Test gas leak detection using a composite membrane
A leak detector includes a leak detector inlet to receive a test gas; a vacuum pump coupled to the leak detector inlet; a test gas sensing unit connected through a passage to the leak detector inlet; and a membrane that is permeable to the test gas disposed in the passage between the leak detector inlet and the test gas sensing unit, the membrane having a permeance to the test gas that varies by less than five percent over a temperature range of T 0 −20K to T 0 +20K, where T 0 is a design temperature. In addition, methods are provided for making a composite membrane that includes a semi-permeable membrane and a porous membrane. Furthermore, a reference leak, which includes a composite membrane is provided.
1. A method for making a composite membrane, comprising;
selecting a semi-permeable membrane having a permeance S to a test gas and a porous membrane having a permeance P to the test gas to satisfy an equation
1.
P
2
ⅆ
S
ⅆ
T
+
S
2
ⅆ
P
ⅆ
T
=
0
,
where T is temperature and wherein the equation is evaluated at a temperature T 0 of about 300 K; and
forming a composite membrane including the selected semi-permeable membrane and the selected porous membrane.
2. The method as defined in claim 1 , wherein selecting comprises selecting materials and thicknesses of the semi-permeable membrane and the porous membrane to satisfy the equation.
3. The method as defined in claim 1 , wherein forming a composite membrane includes forming a silica layer on front and back sides of the porous membrane.
4. The method as defined in claim 1 , further comprising incorporating the composite membrane into a passage of a leak detector between an inlet and a test gas sensing unit.
5. The method as defined in claim 1 , wherein selecting comprises selecting pore sizes of the porous membrane to satisfy the equation.
6. The method as defined in claim 1 , wherein selecting comprises selecting the semi-permeable membrane and the porous membrane such that the net permeance N of the semi-permeable membrane and the porous membrane has zero slope as a function of temperature at the temperature T 0 .
7. The method as defined in claim 1 , wherein the semi-permeable membrane comprises a silica layer and wherein the porous membrane comprises a porous glass substrate.
8. The method as defined in claim 1 , wherein selecting comprises selecting a material and a thickness of the semi-permeable membrane and selecting a material, a thickness and a pore size of the porous membrane.
9. The method as defined in claim 1 , wherein selecting comprises adjusting parameters of the semi-permeable membrane and the porous membrane so that the composite membrane has a permeance to the test gas that varies by less than five percent over a temperature range of T 0 −20K to T 0 +20K.
10. The method as defined in claim 1 , wherein selecting comprises adjusting parameters of the semi-permeable membrane and the porous membrane so that the composite membrane has a permeance to the test gas that varies by less than one percent over a temperature range of T 0 −20K to T 0 +20K.