling period, a freezing period after the cooling period, and a complete freezing time point after the freezing period The amplitude of the first signal generated at the complete freezing time point is greater than the amplitude of the first signal generated during the freezing period. The method for detecting the freezing process of claim 2, wherein the freezing process further comprises a eutectic time point occurring between the cooling period and the freezing period, the first time generated at the eutectic time point Amplitude of a signal Greater than the amplitude of the first signal generated during the freezing period. The method for detecting the freezing process of claim 2, wherein the ultrasonic signal is further reflected by the interface between the material and the air into a second signal, and an average value of the amplitude of the second signal generated during the cooling period. It is greater than the average value of the amplitude of the second signal generated during the freezing period. The method for detecting the freezing process of claim 1, wherein the detecting step is to apply the electrical pulse signal and receive the first signal by an ultrasonic signal transmitting receiver, and display the first signal by a digital oscilloscope. The amplitude of the signal.