Gross leak measurement in an incompressible test item in a film chamber

Abstract

The invention relates to a method for the gross leak measurement of an at least partially incompressible test object (18) in a film chamber (12) that comprises at least one flexible wall region and is connected, in a gas-conducting manner, to a pressure sensor (30), a vacuum pump (26) and, by way of a calibration valve (34), to a calibration chamber (36) enclosing a calibration volume (37), comprising the steps of evacuating the film chamber (12), measuring the pressure curve within the film chamber (12) after the evacuation is completed, connecting, in a gas-conducting manner, the calibration volume (37) to the inner volume of the film chamber (12) during the measurement of the pressure curve, the pressure being measured before the gas-conducting connection is established, and with the gas-conducting connection established, to the film chamber (12), and the pressure in the calibration chamber (36) prior to the connection to the film chamber (12) being higher or lower than the pressure in the film chamber (12), characterized in that the pressure difference p.sub.leer between the pressure before the gas-conducting connection is established and the pressure with a gas-conducting connection to the film chamber (12), in the case of an empty film chamber (12) comprising no test object (18), is compared to the corresponding pressure difference p.sub.Prfling when a test object (18) is present in the film chamber (12).

Claims

1. A method for the gross leak measurement of an at least partially incompressible test object in a film chamber that comprises at least one flexible wall region and is connected, in a gas-conducting manner, to a pressure sensor, a vacuum pump and, by way of a calibration valve, to a calibration chamber enclosing a calibration volume, comprising the following steps: evacuating the film chamber; measuring a progression of a first pressure curve within the film chamber after the evacuation is completed; connecting, in a gas-conducting manner, the calibration volume to the inner volume of the film chamber during the measurement of the progression first pressure curve, pressure being measured before the gas-conducting connection is established, and with the gas-conducting connection established, to the film chamber, and pressure in the calibration chamber prior to the connection to the film chamber being higher or lower than pressure in the film chamber, wherein the pressure difference p.sub.empty is the difference between pressure before the gas-conducting connection is established and pressure with a gas-conducting connection to the film chamber, in the case of an empty film chamber comprising no test object; locating an object in the film chamber; evacuating the film chamber; measuring a progression of a second pressure curve within the film chamber when the test object is present after the evacuation is completed; connecting, in a gas-conducting manner, the calibration volume to the inner volume of the film chamber during the measurement of the second pressure curve, pressure being measured before the gas-conducting connection is established, and with the gas-conducting connection established, to the film chamber, and pressure in the calibration chamber prior to the connection to the film chamber being higher or lower than pressure in the film chamber, wherein the pressure difference p.sub.testing is the difference between pressure before the gas-conducting connection is established and pressure with a gas-conducting connection to the film chamber when a test object is present in the film chamber; comparing p.sub.empty to p.sub.testing.

2. The method according to claim 1, wherein the inner volume V.sub.inside of the film chamber, comprising the test object therein, is calculated as follows: $V_{\mathrm{Inside}}=V_{\mathrm{empty}}\left(\frac{p_{\mathrm{empty}}\left(p_V-p_{G2}\right)}{p_{\mathrm{testing}}\left(p_V-p_{G1}\right)}-1\right)$ or, when p.sub.G1, p.sub.G2<<p.sub.v $V_{\mathrm{Inside}}{V}_{\mathrm{empty}}\left(\frac{p_{\mathrm{empty}}}{p_{\mathrm{testing}}}-1\right),$ where V.sub.inside: is the inner volume of the film chamber comprising the test object therein; V.sub.empty: is the inner volume of the empty film chamber without the test object; p.sub.empty: is the pressure difference between the pressure in the film chamber before connection to the calibration chamber and the pressure in the film chamber after connection to the calibration chamber, with an empty film chamber; p.sub.testing: is the pressure difference between the pressure in the film chamber before connection to the calibration chamber and the pressure in the film chamber after connection to the calibration chamber, when a test object is present in the film chamber; P.sub.G1: is the final pressure that is present with an empty chamber after the calibration volume has been connected to the film chamber; and P.sub.G2: is the final pressure that is present with a chamber that contains a test object after the calibration volume has been connected to the film chamber.

3. The method according to claim 1, wherein the inner volume of the empty chamber is determined by way of a comparative measurement of an empty chamber to a measurement having a know inner volume V.sub.Cal, using the following relationship: $V_{\mathrm{empty}}=\left(\frac{V_{\mathrm{cal}}}{\left(\frac{p_{\mathrm{empty}}\left(p_V-p_{G2}\right)}{p_{\mathrm{testing}}\left(p_V-p_{G1}\right)}-1\right)}\right),$ or when p.sub.G1, p.sub.G2<<p.sub.v $V_{\mathrm{empty}}=\left(\frac{V_{\mathrm{cal}}}{\left(\frac{p_{\mathrm{empty}}}{p_{\mathrm{testing}}}-1\right)}\right)$ where V.sub.cal: is a known calibration volume, having a magnitude between and 1/20 of the inner volume of the empty chamber V.sub.empty.

4. The method according to claim 1, wherein the calibration chamber is provided with a test leak having a known leakage rate, and the inner volume of the film chamber is ascertained taking into consideration the pressure increase that develops after the film chamber has been connected to the calibration volume.

5. The method according to claim 1, wherein the respective pressure differences between the pressure before the gas-conducting connection is established between the film chamber and the calibration chamber, and the final pressure inside the film chamber present with a gas-conducting connection between the film chamber and the calibration chamber are considered.

6. The method according to claim 5, wherein the final pressure that is present inside the film chamber with a gas-conducting connection between the film chamber and the calibration chamber is extrapolated based on an exponential function of the pressure curve that develops, which has at least two pressure values measured by way of the pressure sensor.

7. The method according to claim 6, wherein the following formula is used for the extrapolation of the developing final pressure:
p(t)=(p.sub.Endp.sub.Kam)(1e.sup.t/Tau)+p.sub.Kam, where p(t): is the instantaneous pressure at the point in time t; p.sub.Kam: is the pressure in the film chamber before the gas-conducting connection between the film chamber and the calibration chamber is established; p.sub.End: is the final pressure in the film chamber after the film chamber and the calibration chamber have been connected, preferably at the point in time t=, t: is the time; and Tau: is the time constant of the pressure settling process after the film chamber has been connected to the calibration chamber.

8. The method according to claim 7, wherein, for equal time differences t2t1=t3t2, the final pressure is extrapolated as follows:
p.sub.End=(p2p2p.sub.Kamp2)/(2p2p.sub.Kamp3), where p2: is the pressure in the film chamber at a point in time t2 after the connection between the film chamber and the calibration chamber has been established; p3: is the pressure in the film chamber at a point in time t3 after the point in time t2; and t1: is the point in time at which the calibration valve is opened.

Description

(1) An exemplary embodiment of the invention will be described in greater detail hereafter based on the figures. In the drawings:

(2) FIG. 1 shows a schematic representation in a first operating state;

(3) FIG. 2 shows the pressure curve in the first operating state;

(4) FIG. 3 shows the view according to FIG. 1 in a second operating state;

(5) FIG. 4 shows the pressure curve in the second operating state;

(6) FIG. 5 shows the view according to FIG. 1 in a third operating state;

(7) FIG. 6 shows the pressure curve in the third operating state; and

(8) FIG. 7 shows a detailed view of the pressure curve according to FIG. 6.

(9) The film chamber 12 is composed of two film layers 14, 16, which enclose a test object 18 and are joined to one another in a gas-tight manner in the edge region of the test object 18. The film layers 14, 16 enclose a film chamber volume 20 in the interior of the film chamber 12. In FIG. 1, the film chamber volume 20 is the volume inside the film chamber 12 in the region outside the test object 18.

(10) Via a gas line 22, the interior of the film chamber 12 is connected in a gas-conducting manner via an evacuation valve 24 to a vacuum pump 26, via a measuring valve 28 to a pressure sensor 30, via a vent valve 32 to the atmosphere surrounding the film chamber 12, and via a calibration valve 34 to a calibration chamber 36.

(11) The calibration chamber 36 encloses a calibration volume, which initially is filled with air under atmospheric pressure. The calibration valve 34 is initially closed. The figures show the open state of a valve by way of a solid valve, and the closed state of a valve by a non-solid valve. In the first operating state according to FIG. 1, the measuring valve 28, the vent valve 32 and the calibration valve 34 are thus closed. In contrast, the evacuation valve 24 is open. In the first operating state shown in FIG. 1, the test object 18 is located within the film chamber 12, which is closed in a gas-tight manner, while the vacuum pump 26 evacuates the film chamber 12 via the gas line 22 when the evacuation valve 24 is open.

(12) FIG. 2 shows the pressure curve that develops during evacuation inside the film chamber 12. If the measuring valve 28 were open, the pressure sensor 30 would measure the pressure curve shown in FIG. 2. However, the measuring valve 28 is closed in FIG. 1 during evacuation of the film chamber 12 so as not to damage the pressure sensor 30.

(13) FIG. 3 shows the subsequent operating state after the film chamber 12 has been evacuated. The evacuation valve 24 is closed (shown not solid), and the measuring valve 28 is open (shown solid). The hermetically sealed film chamber volume 20 is thus connected to the pressure sensor 30. As is shown in FIG. 4, the pressure sensor 30 measures an increase in pressure inside the film chamber 12 over the time t. This pressure increase may, on the one hand, result from a leak in the test object 18 and, on the other hand, from offset pressure. The offset pressure increase is an increase in pressure not caused by a leak in the test object 18, but by other physical effects, such as gas molecules outgassing from the film chamber wall.

(14) After the film chamber 12 has been evacuated (first operating state) and the measuring valve 28 has been opened (second operating state), the calibration valve 34 is now also opened. This third operating state is shown in FIG. 5. The air flows out of the calibration chamber 36 through the opened calibration valve 34, via the gas line 22, into the film chamber 12. Due to the large pressure difference between the vacuum inside the film chamber 12 and the atmospheric pressure inside the calibration chamber 36, the pressure in the film chamber 12 rises abruptly after the calibration valve 34 has been opened.

(15) This pressure stroke p is shown in FIG. 6 and is measured by the pressure sensor 30. The pressure stroke p is the difference between the pressure p.sub.G inside the film chamber 12 after the calibration valve 34 has been opened and the pressure p.sub.F inside the film chamber 12 before the calibration valve 34 is opened:
p=(p.sub.Gp.sub.F).
Since the total gas volume in the film chamber 12 and in the calibration chamber 36 remains the same before and after the calibration valve has been opened, the following applies:
p.sub.G(V.sub.F+V.sub.V)=p.sub.FV.sub.F+p.sub.VV.sub.V,
where P.sub.G: is the pressure inside the film chamber 12 after the calibration valve 34 has been opened; V.sub.F: is the film chamber volume 20 to be determined; V.sub.V: is the calibration volume 37 inside the calibration chamber 36 (in the range between 1/1000 and 1/10 of the film chamber volume without the test object); and P.sub.V: is the pressure inside the calibration chamber 36 before the calibration valve 34 is opened (atmospheric pressure, approximately 1 bar).

(16) Based on the pressure stroke p=p.sub.Gp.sub.F, it is possible to calculate the film chamber volume 20 as follows:

(17) $V_F=V_V\frac{\left(p_V-p_G\right)}{\left(p_G-p_F\right)}=V_V\frac{\left(p_V-p_G\right)}{p}.$

(18) The pressure p.sub.G considered in the consideration of the pressure stroke p is preferably the final pressure p.sub.End that develops. The final pressure p.sub.End is the pressure that is present after the pressure equalization has taken place between the film chamber 12 and the calibration chamber 36, which is to say at the end of the settling process of the film chamber pressure after the calibration valve 34 has been opened.

(19) The inner volume V.sub.Innen of the film chamber 12 when a test object 18 is present in the film chamber, such as an at least partially incompressible or even rigid and/or dimensionally stable test object, can be calculated as follows:

(20) $V_{\mathrm{Innen}}=V_{\mathrm{leer}}\left(\frac{p_{\mathrm{leer}}\left(p_V-p_{G2}\right)}{p_{\mathrm{prfling}}\left(p_V-p_{G1}\right)}-1\right)$
and if the developing final pressures P.sub.G1 and P.sub.G2 after pressure equalization are small compared to the pressure in the calibration volume p.sub.V, it can be calculated as follows:

(21) $V_{\mathrm{Innen}}{V}_{\mathrm{leer}}\left(\frac{p_{\mathrm{leer}}}{p_{\mathrm{prfling}}}-1\right),$
where V.sub.Innen: is the inner volume of the film chamber 12 comprising the test object 18 therein; V.sub.leer: is the inner volume of the empty film chamber 12 without the test object 18; p.sub.leer: is the pressure difference between the pressure in the film chamber 12 before connection to the calibration chamber 36 and after connection to the calibration chamber 36, with an empty film chamber 12; p.sub.Prfling: is the pressure difference between the pressure in the film chamber 12 before connection to the calibration chamber 36 and after connection to the calibration chamber 36, when a test object is present in the film chamber 12; P.sub.G1: is the final pressure that is present with an empty chamber after the calibration volume has been connected to the film chamber; and P.sub.G2: is the final pressure that is present with a chamber that contains a test object after the calibration volume has been connected to the film chamber.

(22) The inner volume V.sub.leer is determined by way of a one-time calibration using a known inner volume. For this purpose, a measurement is conducted with an empty chamber, and a measurement is conducted with a known inner volume V.sub.Kal. The inner volume of the chamber is then determined as follows:

(23) $V_{\mathrm{leer}}=\left(\frac{V_{\mathrm{kal}}}{\left(\frac{p_{\mathrm{leer}}\left(p_V-p_{G2}\right)}{p_{\mathrm{prfling}}\left(p_V-p_{G1}\right)}-1\right)}\right),$
or when p.sub.G1, p.sub.G2<<p.sub.v

(24) $V_{\mathrm{leer}}=\left(\frac{V_{\mathrm{kal}}}{\left(\frac{p_{\mathrm{leer}}}{p_{\mathrm{prfling}}}-1\right)}\right)$
where V.sub.Kal: is a known calibration volume. The magnitude of the calibration volume should range between and 1/20 of the inner volume of the empty chamber V.sub.leer.

(25) FIG. 7 shows the settling process of the pressure p during the pressure equalization with an open calibration valve 34. If the pressure stroke p that is considered as the pressure difference P.sub.EndP.sub.Kam, which is to say the difference between the film chamber inner pressure at the end of the settling process and the film chamber inner pressure before the settling process, the pressure curve p(t) can be mathematically described as a function of the time t during the settling process using the following formula:
p(t)=(p.sub.Endp.sub.Kam)(1e.sup.t/Tau)+p.sub.Kam,
where p(t): is the instantaneous pressure at the point in time t; p.sub.Kam: is the pressure in the film chamber (12) before the gas-conducting connection between the film chamber and the calibration chamber (36) is established; p2: is the pressure in the film chamber (12) at a point in time t2 after the connection between the film chamber and the calibration chamber (36) has been established; p3: is the pressure in the film chamber (12) at a point in time t3 after the point in time t2; p.sub.End: is the final pressure in the film chamber (12) after the film chamber (12) and the calibration chamber (36) have been connected, preferably at the point in time t=, t: is the time; and Tau: is the time constant of the pressure settling process after the film chamber (12) has been connected to the calibration chamber (36).

(26) Based on at least two consecutive pressure measurement values p2, p3, and preferably at least three consecutive pressure measurement values p1, p2, p3, the pressure curve p(t) can be extrapolated using the above formula. It is then not necessary to measure the pressure curve and wait until the final pressure p.sub.End has developed once the pressure equalization has taken place. Rather, it is possible to extrapolate the film chamber inner pressure even before the pressure equalization has taken place.

(27) For identical intervals between the points in time t1, t2, and t2, t3, where t1 is the point in time at which the calibration valve 34 is opened, and t2 is between t1 and t3, it is then possible to calculate the final pressure p.sub.End as follows:
p.sub.End=(p2p2p.sub.Kamp2)/(2p2p.sub.Kamp3).

(28) In this way, it is possible to calculate the final pressure p.sub.End that develops inside the film chamber before this final pressure develops with pressure equalization. Based on the final pressure thus calculated, the above-described pressure differences can be ascertained and compared to one another so as to identify a gross leak on an at least partially incompressible test object.