GB2056678A

GB2056678A - Calibration apparatus for vacuum gauges

Info

Publication number: GB2056678A
Application number: GB8023844A
Authority: GB
Original assignee: Balzers AG
Current assignee: OC Oerlikon Balzers AG
Priority date: 1979-08-02
Filing date: 1980-07-21
Publication date: 1981-03-18
Also published as: GB2056678B; FR2463404B3; CH641562A5; DE3025929A1; DE3025929C2; FR2463404A1

Abstract

To obtain a known pressure in the test chamber (1), containing the vacuum gauge (10) to be calibrated, gas is allowed to enter through a first throttling means (8) having a conductance LF, and is pumped out through a second throttling means (3) of a significantly larger conductance LB. To reduce errors the temperatures of the throttling means are maintained equal by mounting them in a body (2) of good thermal conductivity. <IMAGE>

Description

SPECIFICATION A device for the calibration of vacuum gauges The invention relates to a device for the calibration of vacuum gauges which a test chamber, provided with a suction port and a holder for a vacuum gauge to be calibrated, and an inlet for a calibration gas, the test chamber communicating with the suction port and a reference vacuum gauge via respective throttling means.

The so-called expansion method is known for the calibration of vacuum gauges in which a given volume of a calibration gas, the pressure of which is measured by a reference vacuum gauge, is expanded into a space of larger volume, and from the ratio of the two volumes and the- initial pressure of the calibration gas may be calculated the final pressure after expansion to which is exposed the vacuum gauge being calibrated. Such a measurement results in errors due to the inaccuracy of measurement of the ratio of said volumes, and also due to the sorption of the calibration gas on the walls of the chamber which change the pressures, and finally also due to possible desorption of other gases from the walls of the said chamber. Also deviations in the behaviour of the calibration gas from the so-called ideal gas laws may cause errors.

The so-called dynamic expansion method was introduced to reduce some of the sources of errors and also to enable working with very accurate reference vacuum gauges the measuring range of which is however higher than the measuring range of the vacuum gauge to be calibrated. In this-method is measured the pressure drop taking place when the calibration gas flows through a throttling means. The gas inlet through the throttling means into the test chamber of the calibration-device and the exhaustion of the gas therefrom by the attached pump are in dynamic equilibrium, i.e. the amounts of the calibration gas which are supplied to and withdrawn from the test chamber are identical.

In view of the fact that the pumping speed otthe majority of pumps cannot be determined with sufficient accuracy or is not sufficiently constant a further gas throttling means are mostly provided also between the test chamber and the pump in order to eliminate from it errors which would otherwise grow.

If p1 is the pressure of the calibration gas in the inlet, p2 its pressure in the test chamber, and LF and LB, respectively, conductances of the two throttling means, then for the amount O of gas, which flows into the test chamber through LF and which equals the amount of gas pumped from the chamber through L5 (see the accompanying drawing) applies: 0 = LF (P1 - p2) = LB (P2 - 0) (1) when the ultimate pressure of the pump is so low, practically zero, that it is insignificant compared with P2.

Then follows LF (P1 - P2) = L5p2 LFP1 = p2(Lp+L8)

When LF < C LB then approximately

i.e. the pressure of the calibration gas in the test chamber is in the ratio LF/LB reduced compared with the pressure in the calibration gas inlet. The ratio LF/LB represents a constant assuming molecular flow through both the throttling means. Preferably a decimal stepdown ratio is selected; if LF iS e.g. 1000 times smaller than LB, then the same applies also to the pressure p2 compared with p,.

For a pump with a sufficiently low ultimate pressure the step-down ratio P2/P1 is therefore no longer dependent on the conductance of the pump but only on the ratio of the conductances of the two throttling means.

As suitable throttling means porous sintered bodies may be used, particularly for the throttling means between the reference vacuum gauge and the test chamber. It is important to bear in mind that the conductance of a throttling means is proportional to VT/M, where T is the absolute temperature and M the molecular weight. The conductance is also dependent on temperature.

The aim of the present invention is to provide a device for the calibration of vacuum gauges which enables the error due to temperature to be largely eliminated. This is achieved in that both the throttling means are situated in a common temperature equalising body of a material of excellent heat conductivity.

One embodiment of the invention will now be described, by way of example, with reference to the accompanying diagrammatic drawing, in which: Figures 1 and 2 show a cylindrical test chamber respectively in sections perpendicular and parallel to the axis of the cylinder, and Figure 3 shows a detail of Figure 2.

In a test chamber 1 are arranged two vacuum gauges 10 to be calibrated each suspended from a cover 12 in such a way that each molecule of the calibration gas which enters the test chamber 1 at least once collides with a wall before it can enter the active zone of the vacuum gauge.Two impact plates 13 which are semicircular in cross-section are provided for this purpose and a calibration gas inlet 11 opens against an end wall of the cylindrical test chamber 1.

According to the invention the bottom plate 2 of the test chamber 1 is preferably made of copper or aluminium (or an alloy which is correspondingly heat conductive) and in it are situated the two throttling means 8 and 3 (which correspond to LF and L5 in the above equation). The calibration gas is fed to the test chamber through the throttling means 8 via a valve 7, and through the throttling means 3 leaves the chamber and iswithdrawn through the suction port4 by the attached pump 5. The pressure p, of the calibration gas upstreani of the throttling means 8 is measured by a reference vacuum gauge 6.

The common, sufficiently heat conductive bottom plate ensures temperature equalisation and consequently sufficiently equai temperature of the two throttling means so that the dependence of the conductances on the femperatureforthe measurement is no longer of disturbing significance.

The embodiment according to Figure 3 is advantageous for further improvement of the conformity of the teniperature of the throttling means to the tem-perature- of the bottom plate and for maintaining it constant during the measurement. Figure 3 shows a portion of the bottom plate 2 in the. recess of which is inserted a plate 15 from a porous sintered material held in placd by a cover plate 16 screwed to the bottom plate. The cover plate and the bottom plate have respective passages 17 and therethrough for the calibration gas; the calibration inlet 11 (see Figure 2)-is attached as an extension to the passage 18. In this arrangementthe this.arrangemenf the sintered ;3late 15, representing the throttling means proper, is nearly on all sides surrounded by walls of uniform temperature and this ensures quicker and more uniform adjustment of the temperature. This embodiment of the throttling means is recommended particuiarly for LF, while for the throttling means LB a pair of simple holes will suffice, as shown, when pressure reduction according to the above equation 3 is required, i.e. a small ratio of LF to L5 is needed.

Claims

1. A device for the calibration of vacuum gauges, the device comprising a test chamber having a suction port, a holder for a vacuum gauge to be calibrated, and an inlet for a calibration gas the pressure of which is measured by a reference vacuum gauge, the test chamber communicating with the reference vacuum gauge via first gas throttling means, and with the suction port via second throttling means, both the throttling means being situated in a common temperature equalising body of a material of excellent heat conductivity.

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2. A device according to Claim 1 wherein the temperature equalising body is formed by a wall of the test chamber.

3. A device according to Claim 1 or 2 wherein at least the first throttling means is formed by a porous sintered body.

4. A device according to Claim 1,2 or 3 wherein the first throttling means is covered with a cover plate which is in communication with a calibration gas feed pipe.

5. A device for the calibration of vacuum gauges, constructed, arranged and adapted to operate substantially as herein described with reference to, and as shown in, the accompanying diagrammatic drawing.