Abstract: Spectral measurement and spectrometer are important technical measures in detection and monitoring. In order to obtain satisfied measurement accuracy, spectrometer requires high-precision pressure and temperature transducers, actuators and PID controllers, and needs to have the features of a wide range of applications, high precision, easy integration and low cost. This article will focus the characteristics of spectrometer pressure and temperature control, combined with the innovative products of KaoLu FC series electronic needle valve. Also gives a high-precision and low cost high performance spectrum measurement, spectrometer temperature and pressure measurement and control program.
1. The formulation of the question
As a qualitative and quantitative scientific analysis method, spectral measurement has become an important technical measure in various detection and monitoring research due to its advantages of high measurement accuracy and fast response speed. However, in practical applications, the pressure and temperature changes of the sample gas will affect the measurement results. The following are some of the research work done at home and abroad on the temperature and pressure control in the spectral measurement and the characteristics of the impact:
①. Pressure control range
Different spectrometers and spectrometers have different requirements for the pressure control range. For example, the infrared spectroscopy which uses gas curette, the intensity of the absorption peak can be achieved by adjusting the pressure of the sample gas. The general pressure range is 0.5 ~ 60kPa. When using tunable diode laser absorption spectroscopy (TDLAS) technology to measure carbon dioxide concentration in the atmosphere, a stable pressure in the range of 6~101kPa is required. In the precise control of the pressure inside the detector of the X-ray spectrometer, the density of the working gas should be stabilized to ensure the measurement accuracy of the detector. Generally, the pressure should be controlled at around one atmospheric pressure or higher, while the working pressure of the laser-induced breakdown spectrometer should be Up to 275kPa. It can be seen that the pressure control range of the working gas in the spectrometer is relatively wide, generally in the range of 0.1~300kPa, which basically covers the pressure range of 4 orders of magnitude from vacuum negative pressure to 3 times atmospheric pressure.
②. Pressure control accuracy
In the test of spectral, the relationship between the observed spectral line intensity and the real gas density depends on the pressure of the gas sample, so the pressure control accuracy directly determines the spectral measurement accuracy. For example, the pressure control accuracy in the spectrum analyzer of Picarro Company is ±0.0005 atmosphere (fluctuation rate ±0.05%@1 atmosphere pressure). Literature  reported that the absorption tank was controlled when the set pressure was 6.67kPa. After four hours of continuous control, the pressure fluctuation was ±3.2Pa, and the fluctuation rate was ±0.047%. Literature  reported that when gas pressure in a sample cell was also controlled at 6.67kPa, the long-term fluctuation range of the pressure was 7Pa, and the fluctuation rate was ±0.047%. Literature  reported the stability measurement of the laser infrared multi-pass cell pressure control system. The target pressure was set to 60 Torr, the maximum fluctuation was ±0.04 Torr within 150~200s, and the fluctuation rate was ±0.067%. Literature  specially reported the design and research of the high-precision temperature and pressure control system of the spectroscopic measuring instrument. The target pressure value is 18.665kPa, the constant pressure control for 42 hours, the maximum deviation is 5.33Pa, and the fluctuation rate is ±0.014%. Literature  introduced the results of constant pressure control of the detector in the X-ray spectrometer. During the constant pressure of the working gas at 940 hPa, the fluctuation was less than ± 2 hPa, and the fluctuation rate was ± 2%. Literature  introduced the constant pressure control technology of X-ray photoelectric spectrometer in the pressure range of 0.05~30mbar. When the set value is 0.1mbar, the constant accuracy can reach ±0.001mbar, and the fluctuation rate is ±1%.
③. Temperature control accuracy
In the test of spectral, the relationship between spectral line intensity and true gas density also depends on the temperature stability of the gas sample. The stability of temperature would also affect the stability of pressure. Literature  reported that the gas temperature in the sample cell was controlled at room temperature (24°C), the short-term temperature fluctuation is ±0.01°C, the long-term temperature drift is ±0.025°C, and the fluctuation rate is ±0.1%. Literature  reported that high-precision temperature control system of the spectrometer, the temperature is controlled at 45°C, and the temperature fluctuation within 42 hours is ±0.0015°C, and the fluctuation rate is less than ±0.004%.
To sum up, the pressure and temperature changes of the sample gas are the main factors affecting the measurement results. So that in the spectral measurement and various spectrometers, the pressure, temperature adjustment and control of the sample gas has the following requirements:
(1) The pressure control range is very wide (0.1~300kPa), but the corresponding measurement and control accuracy is really high, which make high demands on pressure measurement sensors, control valves, vacuum pumps and corresponding controllers. These four components in this closed-loop control system must match each other; otherwise it is difficult to obtain satisfied results.
(2) Similarly in the process of high-precision temperature control, appropriate temperature sensors, heating devices, power supply and controller. These four components in the temperature closed-loop control system must also match each other.
(3) High-precision controllers would be used in the two closed-loop control systems of pressure and temperature. In order to reduce experimental cost and cost of spectrometer, we want to use a high-precision controller with 2 simultaneous PID automatic control functions.
(4) Focusing on different spectral measurements and spectrometers, their test structures are not the same, which requires the independence of each component in the temperature and pressure control system. In this way, it is beneficial to the integration of the test device and spectrometer.
Overall, in order to obtain the satisfied accuracy of spectral measurement, high-precision pressure and temperature sensors and actuators are required.It has the characteristics of wide application range, high precision, easy integration and low cost.
This article will focus on these features, combine with the innovative products of KaoLu and provide high-precision and cost-effective spectral measurement and spectrometer temperature and pressure measurement and control plan.
2. Spectrometer pressure and temperature integrated measurement and control method
2.1 Control mode design
(1) Pressure Control Mode
As mentioned before, for control range of the spectrometer (0.1~300kPa), the best solution is to select the corresponding measurement and control mode according to the specific pressure range used. As shown in Figure 2-1, it is recommended to use the upstream control mode for the low pressure range. For the high pressure range, the downstream measurement and control mode is adopted; it can also adopt the two-way control mode of simultaneous upstream and downstream control.
Figure2-1 Three mode of pressure control
The upstream control mode is adopted for low pressure, which can repeatedly exert the pumping speed of the vacuum pump, so that the pressure in the vacuum chamber can be quickly and accurately controlled. For high pressure (such as about 1 atmosphere pressure), the downstream control mode can effectively control the pumping speed of the vacuum pump, so that the pressure in the vacuum chamber can be controlled quickly and accurately. At the same time, the sample gas and other work at the air inlet can be avoided. Using FC series electronic needle valve, the vacuum level inside the chamber can be modulated with high resolution.
If there are strict regulations on intake air flow and cavity pressure, they need to be accurately controlled and two-way control mode needs to be adopted. Two-way control mode can control different intake air flows under a constant pressure, but the two-way control mode requires the controller to have two-way control function, which puts forward higher capacity requirements for the controller. The characteristics of the above three control modes are introduced in detail.
(2) Temperature control mode
Also, the temperature measurement and control mode should be selected according to different temperature ranges and temperature control accuracy requirements. For example, when the temperature is near room temperature and the temperature control accuracy is high. It required a two-way control mode with heating and cooling functions. Only this mode to ensure a sufficiently high temperature control accuracy. If it is around the high temperature range, it is also recommended to use a two-way control method, that is, to focus on heating and supply a certain cooling compensation to improve the temperature control accuracy and rapid temperature stability.
2.2 How to choose sensor
The accuracy of sensor is the key to ensure the accuracy of pressure and temperature measurement and control, so selection of sensor is particularly important.
For pressure control in the above range, it is strongly recommended to use the thin-film capacitor vacuum gauge with the highest accuracy. The measurement accuracy of this vacuum gauge can reach 0.2% of its reading, and it has good linearity in the full range, which is very easy to connect. The controller performs linear control with high resolution and small temperature drift. In the actual selection, it is necessary to select a vacuum gauge with a suitable range according to different pressure ranges. For the above-mentioned pressure range of 0.1~300kPa, two types of vacuum gauges, 2 Torr and 1000 Torr, can be selected to achieve accurate pressure range cover.
For temperature control, when temperature is not high, thermistor temperature sensor with the highest measurement accuracy is strongly recommended. High temperature thermistor or platinum resistance temperature sensor is also recommended for higher temperatures. If heating temperature exceeds the use range of thermistor and platinum resistance sensor, it is recommended to use a thermocouple type temperature sensor. These temperature sensors require calibration before using.
2.3 How to choose actuator
The pressure control actuator is the key to determine whether high stability constant control can be achieved. As shown in Figure 2-2, it is strongly recommended to use an electronic needle valve driven by a stepper motor with small linearity and hysteresis, and it is not recommended to use a proportional solenoid valve with large hysteresis and control error. Electronic needle valve can be arranged at air intake and gas outlet, or an electronic needle valve can be arranged according to the selection of upstream or downstream control mode. If the vacuum chamber of the spectrometer is large, electronic needle valve needs to be replaced with an electronically controlled valve with larger diameter and flow rate so as to achieve constant pressure control faster. Please visit https://www.genndih.com/proportional-flow-control-valve.htm
It is recommended to use a semiconductor thermoelectric sheet with Peltier effect for the temperature control actuator. This thermoelectric sheet has a two-way working mode of heating and cooling. With a high-precision thermistor and controller, ultra-high-precision temperature control can be achieved, which is very suitable for spectrometers with temperature control of small working chambers.
If working chamber of spectrometer is large and temperature is below 300 °C, it is recommended to use an external exhaust circulating bath with heating and cooling function for heating. This circulating bath also has heating and cooling function and can achieve high temperature control accuracy.
If the spectrometer works at a higher temperature, it is recommended to use a resistance wire or light heating method, and at the same time, it equipped with a certain ventilation and cooling device to improve response speed of heating, thereby ensuring the stability and speed of temperature control.
2.4 How to choose controller
The controller is the last guarantee to achieve high-precision and high-stability pressure and temperature measurement and control. In pressure control design, controller needs to be selected according to the chosen vacuum gauge and actuator. For the detailed introduction of selection, please refer to the literature . According to the calculation of literature, if the accuracy of pressure measurement and control is to be ensured, a number of digital collector with at least 16 bits must be used. Similarly, the accuracy of temperature measurement and control is also determined by the number of digital collector. Therefore, for the control of pressure and temperature in the spectrometer, it is recommended to use the 24-bit A/D acquisition controller developed by KaoLu with the highest accuracy and cost-effectiveness, and combined with the PID parameter control function.
According to the above selection, the final pressure and temperature measurement and control scheme is shown in Figure 2-3.
In particular, it should be pointed out that the above-mentioned pressure and temperature control basically adopts a two-way control mode. This high-precision controller developed by us has this function. In addition, in practical application of spectrometer, pressure and temperature need to be controlled at the same time. Two controllers controlling electronic needle valve can be used respectively, but overall volume of corresponding spectrometer increases, operation becomes complicated and cost is increased. Now recommended high-precision controller is a dual-channel PID controller. Two channels can independently control different PID parameters and perform auto-tuning of PID parameters at the same time, and each channel has a two-way control function. It effectively simplifies the controller and reduces instrument size and cost.
To sum up, through the analysis of pressure and temperature measurement and control requirements of spectrometer, the detailed temperature and pressure measurement and control technical scheme is determined. The basis for determination of scheme and technical parameters of the corresponding selected components are introduced in detail. Using FC series electronic needle valve in a closed loop modulation allows user to control pressure level in high precision.
The whole technical solution can fully meet the requirements of spectrum measurement and spectrometer for pressure and temperature measurement and control, and has the characteristics of high measurement and control accuracy, powerful functions, wide application range, easy integration and low cost. Except the thin-film capacitor vacuum gauge is an imported product (localized vacuum gauge is also optional), all the selected components and instruments in the scheme are made in Taiwan.