capacitive thin film vacuum gauge structure and working principle
According to the principle of capacitance change caused by the deformation of elastic film under the action of pressure difference, the vacuum gauge is called capacitance film vacuum gauge. It is composed of capacitive film gauge (also known as capacitive pressure sensor) and measuring instrument. According to the different methods of measuring capacitance, there are two kinds of instrument structures: bias method and zero position method. The zero position method is a kind of compensation method with high measurement accuracy. At present, the structure of zero position method is adopted as the secondary standard vacuum gauge of low vacuum in measurement department.
The structure and principle of capacitive film vacuum gauge is composed of capacitive film gauge, measuring bridge circuit, DC compensation power supply, low-frequency oscillator, low-frequency amplifier, phase sensitive detector and indicator.
A metal elastic diaphragm is installed in the middle of the capacitive film gauge, and a fixed electrode is installed on one side of the diaphragm. When the pressure difference between the two sides of the diaphragm is zero, the fixed electrode and the diaphragm form a static capacitance C0, which is connected in series with the capacitance C1 as one bridge arm of the measuring bridge. The other three bridge arms are composed of the series connection of the capacitance C2, C3, C4 and C5.
The metal elastic diaphragm separates the film vacuum gauge into two chambers, which are the measuring chamber connected to the vacuum system to be measured and the reference pressure chamber connected to the high vacuum system (Pb < 10-3pa). A high vacuum valve is arranged on the connecting pipe of the two chambers. When measuring, open the valve first, use the high vacuum pumping system to pump the space on both sides of the inner plate of the gauge to the reference pressure Pb. At the same time, adjust the measuring bridge circuit to make it balanced, that is, the indicating instrument points to zero.
Then, close the valve and connect the measuring chamber to the vacuum system under test. When the measured pressure P1 > Pb, due to the pressure difference P1 Pb in the gauge, the diaphragm strain causes the capacitance C0 to change, which destroys the balance of the measuring bridge circuit, and the indicator also has corresponding indication. Adjust the DC compensation power supply voltage to charge the capacitor C0 to make its electrostatic force equal to the voltage difference. At this time, the bridge circuit reaches the balance again, and the indicating instrument points to zero again. According to the compensation voltage, the measured pressure P1 can be obtained. Therefore, there is the structure principle diagram of the zero position method thin film vacuum gauge: P1 -- measured pressure;
Pb -- reference pressure;
U -- compensation voltage;
K -- regulatory constant. K = C0 / d0, C0 and d0 are the static capacitance and spacing of the fixed electrode and diaphragm under the equilibrium state, respectively. When P1 > > Pb, the measurement result is absolute pressure, that is, the structural schematic diagram of the zero position method thin film vacuum gauge. The measurement range of capacitive thin film vacuum gauge is 10-1 ~ 101pa, and its gauge constant K can be obtained through calibration.