How to read the parameters of film capacitors and the principle of vacuum gauges

Capacitor film vacuum gauge is a vacuum gauge that measures absolute pressure and total pressure. Its principle is that the pressure change applied to the capacitor film produces a change in the distance between the diaphragms, that is, a change in capacitance. The capacitance change is then converted into a change in current or voltage through a frequency detector to form an output signal. Therefore, its measurement directly reflects the change in vacuum pressure and is only related to pressure, not gas composition. That is, the film vacuum gauge is a direct measurement, total pressure type vacuum gauge.

Principle of capacitive thin film vacuum gauge

The vacuum gauge made based on the principle that the elastic film produces strain under the action of pressure difference and causes capacitance change is called a capacitive film vacuum gauge. It consists of a capacitive film gauge (also called a capacitive pressure sensor) and a measuring instrument.According to different methods of measuring capacitance, there are two types of instrument structures: offset method and zero method. The zero method is a compensation method with high measurement accuracy. At present, the low vacuum secondary standard vacuum gauge used in the metrology department adopts the zero method structure.

The figure shows the structural principle diagram of the zero-position method capacitive film vacuum gauge. It consists of a capacitive film gauge, a measuring bridge circuit, a DC compensation power supply, a low-frequency oscillator, a low-frequency amplifier, a phase-sensitive detector and an indicator.

There is a metal elastic diaphragm in the middle of the capacitive thin film tube, and a fixed electrode is installed on the side of the diaphragm. When the pressure difference on both sides of the diaphragm is zero, the fixed electrode and the diaphragm form a "static capacitor Co", which is connected in series with capacitor C1 as the bridge arm of the measuring bridge. The series capacitance of capacitors C2, C3 and C4 and Cs constitutes the other three bridge arms.

The metal elastic diaphragm separates the thin film vacuum gauge into two chambers, namely the measuring chamber connected to the wave vacuum system and the reference pressure chamber connected to the high vacuum system (pb < 10-*Pa). A high vacuum valve 7 is set on the connecting pipe between the two chambers.During measurement, first open valve 7, use a high vacuum pumping system to evacuate the space on both sides of the diaphragm in the gauge to the reference pressure pbe, and at the same time adjust the measuring bridge circuit to balance it, that is, the indicator points to zero.Then, close valve 7 and connect the measuring chamber to the measured vacuum system. When the measured pressure P>Pb, due to the pressure difference P1-Pb in the gauge, the diaphragm is strained, causing the capacitance Co to change, destroying the balance of the measuring bridge circuit.

pb - reference pressure

U——compensation voltage

K is a regulating constant, and its value is K=Co/d, where Co and do are the static capacitance and distance between the fixed electrode and the diaphragm in equilibrium state, respectively.

When p1>po, the measurement result is absolute pressure, that is, p1=KU? (13)

The measuring range of the capacitive diaphragm vacuum gauge is 10^1~ 10“Pa, and its regulation constant K can be obtained through calibration.

In recent years, capacitive diaphragm vacuum gauges have made significant progress. The advent of new dual-capacitive diaphragm vacuum gauges has improved the accuracy of this type of vacuum gauge and expanded its measurement range, allowing its lowest measurement limit to reach l0^3Pa.

The single-sided double capacitance diaphragm vacuum gauge has the characteristics of high sensitivity, constant dielectric constant of gas, pressure reading is completely unaffected by gas composition, fast response speed, etc. If a degasser is added to the reference chamber of the gauge and the pressure is pumped to ≤10°5Pa, the absolute pressure of 210”Pa can be measured.

Disadvantages of Film Vacuum Gauge

Film vacuum gauges are not without disadvantages. First, they are expensive. Domestic ones are generally 4,000 to 6,000 yuan, and those with low quantity limits are more expensive. Imported ones are about 16,000 to 25,000 yuan per piece.Secondly, the old-fashioned domestic thin-film gauges are prone to zero drift and need frequent calibration, especially those with high requirements. This is mainly due to outdated technology and relatively backward domestic material science, resulting in poor temperature stability; another reason is that the range span is small, and the domestic ones do not exceed 3 orders of magnitude.The third is that the old-style capacitors are very delicate and easy to break. Those who have used them know that it is difficult to clean out the oil and dirt. You need to use acetone to clean and save 1/3 of them. Often, they will be scrapped, and 2/3 of the small-range ones will break.

Film capacitor parameters related

The capacitance of a capacitor is a parameter that characterizes its ability to store charge. It is numerically equal to the ratio of the amount of charge Q stored on the plates to the voltage U applied to the plates.

The 684 on the capacitor uses a digital counting method, which is generally a 3-digit counting method. The leftmost digit and the middle digit are valid digits, and the rightmost digit represents the multiple, that is, the power of 10. If the unit is not indicated, it is generally assumed to be pF.If it is a number with a decimal point and no unit is indicated, the unit is uF.The reading of film capacitors is similar to that of color ring resistors. The first two digits of the three-digit number are the coefficient, and the last digit is the exponent of 10. 104 is 10 times 10 to the fourth power, which is equal to 100000pF, or 100nF.