The principle and calibration method of resistance vacuum gauge

Resistance vacuum gauge is a vacuum gauge that uses resistance technology. Generally, the zero point is calibrated first, and then the full scale is calibrated.

principle

The structure of the Pirani vacuum gauge is shown in the figure. There is a platinum wire inside the metal cylinder, with electrodes connected at both ends. When providing current to the platinum wire through the electrode, the platinum wire will generate heat, and gas molecules will collide with the platinum wire or undergo thermal radiation or solid thermal conduction, taking away the heat from the platinum wire. The heat absorbed by the above three methods per unit time is Qg, Qr, Qs, and the following formula holds in equilibrium state

Q=I2R=Qg+Qr+Qs (1)

Q is the heat released by a thin wire per unit time, R is the resistance of the wire, and I is the current of the wire.

When the average free stroke of a gas is much larger than the diameter of a thin line, Qg is represented by the heat conduction through free molecules

Qg= α Λ π da (T-T0) p (2)

As shown in the figure, T and T0 are the temperatures of the thin line and the metal cylinder, respectively, P is the gas pressure, and a is the length of the thin line. The remaining Qs and Qr can be represented as follows, respectively

Qs=S κ (T-T0)/L (3)

Qr=π da σε (T4-T04) (4)

(3) It is the thermal conductivity of the electrode, where S is the cross-sectional area of the thin wire, κ It is the conductivity of the solid, and L is the length of the electrode.

(4) The equation represents thermal radiation, σ and ε They are respectively referred to as constants and solid emissivity. If T and T0 are kept constant, then equations (3) and (4) are constants. If I02R is used to represent a certain amount of solid heat conduction and radiation, then Equation 1 can be expressed as

I2R=Ap+I02R (5)

A= α Λ π da (T-T0) (6)

I0 is the current of a thin wire when the pressure is 0, which compensates for the heat loss caused by solid thermal conduction and radiation. Equation A is a constant that does not depend on pressure. If the resistance R, current I0, and constant A of the thin wire are known, the pressure P can be obtained through equation (5).

calibration

Generally, calibrate the zero point first, and then perform full scale calibration.

(1) Calibrate the zero point. To calibrate the zero point, the vacuum system being tested needs to be evacuated. Once the vacuum degree measured by the ionization gauge is higher than 1.0 ^ -1Pa (such as 9.9 X10 ^ -2Pa), press the "zero calibration" button, and the corresponding indicator light will light up or flash quickly. The vacuum gauge will automatically calibrate the zero point. When it displays 1.0 ^ -1Pa, it indicates that the zero point of the gauge has been calibrated. For resistance vacuum meters without automatic zero calibration, manual adjustment can be made through the potentiometer knob on the operating panel.

(2) Full scale correction.