DE2336985A1 - Oxygen in liquid determination by semiconductor - this being separated from liquid by membrane only permeable to gas - Google Patents

Abstract

The O2 concn. in a liq., specif. its partial pressure, can be determined by using a semi-conductor device whose conductivity varies with the gas partial pressure and which can give a reading on a detector. The semiconductor is shielded by a membrane only permeable to gases. This membrane is a plastic (e.g. PTFE). Specif. a luminous diode can be used as energy source. Can be used by unskilled personnel to determine O2 concns, in effluents and rivers etc.. Gives reliable, reproducible results as compared with methods involving electrolytes.

Description

BROWN, BOVERI & CIE · AKTIENGESELLSCHAFT

MANNHEIM BROVM BOVERl

Mp.no. 585/73 Mannheim, July 5, 1973 I.

PAT-Ka / Bt

Sensor for measuring the partial pressure of oxygen in liquids.

The invention relates to a measuring transducer for measuring the oxygen partial pressure in liquids with a Semiconductor element whose conductivity changes due to the partial pressure of the surrounding gases and via a measuring device delivers the measured value.

For such.Meßauf taker there is one in measurement technology wide scope. These sensors are of particular importance in water measurement technology. In all phases of the i

Wastewater treatment depends on the biochemical process; the presence of dissolved _{O 2} components in the wastewater. Be it in the receiving water, where predominantly natural aerobic effects reduce the wastewater pollution, be it in the sewage treatment plant, where similar processes take place in the aeration basin - you have to know about the oxygen content if you want to monitor or influence wastewater and receiving water.

The requirements of wastewater practice are given by the task: O ₂ measuring devices and arrangements should be suitable for rough operational practice, they must be clear and easy to use, without the need for special prior knowledge of the operating personnel, they must be adapted to the conditions of wastewater technology and Above all, bring the level of reliability and accuracy that is absolutely necessary for monitoring tasks.

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585/73

In order to accomplish this task, two measuring methods with appropriate measuring sensors have been in use for a long time. One of these is the electrochemical (galvanic) method, as known, for example, from US Pat. No. 3,21824-2 and the membrane polarographic method according to US Pat. No. 2,913,386.

However, both methods have a weakness, which means that they cannot fully meet the requirements set out above.

The disadvantage of the methods is the consumption of the electrolyte. The electrolyte needs to be replaced from time to time, what inevitably causes an interruption of the measurements and requires trained specialists for maintenance.

The object of the invention is to provide a transducer which eliminates the aforementioned disadvantage and thus ¹ reaches a high service life and low maintenance with untrained personnel and without electrolyte consumption.

To achieve this goal, the invention makes the knowledge the so-called chemisorption to use. Especially with systems that conduct the surface layer (semiconductors with surface layer formation) special oxygen pressure dependencies of the conductivity occur. The absolute value of the total conductivity, the from the conductivities of the surface layer and the inner phase in relation to that of the surface layer and inner phase Composed of parts of the current path, both in the case of an excess electron conductor and an electron

defective conductor with poorly conducting impoverishment edge layer practically solely due to the conductivity of the surface layer

-3- I.

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PaI 4 F IfIi ^- ? 11S "f-KE ·

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give. In the case of the formation of highly conductive enrichment edge layers however, the opposite can also occur. In the former case, it becomes the oxygen pressure dependency the total conductivity due to the oxygen ; pressure dependency of the surface layer conductivity, j because the inner phase is firstly much better conductive and for Other than that, the influence of oxygen on the conductivity of the surface layer remains limited. The total conductivity \ f _-

I. · Egg.

j of a polycrystalline oxide, as it is for a measuring transducer ! of the type according to the invention is used, looks according to Engell and Hauffe as follows:

! . 1 a - 2.1 /1._. . 2_1 _c . 1

where 1 is the length of the crystal lozenge, a is the mean edge length of a crystallite, c is a material constant, ^ 5 'the; The conductivity of the inner phase of the lozenge and p _{Q represent} the partial pressure of oxygen surrounding the lozenge. In c all constant terms are combined. An application of «

i Λ lip Λ 'p-p-p

; against Pq - 'or p _Q should result in an I straight line from case to case.

1 In FIG. 1, the course of the oxygen pressure dependency is shown for illustration the conductivity of ZnO at 25 ° C.

I These physical principles led to the invention , Formation of a measuring transducer, which is characterized by

that the semiconductor element is provided with a layer which only allows gases to pass through. The layer consists of a plastic, j such as polytetrafluoroethylene, polyethylene, etc. j

I ■ ι

So far, the effect theoretically shown above has been possible : cannot be used to measure oxygen partial pressure, as j conductivity measurement e.g. in corrosive, humid gases j and is completely disturbed in water or in water polluted by acids and bases. The sorbent (the semiconductor) is destroyed by these substances. First the protective layer

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Conducts an oxygen-dependent conductivity measurement via the semiconductor element possible in the water. The semiconductor element can only be covered by this layer or directly with it Layer be coated. ]

In a further embodiment of the invention, an energy supply in the form of heating, irradiation or exposure is provided to accelerate the measurement, such as for desorption. A luminescent diode has proven to be particularly advantageous for this purpose.
ι

j The special gate parts of the sensor according to the invention lie in the fact that the components no longer have to be serviced, exchanged or renewed, as is the case with polarographic

the electrolyte, anode and cathode are the oxygen electrodes, j that wear out over time (3-6 months). j

One finds particular interest in such a maintenance-free one Measuring sensor in sewage treatment plants, where a completely continuous I measurement of oxygen is required. Another benefit is there 'in that you can design the transducer tubular and both the inner wall of the pipe and the outer wall of the pipe can be designed as a membrane that is permeable to acid. The sorbent (the semiconductor) is in both cases in the wall of the pipe

i body. j

The method is explained in more detail using three exemplary embodiments:

Fig. 2 shows a tube 1, which at the lower end with a porous body 2, which is covered with a layer 3 (Kunstj fabric membrane) was covered, is provided. Inside the I tube 1 is a semiconductor element 4 (sorbent) on which ίtwo metallic electrodes 5, 5 'are attached separately to with a conductivity measuring bridge 6 the conductivity of the Sor-! bens 4 as a function of the out of the water through the plastic membrane 3 diffusing oxygen 7 to measure. ;

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Fig. 3 shows a tube 11 open at both ends, the inside! lined with the sorbent 14 and removed from the pure body 12 is fully or partially covered. The body or the porous tube 12 is coated on the inside with the plastic membrane 13 .; On the end faces of the tubular sorbent 14 there are two metallic electrodes 15, 15 'in order to increase the conductivity analogously to FIG to eat. The water, from which the oxygen has to diffuse through the plastic membrane 13, flows through the Tube 11.

Of course, you have to imagine the measuring transducer encapsulated in a watertight manner!

4 shows a tubular piece 21 which is open at both ends. The porous body 22 is now seated outside over the pipe 21. Between the pipe 21 and the pDröse body 22 is located the sorbent 24, on whose end faces the electrodes 25, 25 ' are arranged. The water now flows past the plastic membrane 23 outside the pipe.

One can also imagine a double-walled measuring sensor according to FIG. 5, in which the water is between the two walls 26 and 27 flows through it. The water inlet is denoted by 38, the outlet by 39 and the gas path by 40. The other one Structure is identical to one of the previous figures.

409886/1182

Pat 4 F 1 (6 »0 10000 / KE)

Claims (4)

585/73 claims 1.) Sensor for measuring the partial pressure of oxygen in Liquids with a semiconductor element whose conductivity is determined by the partial pressure of the surrounding i Gases changes and the measured value | supplies, characterized in that the semiconductor element <is provided with a layer which only allows gases to pass through. ^! 2. Sensor according to claim 1, characterized in that the layer consists of a plastic such as polytetrafluoroethylene. 3. Measuring transducer according to claim 1 or 2, characterized in that that an energy supply in the form of a heater, Be j radiation or exposure is provided. 4. Sensor according to claim 3 »characterized in that a light emitting diode is provided for supplying energy. 409886/1182 PaI 4 f 1 (172! J <". 0O.'KF> Blank page