Analysis of Temperature Characteristics of Digital Mass Thickness Sensor

1 Introduction

Current electronic instruments and equipment are mainly composed of electronic components and semiconductor devices. There are two problems: first, many components, especially semiconductor devices, such as PBs, are seriously affected by temperature; second, electronic components have a temperature limit, and their reliability is inversely proportional to the temperature. When this limit is exceeded, the reliability and mean time between failures will drop sharply. Studies have shown that if the temperature cycle of the instrument is deliberately exceeded by 20°C, its failure rate will increase by 8 times [1].

It can be seen that temperature control of instruments is very necessary in many cases. In the papermaking process,

the sensor for detecting the basis weight, moisture content and ash content of paper is located after the drying cylinder of the paper machine, where the temperature is very high. For paper machines that produce cigarette paper, there is no cooling cylinder at the tail end. At the sensor, the temperature of the paper is about 65-75°C, and the heat dissipation of the instrument is very important [2]. At present, from the perspective of instruments used for papermaking process control, imported instruments almost all use water cooling to dissipate heat [3]. Domestic instruments have almost no heat dissipation measures. Water cooling has certain requirements for water temperature. At the same time, there are problems such as difficult water pipe sealing, easy water droplets, difficult maintenance, and high cost, which bring inconvenience to practical applications; domestic instruments are difficult to meet the requirements in terms of performance and reliability [2]. In view of the above situation, through the analysis and research of modern thermal control technology, we proposed a heat pipe and semiconductor cooling combined heat dissipation and temperature control solution [4]. 2 Introduction to digital mass thickness sensor The basic principle of the digital nuclear radiation mass thickness sensor is shown in Figure 1.

Figure 1 Basic structure of digital mass thickness sensor

Its basic working principle is: the radiation beam of the radiation source radiates to the object to be measured. In addition to being partially absorbed and partially scattered, the β rays that pass through the measured material radiate to the scintillator of the receiver. The scintillator produces fluorescence. The fluorescent photons are converted and amplified by the photomultiplier tube to form a pulsed electron flow, which is then amplified, shaped and driven and sent to the microcomputer in the form of pulses. The microcomputer counts them and then calculates the mass thickness of the measured material.

The scintillator used for nuclear light conversion should be used at a temperature below 75-80℃, otherwise it will affect the accuracy and service life. The photomultiplier tube is also extremely sensitive to temperature changes. As the temperature increases, the gain of the photomultiplier tube decreases, the signal-to-noise ratio increases, and the dark current increases. These factors affect the sensitivity and stability of the sensor. Therefore, the photomultiplier tube is best kept at room temperature. When used in a high temperature environment, the temperature control technology of heat pipes and semiconductor cooling and heat dissipation can be used. Experiments have shown that the effect is very good.

3 Temperature characteristic analysis

When β rays pass through a substance, low-energy β is quickly absorbed due to ionization and excitation and bremsstrahlung radiation. For the main part of the β spectrum, the absorption curve is approximately an exponential decrease:

I = I0e-μmxm (1)

where I0 and I are the β radiation before and after passing through the xm thick material, and μm is the mass absorption coefficient.

At 0℃, the air density is 1.2929kg/m3. Assume that the air gap height of the mass thickness measuring instrument is 12mm. The diameter of the radiation source and receiver is 40mm. In the range of 0-60℃, the air density changes by an average of about 3.3% for every 10℃ change in temperature. The air gap height is 12mm, and its air equivalent quantitative is about 1.2929×12=15.6g/m2, and the air gap quantitative change is 15.6×3.3%=0.515g/m2.

It can be seen that the change of air gap temperature has a great influence on the accuracy of the measuring instrument, which can be eliminated by temperature measurement and software compensation.

In addition to being affected by the processing and raw materials of the tube, the dark current and thermal noise are mainly affected by temperature changes. Their temperature relationship curves are shown in Figures 2 and 3 respectively.

Fig.2 Curve of the effect of temperature change on the dark current of photomultiplier tubeFig.3 Curve of the effect of temperature change on the thermal noise of photomultiplier tube

It can be seen that cooling or temperature control is very necessary to improve the performance of scintillation detectors.

4 Heat pipe temperature control technology

The heat pipe is an efficient heat transfer device with excellent thermal conductivity. It can efficiently transfer heat over a long distance under a very small temperature difference without any external pressure transmission power.

Figure 4 is a typical heat pipe structure. It is a closed container. The entire heat pipe can be divided into an evaporation section, an adiabatic section and a condensation section in the longitudinal direction; it can be divided into a liquid working fluid, a tube core and a tube shell in the radial direction. The tube core is used to saturate the liquid phase of the working fluid, and the rest of the tube shell contains the vapor phase of the working fluid.

Figure 4 Schematic diagram of the working principle of the heat pipe

Heat is input from the evaporation section and discharged from the condensation section. When the evaporation section is heated, the liquid working medium in the core material evaporates. On the one hand, a pressure difference is established between the evaporation section and the condensation section, and the steam is driven from this section to the condensation section. As long as the temperature of the condensation section is lower than the saturation temperature of the steam, it condenses in this section and transfers the latent heat of vaporization to the external radiator for dissipation. On the other hand, the liquid in the core material of the evaporation section evaporates, causing the liquid-vapor interface of this section to shrink into the core, reducing the radius of curvature of the interface to generate capillary pressure. The capillary pressure draws the liquid working medium in the condensation section back to the evaporation section, causing it to evaporate again. This process repeats itself, efficiently transferring heat from the evaporation section to the condensation section.

Since heat pipes transfer heat by latent heat of phase change, the temperature difference between the two ends of the heat pipe is very small, generally between a few tenths and a few degrees. The thermal conductivity of heat pipes is extremely high, which is 103-104 times that of good metal conductors.

5 Heat dissipation and temperature control scheme

The instrument we want to dissipate heat and control temperature is an online detection instrument, which is characterized by a high ambient temperature. Heat dissipation and temperature control measures should be simple and reliable, and it is best not to bring some auxiliary equipment, because the detector generally moves with moving parts such as the scanning frame probe. In addition, some components in the instrument have very high requirements for heat dissipation and temperature control accuracy, and at the same time, the heat dissipation and temperature control device is required to dissipate the heat brought by changes in the external environment and the heat generated by itself as quickly as possible, that is, a certain degree of rapidity is required.

Based on the above situation, we use local semiconductor cooling, use an insulated tube to transfer the heat from the hot end of the semiconductor cooler to the outside of the instrument, and use an expanded area for natural cooling or forced air cooling to dissipate the heat. When the heat dissipation required is not very large, this scheme can use the structure shown in Figure 5.

Figure 5 Heat pipe semiconductor cooler heat dissipation and temperature control scheme

For the influence of air gap temperature, we use temperature sensor detection and software compensation method to eliminate it. The photomultiplier tube adopts the method of heat dissipation and temperature control by combining semiconductor cooler and heat pipe.

The introduction of heat pipe has solved the problem of heat dissipation and temperature control of the instrument more thoroughly.

Performance indicators: the operating temperature range is 10-60℃; the temperature control accuracy is ±2℃.

Figure 6 shows the eight-hour drift curve of the digital mass thickness sensor after using the above heat dissipation and temperature control technology, and its mean square error is 0.1026g/m2; when no temperature control is added, it is generally around 0.6-1.0g/m2. It can be seen that the addition of the temperature control system greatly improves the performance of the instrument.

Figure 6 Eight-hour drift curve of mass thickness sensor

6 Conclusion

Temperature changes affect the sensitivity and stability of mass thickness sensors. By utilizing the heat pipe's high efficiency and excellent thermal conductivity, and using semiconductor coolers and heat pipe heat dissipation temperature control technology, the performance of mass thickness sensors has been greatly improved. This technology can be widely used in various online sensors and has great promotion value.