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The present invention provides apparatus and methods for measuring the optical density of a fluid by measuring the light transmission through a reconfigurable optical path. The optical path is reconfigured by the deflection of an elastic membrane wherein the optical path length through the fluid is

The present invention provides apparatus and methods for measuring the optical density of a fluid by measuring the light transmission through a reconfigurable optical path. The optical path is reconfigured by the deflection of an elastic membrane wherein the optical path length through the fluid is determined by the position of the membrane relative to a fixed surface. The optical density can be determined from two measurements with different optical path lengths through the fluid.

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1. An apparatus for measuring the optical density of a fluid comprising: a light source;a light detector;an elastic membrane;a first deformable portion of the elastic membrane with a first side fluidically isolated from a second side that is in contact with the fluid;a first surface facing the secon

1. An apparatus for measuring the optical density of a fluid comprising: a light source;a light detector;an elastic membrane;a first deformable portion of the elastic membrane with a first side fluidically isolated from a second side that is in contact with the fluid;a first surface facing the second side of the first deformable portion of the elastic membrane with at least a portion of the first surface in contact with the fluid;a second surface facing the first side of the first deformable portion of the elastic membrane; anda reconfigurable optical path from the light source to the light detector that passes through the fluid in at least one configuration of the reconfigurable optical path;wherein: the optical path length through the fluid is determined by the position of the first deformable portion of the elastic membrane relative to the first surface;in a first configuration of the reconfigurable optical path the elastic membrane substantially contacts the second surface and the fluid is included in the reconfigurable optical path; andin a second configuration of the reconfigurable optical path the elastic membrane substantially contacts the first surface and the fluid is substantially excluded from the reconfigurable optical path. 2. The apparatus of claim 1 wherein the first deformable portion of the elastic membrane and the first surface are transparent to the light from the light source. 3. The apparatus of claim 1 wherein one or more of the first deformable portion of the elastic membrane and the first surface reflects the light from the light source. 4. The apparatus of claim 1 wherein the first surface and the first deformable portion of the elastic membrane are substantially planar. 5. The apparatus of claim 1 wherein a cross section of the first deformable portion of the elastic membrane on a section plane forms a closed contour. 6. The apparatus of claim 1 further comprising one or more aperture(s) wherein the reconfigurable optical path passes through the aperture(s). 7. The apparatus of claim 1 further comprising: a first substantially closed surface defining a first volume, wherein a portion of the first substantially closed surface comprises the first side of the first deformable portion of the elastic membrane; anda conduit to the first substantially closed surface. 8. The apparatus of claim 7 further comprising: a first source of a reference fluid at a first pressure;a second source of a reference fluid at a second pressure; anda valve selectively connecting the conduit to the first substantially closed surface to the first source at the first pressure, or to the second source at the second pressure. 9. The apparatus of claim 8 wherein the first pressure is sufficiently different from the hydrostatic pressure of the fluid wherein the first deformable portion of the elastic membrane substantially contacts the second surface in a first position when the conduit to the first substantially closed surface is connected to the first source at the first pressure; and where the second pressure is sufficiently different from the hydrostatic pressure of the fluid wherein the first deformable portion of the elastic membrane substantially contacts the first surface in a second position when the conduit to the first substantially closed surface is connected to the second source at the second pressure. 10. The apparatus of claim 7 further comprising: a second deformable portion of the elastic membrane with a first side fluidically isolated from a second side that is in contact with the fluid;a second substantially closed surface defining a second volume, wherein a portion of the second substantially closed surface comprises the first side of the second deformable portion of the elastic membrane;a conduit to the second substantially closed surface;a third substantially closed surface defining a third volume, wherein a portion of the third substantially closed surface comprises the second side of the first deformable portion of the elastic membrane and the second side of the second deformable portion of the elastic membrane and the first surface; anda conduit to the third substantially closed surface. 11. The apparatus of claim 10 further comprising: a third deformable portion of the elastic membrane with a first side fluidically isolated from a second side that is in contact with the fluid;a fourth substantially closed surface defining a fourth volume, wherein a portion of the fourth substantially closed surface comprises the first side of the third deformable portion of the elastic membrane;a conduit to the fourth substantially closed surface;the third substantially closed surface further comprising the second side of the third deformable portion of the elastic membrane; anda source of vacuum pressure connected to the conduit to the fourth substantially closed surface. 12. A method to measure optical density of a fluid comprising the steps of: a) configuring a first configuration of a reconfigurable optical path from a light source to a light detector that includes a first optical path length through the fluid defined by a first surface and a second side of a deformable portion of an elastic membrane, wherein the deformable portion of the elastic membrane is deflected in a first position where a first side of the deformable portion of the elastic membrane substantially contacts a second surface;b) activating the light source and the light detector;c) measuring the quantity of light transmission, T1, through the reconfigurable optical path, configured in the first configuration, from the light source to the light detector that includes the first optical path length through the fluid;d) configuring a second configuration of the reconfigurable optical path from the light source to the light detector that substantially excludes the fluid, wherein the deformable portion of the elastic membrane is deflected in a second position where the second side of the deformable portion of the elastic membrane substantially contacts the first surface; ande) measuring the quantity of light transmission, T2, through the reconfigurable optical path, configured in the second configuration, from the light source to the light detector that substantially excludes the fluid. 13. The method of claim 12 comprising the additional step of a) computing the optical density using the formula: optical_density=A*log10(T2/T1)+B, where A and B are constants.