DE102020101219A1 - Improved luminescence based oxygen sensor - Google Patents

Abstract

The invention relates to a luminescence-based oxygen sensor comprising a housing (2) with a housing interior (20) which forms a measurement volume and with an inlet (21) and an outlet (22), a light source (3) which emits excitation light in a first wavelength range can, a fluorophore (4) which contains an oxygen-sensitive fluorescent material and is arranged in the measurement volume so that the fluorescent material is in direct contact with the fluid and can be exposed to the excitation light emitted by the light source (3), wherein the Fluorophore (4) thereupon emits luminescent light in a second wavelength range with a response delay as a response to the excitation light, a light detector (5) which can detect at least light within the second wavelength range and outputs an analog detector signal representing a detected light intensity, in particular proportional thereto, and a e control (6), which controls the light source (3) and derives an analog or digital measurement signal representing the oxygen content of the fluid from the detector signal, the control (6) being prepared for this by means of the light source (3) the excitation light in the form of a short To emit light pulse, and to derive the measurement signal exclusively from the response delay.

Description

The present invention relates to a luminescence-based optical oxygen sensor according to the preamble of claim 1 and to a method for operating such an oxygen sensor.

The problem of a quick and reliable measurement of the oxygen content of a fluid arises in various areas of science, technology and medicine, for example the determination of the oxygen content in the breath, in the blood or in the exhaust air of an internal combustion engine. In the case of bioreactors, too, monitoring of the supply and / or exhaust air is very important in order to be able to follow and regulate the process or growth parameters and the material balance as precisely as possible. Particularly in this application, in addition to a sufficiently fast response time, compactness and reliability, even under adverse conditions, such as highly reactive components of the exhaust air, are essential criteria for an oxygen sensor.

Various types of oxygen sensors are known which are based on different physical processes such as oxygen-dependent electrical conductivity, electro-galvanic processes or optical processes. Among the oxygen sensors based on optical processes, the luminescence or fluorescence-based sensors are of particular interest.

Luminescence and fluorescence are essentially used synonymously in the following, even if luminescence is actually a broad generic term which, in addition to the fluorescence falling under the more restricted term photoluminescence, also includes other effects with spontaneous photon emission by an excited medium, in which the excitation is not by photons but took place in other ways, for example mechanically (mechanoluminescence), chemically (chemiluminescence) or electrically (electroluminescence).

Luminescence or fluorescence-based oxygen sensors use oxygen-sensitive fluorescent materials to measure the oxygen concentration of a fluid to be examined, i.e. a gas mixture or a liquid. A fluorescent material emits luminescent light of a second, usually higher (ie lower-energy) wavelength through spontaneous emission of photons after its molecules have been excited by excitation light of a first wavelength or a first wavelength range. This spontaneous emission takes place with a certain delay, which corresponds to the lifetime of the excited states occupied by the excitation light. Both the intensity and the delay of the spontaneous emission, i.e. of the luminescent light, can depend on environmental parameters. In the case of an oxygen-sensitive fluorescent material, the intensity and delay are dependent on the oxygen content of the material. Oxygen can diffuse into the material from the environment or, conversely, diffuse out of the material into the environment. Intensity and delay can therefore be a measure of the oxygen content in the vicinity of the material.

The international published application WO 2007/027116 A1 describes a method for luminescence-based oxygen measurement, which uses a zirconium dioxide single crystal as the fluorescent material. Here, however, ultraviolet excitation light is used and the ZrO2 crystal has to be heated to high temperatures of a few hundred degrees Celsius. As a result, this method is not suitable for use in a simple, inexpensive sensor.

The publication has similar disadvantages CA 1243351 A1 which proposes an oxygen sensor package which comprises a solid electrolyte sensor and a heating element made of silicon carbide with a convex surface, which surrounds the solid electrolyte and heats it on all sides.

The disclosure document EP 3 121 589 A1 presents an oxygen sensor which contains a fluorophore with aluminum nitride nanopowder beads as a fluorescent material. Heating is not necessary here, but the sensor there derives the oxygen concentration from the (extinction of) fluorescence intensity. This has the disadvantage that the detected fluorescence intensity depends not only on the oxygen content but also on numerous other variables, such as excitation intensity, transmissivity / transparency of the fluid or detector sensitivity. Not all of these variables can be reliably and precisely controlled, for example the transparency of the fluid. Others can only be kept under control with great effort, such as excitation intensity, which makes an exact control of the operating voltage and temperature of the light source necessary. The result is an unfavorable trade-off between the complexity and costs of the oxygen sensor and the measurement accuracy.

One object of the present invention is therefore to improve the known luminescence-based oxygen sensors in such a way that increased reliability and accuracy are achieved. Another task is to find a simplified and more compact construction.

This object is achieved by a device according to claims 1-9, which is operated according to the method according to claims 10-13.

The main innovation compared to the known luminescence-based oxygen sensors is the derivation of the measurement signal exclusively from the response delay, i.e. the time between the emission of the excitation light by the light source and the detection of the luminescence light that the fluorophore sends out as a response. The present invention advantageously uses the knowledge that the response delay is also dependent on the oxygen content of the fluid, but is essentially not influenced by the intensity of the excitation light. As a result, the fluctuations in the intensity of the luminescent light that lead to measurement errors are accompanied by fluctuations in the intensity of the excitation light received by the fluorophore, which can be attributed, for example, to fluctuations in the supply voltage or the temperature of the light source or to fluctuations in the transmissivity of the medium due to suspended matter or dissolved gases , advantageously avoided.

In addition to the actual, material-dependent response delay of the fluorescent material, the measured response delay also includes the transit time of the light from the light source to the fluorophore and from this to the light detector. Due to the close structural proximity of the components - the distances are in the range from millimeters to a few centimeters at most - this transit time delay is even when measuring oxygen in liquid media, in which, due to the refractive index> 1, the speed of light can be significantly reduced compared to its value in a vacuum , in the range of at most a few hundred picoseconds and is therefore negligible in terms of measurement accuracy.

In contrast, the effects of the finite rise times of the light source and light detector are not negligible, i.e. the fact that, in the case of the light source, the intensity of the excitation light pulse and in the case of the light detector the detector signal builds up only with a finite delay. In preferred embodiments of the invention it is therefore proposed to measure the response time between corresponding intensity maxima in excitation and luminescence response light pulses. In this case, only the maxima of the luminescence response light pulse are determined by actually detecting the light pulse. The maxima of the excitation pulse, on the other hand, are derived from the control voltage and the known characteristics of the light source. In this way, indirect intensity effects can be avoided, as would be the result, for example, with a response time measurement between two defined intensity thresholds. In order to make the assignment easier for the controller, the excitation pulse should only have a single maximum. The same then usually follows automatically for the luminescent light pulse, unless two (or more) different fluorescent materials are used with material-dependent response times that differ significantly from one another.

In contrast to known oxygen sensors, which work with the detection of the oxygen concentration-dependent luminescence intensity, whether with or when the luminescence is extinguished, the present invention offers the advantage of a fundamentally more reliable oxygen content measurement. In addition to being independent of uncontrollable fluctuations in intensity, the light sources and environmental parameters do not have to be controlled as precisely and reproducibly as with known, intensity-based optical oxygen sensors. In addition, the control electronics can also be made simpler, since no high-quality amplifiers with a wide linear range have to be used to control the light source and light detector. As a result, the sensor can be made more compact, with lower tolerances and cheaper components, and thus more cost-effectively overall.

Advantageous further developments of the present invention, which can be implemented individually or in combination, provided that they are not obviously mutually exclusive, will be briefly presented below.

The fluorophore arranged in the measuring volume is preferably plate-shaped and coated with a fluorescent material at least on the side facing the light source and the light detector. Alternatively, the entire fluorophore plate can also be made from the fluorescent material. It is also preferably attached to an inner wall of the housing that delimits the measurement volume.

It is proposed to use a material containing a ruthenium complex as the fluorescent material of the fluorophore, because here both the excitation and fluorescence wavelengths lie in the visible spectrum. In addition, the lifetimes of the excited states are neither so short that an accurate response delay measurement would be difficult as a result, nor so long as that a repetition rate of the measurement would be greatly reduced. The response delay is between approx. 2 to approx. 10 µs, depending on the oxygen concentration.
The use of tris (4,7-diphenyl-1,10-phenantroline) ruthenium (II) perchlorate is particularly preferred, which when excited with light has a range of 520 nm and emits fluorescent light at 620 nm.

The light source and / or the light detector can be arranged outside the measurement volume and optically communicate with the fluorophore via a window made of material that is transparent for the wavelength ranges of the excitation and luminescent light. A particularly compact and simple embodiment of the oxygen sensor according to the invention results, however, when at least the light detector, but particularly preferably the light detector and light source, are arranged within the measurement volume in the interior of the sensor housing.
In the first case, the light detector can be placed directly in front of the fluorophore, so that a sufficient intensity can be measured at the location of the detector even without additional optics that focus the luminescent light. In this context, immediately before means at a distance which is smaller than the lateral extent of the fluorophore, particularly preferably in a distance which corresponds to between one third and two thirds of the lateral extent. The lateral extent of a plate-shaped fluorophore is given by the length or width of this plate.

If the light source is also arranged in the measuring volume, a transparent window can be dispensed with and the housing can be manufactured in one piece from any material that can withstand the chemical stress caused by the fluid to be analyzed, such a material usually being nontransparent.

It is also proposed for particularly preferred embodiments to use a laser diode with an emission frequency in the excitation spectral range of the fluorescent material. The advantages here are that a laser diode, even without additional measures, firstly emits light only in a narrow spectral range, which on the one hand makes optical filters unnecessary and on the other hand lowers energy consumption and, secondly, emits a sufficiently bundled light beam, which makes it unnecessary allows focusing optics. These advantages also allow a particularly compact oxygen sensor according to the invention which is inexpensive not only to manufacture but also to operate.

In preferred embodiments, the control comprises an amplifier connected upstream of the light source. In some embodiments, an amplifier connected downstream of the light detector can also be present for amplifying the analog detector signal. This is then preferably converted into a digital detector signal by means of an analog-digital converter. The digital detector signal can then be fed to a microcontroller for evaluation and derivation of the measurement signal.

The oxygen sensor according to the invention is preferably designed in such a way that, in addition to gas mixtures, liquids can also be analyzed, at least insofar as these are at least largely transparent. For this purpose, in the embodiments in which the light source and / or light detector are arranged in the measuring volume, the electrical supply lines are insulated in a liquid-tight manner or the light source and / or the light detector as a whole, including their respective supply lines, are protected in a liquid-tight envelope.

Encapsulation is also recommended for use with gaseous fluids, where these can contain aggressive, highly reactive chemicals, such as when used for oxygen measurement in the exhaust air of combustion engines or bioreactors. Such a protective cover should therefore be as chemically inert as possible, but at the same time also be inexpensive. It is therefore proposed in particular to enclose or encapsulate the light source and / or light detector in glass or quartz glass in order to protect them from direct contact with the fluid.

The housing is preferably cylindrical or cuboid. Inlet and outlet can be arranged on the same or on opposite end faces. In preferred embodiments, a suspended matter filter is connected upstream of the inlet in order to prevent possible turbidity of the fluid which would falsify the measurement. Due to the oxygen fraction determination based exclusively on response delay according to the invention, however, this is less critical than in the case of intensity-based sensors.

For preferred embodiments of the method according to the invention, it is proposed to emit short excitation light pulses with a duration between 0.1 and 1 microsecond, in particular about 0.3-0.5, particularly preferably 0.4 microseconds.

Each pulse can have several intensity maxima, the response delay between corresponding maxima, e.g. between the first maximum of the excitation pulse and the first maximum of the response pulse, being measured.

However, to simplify the response delay measurement, it is proposed to emit pulses with a sharply defined maximum, such as sawtooth, Lorentz or Gaussian pulses.

The controller can generally determine the oxygen content for each measurement from the response delay measured by means of a single excitation pulse. However, in order to increase the accuracy and to reduce inevitable statistical errors, it is proposed in to emit several excitation pulses defined by a pulse repetition time and to determine the oxygen concentration on the basis of the average response delay. A time between 1 to 100 ms, preferably 5 to 20 ms, is suggested as the pulse repetition time. The number of pulses is in particular between 2 and 100, preferably between 10 and 30, particularly preferably 20.

Further details, advantages and features of the present invention emerge from the exemplary embodiments explained in more detail below with reference to the figures. These are only intended to illustrate the invention and in no way limit its generality.

• 1: eine schematische Darstellung eines Sauerstoffsensors gemäß einer bevorzugten Ausführungsform der Erfindung

It shows:

• 1 : a schematic representation of an oxygen sensor according to a preferred embodiment of the invention

In 1 a preferred embodiment of an oxygen sensor according to the invention is shown.
The oxygen sensor 1 includes the housing 2 with one inlet each 21 and an outlet 22nd on an upper face. The inlet 21 is a particulate filter 23 upstream. Within the the measuring volume 20th forming the interior of the housing are arranged laser diode 3 as an excitation light source, a plate-shaped fluorophore 4th and a light detector 5 consisting of a photodiode 51 behind a plate-shaped optical filter 52 . The filter 52 is a low-pass filter that ensures that the excitation light that is elastically scattered by the fluorophore, which does not experience any change in wavelength, is passed on to the broadband photodiode 51 does not directly stimulate.
The one here schematically outside the case 2 control shown 6th can be in one of the measuring volume 20th separate chamber within the housing, but also within the measuring volume 20th to be ordered. As shown, it includes a preamplifier 63 to amplify the control signal of the laser diode 3 , an amplifier 65 to amplify the detector signal of the photodiode 51 . The amplified detector signal is generated by means of the A / D converter 66 digitized and the microcontroller 61 supplied, which from it with recourse to stored calibration values, which the relationship between the oxygen concentration in the fluorophore 4th and the expected response delay relate to the oxygen concentration in the fluid.

In response to a control signal from the microcontroller 61 the control 6th the laser diode sends out 3 a short pulse lasting about 0.4 microseconds, naturally very sharply bundled and narrow-band excitation light with a wavelength of approx. 520 nm on the fluorophore 4th . With one of the oxygen concentration in the (fluorescent material of the) fluorophore 4th Depending on the response pulse delay, it emits a luminescence light pulse with a light wavelength of approx. 620 nm. This emission occurs isotropically in the front half-space of the photodiode 51 facing side of the fluorophore 4th . The luminescence light pulse can use the appropriately selected filter 52 pass and is from the photodiode 51 detected, which outputs an analog detector signal proportional to the detected intensity. This is done by the amplifier 65 amplified, then from the A / D converter 66 digitized and then the microcontroller 61 fed, which therefrom a measurement signal representing the oxygen concentration M. calculates and outputs.

The supply lines 31 and 59 to the light source or light detector are protected from direct contact with liquid by the liquid-tight envelopes 32 or 58. The sensor is therefore also suitable for measuring oxygen in, at least sufficiently, transparent liquids.

In conventional oxygen sensors, optics are often required to focus the luminescent light on the light detector. Here the photodiode 51 but immediately in front of, that is, within a distance which is smaller than the length or width of the fluorophore plate 4th , is arranged in front of this, no focusing optics are provided in this preferred embodiment. This is also the case for the laser diode due to the natural beam bundling 3 not necessary, as this emits a beam with an elliptical cross-section and an opening angle of approx. 5 ° x 20 ° even without optics. An optical filter for the laser diode can also be dispensed with. Overall, a significantly simpler and more compact construction of the oxygen sensor is made possible in this way.

If the actual oxygen concentration changes, there is a delay between the actual oxygen concentration and the oxygen concentration measured in this way because the oxygen concentration in the fluorophore first has to adapt to the changed concentration of the fluid. Due to the direct contact between fluorescent material of the fluorophore 4th and for the fluid, however, this delay, known as the (sensor) response time, is comparatively small and ranges from approximately one second to a few tenths of a second.

List of reference symbols

1

Oxygen sensor

2

casing

20th

Measurement volume

21

inlet

22nd

Outlet

23

HEPA filters

3

Laser diode

31

Lead to 3

4th

Fluorophore plate

5

Light detector

51

Photodiode

52

optical low pass filter

58

Wrapping of 59

59

Lead to 51

6th

steering

61

Microcontroller

63

Preamplifier for 3

65

Amplifier for 5

66

A / D converter

M.

Measurement signal

QUOTES INCLUDED IN THE DESCRIPTION

This list of the documents listed by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent literature cited

• WO 2007/027116 A1 [0006]
• CA 1243351 A1 [0007]
• EP 3121589 A1 [0008]

Claims (13)

Device for luminescence-based measurement of the oxygen content of a fluid, comprising - a housing (2) which encloses a measurement volume (20) and with mit an inlet (21) through which the fluid can enter the measurement volume (20), and ◯ an outlet ( 22), through which the fluid can exit the measuring volume (20), - a light source (3) which can emit excitation light in a first wavelength range, - a fluorophore (4) which contains an oxygen-sensitive fluorescent material and so in the measuring volume is arranged so that the fluorescent material is in direct contact with the fluid and can be exposed to the excitation light emitted by the light source (3), the fluorophore (4) thereupon emitting luminescent light in a second wavelength range in response to the excitation light, where o an intensity of the luminescent light is dependent on an intensity of the excitation light and the oxygen content of the fluid es, and o a response delay of the luminescent light is dependent on the oxygen content of the fluid but is essentially independent of the intensity of the excitation light, - a light detector (5) which can detect at least light within the second wavelength range and a light intensity that represents a detected light intensity, in particular outputs a proportional analog detector signal, and - a controller (6) which controls the light source (3) and derives an analog or digital measurement signal representing the oxygen content of the fluid from the detector signal, characterized in that the controller (6) is prepared for this - to emit the excitation light in the form of a short light pulse by means of the light source (3), - to measure the response delay as the time between emission of the excitation pulse and detection of a luminescent light pulse, and - to derive the measurement signal from the response delay. Device according to Claim 1 , characterized in that the short light pulse has a pulse length of between 0.1 to 1 microseconds, in particular 0.4 microseconds. Device according to Claim 1 or 2 , characterized in that the fluorophore (4) - is coated with the fluorescent material or consists of such a material, and / or - is designed in the form of a plate, and / or - is attached to an inner wall of the measuring volume (20). Device according to one of the preceding claims, characterized by a fluorescent material containing a ruthenium complex, in particular tris (4,7-diphenyl-1,10-phenantroline) ruthenium (II) perchlorate. Device according to one of the preceding claims, characterized in that the light source (3) - arranged within the measurement volume, and / or - a laser diode, and / or - attached to an inner wall of the housing (2), and / or - with a transparent one , chemically inert material, in particular glass or quartz glass, is encased. Device according to one of the preceding claims, characterized in that the light detector (5) - arranged within the measurement volume, and / or - a photodiode and through an optical low-pass filter which filters out light of the first wavelength range, but allows light of the second wavelength range to pass, and which is arranged in particular between the photodiode (5) and the fluorophore (4) and / or the light source (3), is protected from direct exposure to excitation light, - is arranged immediately in front of the fluorophore (4), i.e. at a distance which is smaller than a lateral extent of the fluorophore (4), in particular half or less, preferably 1/4 or less of this lateral extent, and / or - is encased with a transparent, chemically inert material, in particular glass or quartz glass. Device according to one of the preceding claims, characterized in that the controller (6) - an amplifier (63) connected upstream of the light source (3), and / or - an amplifier (65) connected downstream of the light detector (5), and / or - a A / D converter (66) for digitizing the analog detector signal and / or - a microcontroller (61) for controlling the light source (3) and evaluating the digitized detector signal. Device according to Claim 2 , characterized in that the device is suitable for measuring oxygen in transparent liquids by isolating electrical leads to the light source (3) and / or the light detector (5) or isolating the light source (3) and / or light detector (5) together with their respective leads are protected from direct fluid contact by a transparent, liquid-tight cover. Device according to one of the preceding claims, characterized in that the inlet (21) - is arranged on the same side or on opposite sides of the housing (2) as the outlet (22), and / or - has an upstream fluid filter for filtering out suspended matter , Method for operating a luminescence-based oxygen measuring device according to one of the Claims 1 - 9 for measuring the oxygen content of a fluid, characterized in that the controller (6) (a) acts on the fluorophore (4) with a short excitation light pulse by means of the light source (3), (b) by means of the light detector (5) a luminescence light pulse of the Fluorophore is detected as a response to the excitation light pulse, (c) the response delay, i.e. the time between emission of the excitation light pulse and detection of the luminescence light pulse, and (d) the oxygen content of the fluid is determined from this and a measurement signal representing this is output. Procedure according to Claim 10 , characterized in that steps (a) - (c) are repeated several times and the oxygen content is determined in step (d) on the basis of an average response delay. Procedure according to Claim 11 , characterized in that steps (a) - (c) are repeated between 1 and 99 times, in particular between 9 and 29 times, preferably 19 times. Method according to one of the Claims 10 - 12th , characterized in that the response delay is measured in step (c) between corresponding intensity maxima of the excitation light pulse and the luminescence light pulse.

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