Patent Application
20050145330
Electro-chemical sensors are used extensively for the detection of industrial toxic gases. Toxic gases are typically measured in the parts per million range (ppm) for human safety. Typical gases and instrument ranges (full scale) include: carbon monoxide (100 and 500 ppm), chlorine (10 and 20 ppm), chlorine dioxide (3 ppm), hydrogen chloride (20 ppm), nitric oxide (100 ppm), nitrogen dioxide (20 ppm), ammonia (50 and 100 ppm), ozone (1 ppm), sulfur dioxide (20 ppm) and hydrogen sulfide (20 ppm). In addition, electro-chemical sensors are used for the monitoring of oxygen deficiency for personal safety. Industrial safety instrumentation based on these sensors can produce an alarm when the oxygen level drops below a certain percentage, for example, 19.5% by volume is considered the minimum safe level of oxygen.
A typical electro-chemical sensor includes a plastic enclosure, which houses such elements as electrodes, electrolyte, various membranes and wicking material. The housings of currently available electro-chemical cells include a bottom part and a top cover. These two parts are fused together to form a cell housing; the gas diffuses through openings and membranes in the top cover.
Cell performance and integrity strongly depends on the design of the housing and the type of encapsulation. There are a number of methods that are currently used for the fabrication of electro-chemical sensors, including: mechanical “snap fit”, ultrasonic welding of the top and bottom parts, epoxy and chemical solvent bonding. All of these techniques have significant shortcomings.
Features and advantages of the disclosure will readily be appreciated by persons skilled in the art from the following detailed description when read in conjunction with the drawing wherein:
In the following detailed description and in the several figures of the drawing, like elements are identified with like reference numerals.
Nd:YAG and diode lasers produce optical energy in the near-infrared wavelength region of 800-1100 nanometers. Radiation at these wavelengths can actually penetrate inside a plastic part. The depth of the laser penetration is a function of the laser wavelength and the optical properties of the selected plastic. Currently, high power diode lasers are available at wavelengths of 808 nm, 940 nm and 980 nm, and can deliver the 5 to 50 Watts of energy for welding plastics. The laser beam can be transmitted using a wide spectrum of optical systems, including reflective optics, refractive optics, and fiber-optic elements. To assure optimal energy absorption conditions, the optical properties of a polymer can be altered by the addition of an absorber (carbon black or absorbing pigments).
Precision laser welding of thin and/or thick plastic parts and components has the advantage that laser welding can be performed exactly where it is required at the interface between pre-assembled parts in a well-defined manner. Direct contact between parts held under pressure ensures heating of both materials at the interface; welding occurs upon the localized melting and fusion of materials at the interface.
A plastic electrolyte cover 22, which in an exemplary embodiment has an inverted cup-like shape with an opening 22A, is placed in the bottom housing, forming an electrolyte reservoir 42 holding an electrolyte liquid or gel 44. A layer portion 46A of wicking material is positioned at the top of the reservoir, with another layer portion 46B positioned in the opening 22A. A third layer portion 46C of wicking material is positioned over the opening 22A in space 56. The wicking material can comprise a cotton wick or glass fiber wick, by way of example. An electrically conductive sensing electrode (with catalyst) having, in an exemplary embodiment, a generally circular shape, is fabricated, e.g. screen printed, on a Teflon membrane, collectively shown as layer 48, forming a top electrode structure. In an exemplary embodiment, an electrically conductive reference electrode 52 and an electrically conductive spaced counter electrode 54 are fabricated on another Teflon membrane, forming a bottom electrode structure. The top and bottom electrode structures are separated by the layer portion 46C. In this exemplary embodiment, the reference electrode and the counter electrode each have a generally half-circular configuration, with a gap formed between adjacent edges. The opening 22A in the cover 22 permits the electrolyte 44 to wick by capillary action from the reservoir into the wicking material layers, and provide an electrically conductive path between the electrodes 48 and 52, 54. In an exemplary embodiment, the conductivity of the path is increased by the reaction of the gas with the electrodes.
An optically transparent clear top housing cover 30 is sealed to the bottom housing structure 20 by laser welding, as will be described below. The top cover 30 has an opening formed there through, permitting ambient air to pass through the cover to the interior 56 of the housing structure 20. The diameter and length of the opening controls the flow of ambient air into the sensor. A dust cover (not shown) may be placed over the opening 32 in some applications, while still allowing gas to pass into the housing.
Electrical contact to the electrode structures 48, 52, 54 is made, in this exemplary embodiment, by wire ribbons 70, 72, 74, fabricated of an electrical conductive material such as platinum. The wire ribbons are passed through holes or channels 21A, 21B, 21C formed in the housing structure 20 to make electrical contact between terminals 70A, 70B, 70C and respective ones of the electrode structures 48, 52, 54. The holes or channels 21A, 21B, 21C are filled with epoxy after the wires are inserted to prevent electrolyte leakage.
The sensor components disposed within the housing 20 thus include in this exemplary embodiment the electrolyte 44 disposed in the reservoir 42, the reference and counter-electrode structure 52, 54, wicking material 46A-46C, and the sensing electrode (with catalyst) 48. It will be appreciated that these components are merely exemplary; other electro-chemical sensors may have a different set of components.
An elastomeric o-ring seal 58 is positioned at the top of the structure 20, forming a seal between the body structure 20 and the cover 30. The o-ring seal 58 prevents the leakage of electrolyte from the housing structure.
The cover 30 in this exemplary embodiment is joined to the body structure 20 at laser welding area 60 by a process described below. The laser weld bond is at the mating surface between the top cover and the bottom housing, covering a one to two millimeter wide annular ring at the outer diameter of the package in an exemplary embodiment. The thickness of the cover in this embodiment is reduced at the periphery to form a step or shoulder 34. Other configurations for the body structure and cover may alternatively be employed; for example the body structure and cover may have a rectilinear footprint rather than a circular configuration as shown in
The top cover is attached to the bottom housing by a laser welding processes. Edges of the Teflon membranes are positioned on a shoulder area 20A1 of the wall 20A. The o-ring 58 is compressed between the cover and the Teflon membranes carrying the electrode structures supported by the shoulder area 20A1, providing a fluid seal between the housing 20 and cover 30. In one exemplary embodiment, the weld between the cover and body structure is made for mechanical attachment, with the o-ring 58 providing the fluid seal. Providing a continuous weld bond about the periphery is good manufacturing practice, and increases reliability and bond strength. In an exemplary embodiment, the weld is a continuous one about the entire 360 degree perimeter, with a weld overlap of 5 to 10 degrees; i.e. the end of the weld process finishes 5 to 10 degrees past its start point so that the end of the welding process overlaps onto areas already welded. For some applications, the o-ring 58 may be omitted, with the laser weld joint a continuous peripheral joint to provide a fluid seal.
The housing 20 is made of an optically absorbing material, e.g. a plastic material having dyes or carbon black added to make the material opaque. One exemplary material suitable for the purpose is black ABS, which has high optical absorption in the spectral bands of the laser radiation. This high optical absorption is due to the pigments present in the plastic. The body structure can be fabricated of plastic materials with additives to make it opaque to the laser radiation, and with an appropriate melting temperature to support consistent fusion of the parts.
The plastic cover 30 is made of an optically transparent plastic, such as, by way of example, polycarbonate, acrylic or clear ABS. Optically translucent or semi-opaque plastics such as natural ABS can also be used, provided the material and thickness are such as to transmit sufficient laser energy to the weld interface.
It is possible to select plastic materials that satisfy chemical resistance requirements, which are mandatory for industrial electro-chemical sensors, and also satisfy the optical transmittance characteristics required for laser welding. For adequate laser energy transmission, the top cover 30 is typically no more than a few millimeters thick, and in an exemplary embodiment, from 1 mm to 3 mm at the weld joint area, i.e. the area of the cover through which the laser beam is transmitted to reach the joint. In the embodiment shown in
Exemplary sensor laser welding examples include the following. For an exemplary carbon monoxide sensor, the bottom housing is fabricated of black ABS, and the top cover is fabricated of clear polycarbonate. A 50-Watt solid-state laser at 940 nm is the welding laser, with a spot size of 0.6 mm. The speed of relative motion between the laser beam and the part is typically about 36 degrees per second of rotational motion, with a clamping pressure of 100 psi and laser power of 10 Watts. For an exemplary oxygen sensor, the top cover is fabricated of natural ABS, which scatters as well as absorbs some laser radiation; the bottom housing is fabricated of black ABS. Since the cover scatters and absorbs some laser radiation, greater laser power, e.g. 25 Watts, is employed. The parameters will also depend on the laser spot size.
Compared to other methods of bonding such as ultrasonic welding, transmission laser welding has several advantages for electro-chemical sensors, including the following:
Although the foregoing has been a description and illustration of specific embodiments of the invention, various modifications and changes thereto can be made by persons skilled in the art without departing from the scope and spirit of the invention as defined by the following claims.
