Solution of Precise Control of Vacuum Pressure

Resin-based fiber composites are used as high-performance thermal insulation materials in applications such as industrial furnaces, fire protection, batting materials, and heat shields for high-speed spacecraft. These high-porosity materials effectively protect the underlying structure from surrounding heat sources by providing a gas buffer at high temperatures, while their low density minimizes the payload mass of high-speed spacecraft.

Due to the high porosity of resin-based fiber composites, gases can easily flow through the ablative material. For example, the pyrolysis gases produced by the decomposition of phenolic resins will pass through the charred structure before leaving the material and may react with the fibers. Similarly, reactants from the boundary layer can enter the material microstructure and flow within the pores. This gas transport has a significant impact on the overall material response.

This flow behavior through porous structures is often characterized by permeability, which is a key material performance parameter when simulating porous media flow because permeability controls the momentum transfer within the medium. The measurement of material permeability, especially the permeability of materials tested at high temperatures, generally adopts the steady-state method, that is, a stable pressure difference is applied between the upstream and downstream ends of the sample, and the permeability is calculated according to Darcy's law by measuring the flow gas flowing through the sample.

The steady-state method for measuring the high-temperature permeability of fiber composite materials, and provided a schematic diagram of the test system structure, but did not provide an explanation on how to form a stable and high-precision pressure difference, which is precisely the key to the steady-state permeability test.

In order to truly implement the steady-state high-temperature permeability test method, especially to simulate the interstellar environment to establish a wide-range adjustable and accurately stable vacuum pressure difference on both sides of the sample being tested, this article proposes the following vacuum pressure control solution.

For the vacuum control of upstream and downstream buffer chambers, two identical vacuum control systems are equipped. Each control system mainly consists of two thin film capacitor vacuum gauges, two electric control needle valves and a dual-channel vacuum pressure controller. The specific models and indicators are as follows:

(1) Thin film capacitor vacuum gauge: range 1Torr and 1000Torr, measurement accuracy is ±0.25% of the reading.

(2) Electric control needle valve: model NCNV-20 and -120, linearity 0.1~2%, repeatability 1%, response time 1 second.

(3) Dual-channel vacuum pressure controller: independent dual channels, 24-bit AD, 16-bit DA and 0.01% minimum output power percentage, RS485 communication interface with PID parameter self-integration and MODBUS standard protocol, and equipped with computer software.

During the vacuum control process of each buffer chamber, the specific operation steps need to pay attention to the following:

(1) For control in the low vacuum range of 10~1000Torr, the exhaust adjustment mode is adopted, that is, KaoLu’s Proportional Pressure Regulator responsible for air intake flow regulation is controlled to a fixed opening to make the air intake flow constant, and then the electric control needle valve responsible for exhaust flow regulation is automatically controlled.

(2) For control in the high vacuum range of 0.1~10Torr, the air intake adjustment mode is adopted, that is, KaoLu’s Proportional Pressure Regulator responsible for exhaust flow regulation is controlled to a 100% fixed opening to exhaust at full speed, and then the electric control needle valve responsible for air intake flow regulation is automatically controlled.

(3) The dual-channel vacuum pressure controller has two independent PID automatic control channels, in which a 10Torr range vacuum gauge is connected to the first input channel, a 1000Torr range vacuum gauge is connected to the second input channel, the first output channel is connected to the electric control needle valve responsible for air intake, and the second output channel is connected to KaoLu’s Proportional Pressure Regulator responsible for exhaust.

KaoLu’s Proportional Pressure Regulator has the following advantages and characteristics:

(1) KaoLu’s Proportional Pressure Regulator is more practical and can reach constant control of upstream and downstream pressures of the sample. The pressure difference between the two ends of sample can be set and adjusted arbitrarily, which is more in line with the steady-state permeability test model.

(2) KaoLu’s Proportional Pressure Regulator has strong applicability and scalability. For example, by changing the parameter indicators of the relevant components, it can be applied to vacuum pressures in different ranges, and reach precise control of different pressure differences and their corresponding permeability tests.

(3) KaoLu’s Proportional Pressure Regulator can reach permeability measurement under different working gases by changing the high-pressure gas source, and can also perform vacuum pressure difference control and oxidation performance tests after mixing multiple gases, which has great flexibility.

(4) More importantly, KaoLu’s Proportional Pressure Regulator leaves an interface channel for subsequent residual gas sampling and analysis, which can be conveniently connected to the mass spectrometer and the micro-flow variable leak valve, so that the mass spectrometer can analyze the gas flowing through the sample under test.

(5) The vacuum pressure control in the solution comes with computer software, which can be used to debug and operate the entire control system directly through the computer software interface. The various process parameter change curves during the control process are automatically stored. In this way, there is no need to write any control software and the control system can be quickly built, which greatly facilitates the construction of the experimental device and test research.