The present invention pertains to an adapter for an electrochemical gas sensor with a housing accommodating an electrolyte, at least one measuring electrode arranged therein, and a membrane that screens the measuring electrode from a measured gas and is permeable to the measured gas.
In an electrochemical gas sensor, the measured gas diffuses through a membrane into the electrolyte of the sensor. A measuring electrode, at which the measured gas is electrochemically converted, is located in the electrolyte. A current, which generates a measured signal, flows through the electrochemical gas sensor. The value of the measured signal is determined by the rate of the signal-generating process taking place at the electrode. It depends on the so-called mass transport (diffusion and optionally convection), on the one hand, and, on the other hand, on the resulting overall rate of the reaction taking place at the electrode, which may consist in turn of a plurality of partial steps (adsorption/desorption, homogeneous reactions, heterogeneous reactions, charge transfer, phase formation).
Electrochemical gas sensors are designed, in general, such that the mass transport is the rate-determining partial step, because a linear concentration dependence of the signal can thus be achieved and the sensor has a substantially higher long-term stability.
In a transport-controlled process taking place at the electrode, the resulting sensor current depends on the concentration gradient or the layer thickness of the depletion zone in front of the measuring electrode. If the sensor is operated in the diffusion mode, a spherical depletion zone is formed, which greatly limits the signal amplitude and may also be interfered with by ventilation.
If gas is actively admitted to the sensor, more or less linear gradients are formed depending on the geometry and the velocity of flow.
U.S. Pat. No. 4,017,373 shows an electrochemical gas sensor with a flow gap for the measured gas, wherein the feed and removal of the measured gas to and from the measuring electrode take place via thin pipelines.
DE 196 19 169 C2 describes an electrochemical gas sensor with at least two electrodes, with an electrode carrier and with an electrolyte in a housing made of a material that is impermeable to electrolyte. The otherwise closed housing has an inlet capillary and an outlet capillary for the measured gas, so that an interaction is ensured between the measured gas sample and the measuring electrode in the electrochemical gas sensor, but the diffusion of moisture from the environment into the interior of the housing is prevented from occurring at the same time. The drawback of the prior-art electrochemical gas sensor is that the measuring sensitivity is low, so that low gas concentrations to be measured cannot be determined.
The object of the present invention is to increase the measuring sensitivity of an electrochemical gas sensor.
According to the invention, an adapter is provided intended for an electrochemical gas sensor with a housing accommodating an electrolyte. At least one measuring electrode and a membrane are arranged in the housing. The membrane screens the measuring electrode from a measured gas and is permeable to the measured gas. The measured gas will be defined below as both a measured gas in a gas mixture and a measured gas dissolved in a liquid. An inlet for feeding the measured gas to the side of the membrane located opposite the measuring electrode is formed either by the adapter or by the housing or by the adapter and the housing together. If the inlet for feeding in the measured gas is formed by the adapter and the housing together, the two parts have a geometry that releases the inlet for the measured gas when the parts are fitted together.
An outlet for removing the measured gas from the side of the membrane located opposite the measuring electrode is correspondingly formed by the adapter or by the housing or by the adapter and the housing together.
Between the inlet and the outlet, the adapter has a gas-impermeable surface extending in parallel to and at a spaced location from the membrane, so that the adapter and the membrane form a flow gap for the measured gas, a so-called capillary gap. A corresponding adapter is also called a capillary gap gas distribution adapter. The adapter can be connected to a pump, so that the measured gas fed in is sent through the gap by means of the pressure generated by the pump when the pump is arranged upstream or by the suction when the pump is arranged downstream. Due to the fact that the measured gas is passed through a flow gap, which is limited by two surfaces, namely, the gas-impermeable surface of the adapter and the membrane, good diffusion of the measured gas through the membrane and thus a measuring sensitivity of the electrochemical gas sensor that is improved many times is guaranteed.
In a first embodiment, the flow gap has an essentially parallelepipedic shape, with two lateral limiting walls extending vertically between the flat membrane and the gas-impermeable surface.
In a second embodiment, the flow gap is of a radially symmetrical design. The membrane and the gas-impermeable surface have the shape of circular disks. A central opening in the gas-impermeable surface acts either as an inlet for feeding in the measured gas or as an outlet for removing the measured gas. A ring-shaped gap is provided between the edge of the gas-impermeable surface shaped as a circular disk with a central opening and the housing. The measured gas is fed in via the central opening in the gas-impermeable surface, after which it is passed radially to the outside through the flow gap and is removed via the ring-shaped gap.
This is especially advantageous because the velocity of flow decreases in the radial direction because of the continuity equation. As a result, the residence time of the measured gas at the membrane increases with increasing depletion due to the reaction with the electrolyte, so that little time is available for the diffusion of the gas through the membrane in areas with high measured gas concentration, and much time is available for this diffusion in areas with low concentration.
However, it is conversely also conceivable that the measured gas is fed in via the ring-shaped gap. It is then passed through the flow gap radially in the inward direction and is removed via the central opening in the gas-impermeable surface.
A third embodiment provides for a rotationally symmetrical design of the flow gap with the membrane formed as an outer cylinder jacket around the axis of rotation and with the gas-impermeable surface, which is formed as an inner cylinder jacket and is arranged coaxially to the outer cylinder jacket. An alternative variant of the third embodiment provides for the membrane being designed as an inner cylinder jacket and the gas-impermeable surface as an outer cylinder jacket. The measured gas is passed through the flow gap in parallel to the axis of rotation in both cases.
In a special design of the third embodiment and its alternative variant, the outer cylinder jacket is limited by a gas-impermeable outer cylinder bottom extending at right angles to the axis of rotation. The inner cylinder jacket is limited by an inner cylinder bottom, which is located at a spaced location from the outer cylinder bottom. A central hole, through which measured gas is either fed in or is passed to the outside to the flow gap or is, conversely, removed, after it was passed radially in the inward direction from the flow gap, is located in the inner cylinder bottom.
An additional embodiment is represented by an adapter detachably connected to the housing of the electrochemical gas sensor. A plug-type connection may be provided which optionally snaps in, or has a screw connection.
As an alternative to this, the adapter is firmly connected to the housing of the electrochemical gas sensor, especially in one piece as part of the housing of the electrochemical gas sensor.
In a preferred embodiment, the flow gap has a gap width of at most 2.5 mm. Optimal mass transport of the measured gas being sent by the membrane due to diffusion is guaranteed in this size range.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which preferred embodiments of the invention are illustrated.