FI120663B - Device and method for optical concentration measurement - Google Patents

Description

Apparatus and method for optical concentration measurement

The invention relates to an apparatus for optical concentration measurement according to the preamble of claim 1.

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The invention also relates to a method for optical concentration measurement.

According to the prior art, the concentrations of gases and liquids are measured optically by applying light from the first end of the measuring chamber to a detector at the other end of the measuring chamber 10. The result of the measurement is based on the spectral properties of the substance being measured. In other words, for example, the CO2 content of carbon dioxide can be determined by attenuating the signal at the absorption wavelength of this gas in the measuring chamber.

15 In known solutions, the material to be measured, liquid or gas, passes through the measuring chamber. In this way, a responsive measuring device is provided.

Because the flow forms a mass flow, dirt particles are also carried along with the mass flow. Thus, the measuring chamber is exposed to any dirt carried by the gas or liquid to be measured, which immediately begins to contaminate both the transmitter and receiver surfaces as well as the walls of the measuring chamber.

The obvious solution to this problem is to filter the incoming flow. However, this requires constant maintenance as sooner or later the filters become clogged. In practice, problems 25 begin much earlier because the flow rate through the measuring chamber is reduced and thus the response times of the measurement are increased. In general, therefore, the measurement situation is not stable, but the various disturbance parameters (fouling, response times) are constantly changing during the measurement.

The object of the invention is to solve the problems described above and to provide an entirely new type of apparatus and method for optical concentration measurement 2 when it is desired to introduce the gas or liquid to be measured into the apparatus by means of a closed flow path (tube, etc.).

The invention is based on the fact that the apparatus and method are implemented in such a way that there is no substantial flow in the measuring chamber 5 during the measurement.

According to the invention, the gas or liquid to be measured passes through a by-pass path in the sensor, one of which is a diffusion unit into the sensor measuring chamber. The concentration of the substance to be measured in the measuring chamber by diffusion is the same as in the by-pass.

More specifically, the apparatus according to the invention is characterized in what is stated in the characterizing part of claim 1.

The method according to the invention, in turn, is characterized by what is stated in the characterizing part of claim 15.

The embodiments of the invention provide significant advantages.

Because in the pure diffusion chamber no flow occurs during the measurement 20, the dirt particles will not migrate there either. If necessary, the inlet of the measuring chamber may be provided with a particle filter which, for the reasons stated above, remains clean for a considerable period of time, since in the absence of a substantial flow dirt particles are adhered to the filter only by chance.

25 Since no contamination occurs, the instrument has very good stability and very little maintenance.

The invention will now be described by way of example and with reference to the accompanying drawings.

Figure 1 is a schematic side elevational view of one prior art measuring apparatus.

Figure 2 is a schematic side view of another prior art measuring apparatus.

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Figure 3 is a schematic side view of one measuring apparatus according to the invention. 5

Figure 4 is a schematic side view of a flow modeling of the solution of Figure 3.

Figure 5 is a schematic side view of a general solution of the invention.

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In the following description, the following terminology will be used: 1 measuring chamber 2 light source 15 3 light detector 4 flow inlet 5 flow outlet 6 bypass path 7 inlet 20 8 output 9 diffusion connection 10 measuring sensor 11 first diffusion opening 12 second diffusion opening 25

1, a measuring chamber 1 is arranged between the light source 2 and the light detector 3. The light source 2 is disposed at the first end of the measuring chamber 1 and the light detector 3 at the second end of the measuring chamber 1. At the end of the light source 2, two diffusion openings 11 and 12 are provided. Due to the two diffusion openings 30, a flow is created in the measuring chamber 1 if a pressure difference is formed between the openings 4 and 5.

The prior art according to Figure 2 has a solution in which the flow of gas or liquid (fluid) is arranged through the entire measuring chamber 1.

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The solution of Figure 1 does not apply to applications where gas must be introduced into the sensor by means of a closed flow path (e.g., a pipe). If the solution of Fig. 1 encounters a measurement situation with strong currents, a pressure difference is easily created between the diffusion openings 4 and 5, which in turn causes the flow into the measuring chamber 1. As a result of the flow, the measuring chamber 1 is exposed to dirt. Also in the solution of Figure 2, both the light source 2, the light detector 3 and the measuring combs are exposed to 10 dirt and over time become dirty. Some of the contamination can be offset by software, but the measuring equipment must be cleaned before long.

According to Fig. 3, in the solution according to the invention, the measuring chamber 1 with the light source 2 and the light detector 3 at its ends is connected to the measurable bypass flow 15 (gas or liquid) with only one connection, diffusion connection 9. The diffusion line 9 connects the measuring chamber to the gas or fluid passing through the bypass 6 so that it does not interfere with the optical measurement.

Preferably, for diffusion reasons, the diffusion coupling 9 is located in the vicinity of the inlet 7 or outlet 8.

Fig. 4 shows a computational model of the flows and modeling of the apparatus of Fig. 3, which clearly shows that no flow occurs in the measuring chamber 1 or the diffusion connection 9.

Solutions other than those described above may also be contemplated within the scope of the invention.

Typically, the inlet and outlet terminals 8 and the bypass path 6 are included in the measuring apparatus, but in some embodiments of the invention, the diffusion tube 9 may also be connected to a sensor operating on another principle, e.g. a capacitive humidity sensor. In this case, the same advantages apply to such a sensor with respect to contamination, since there is no flow in the vicinity of the sensor.

The gas flow from the inlet connection 7 to the outlet connection 8 may be the result of differential pressure 5 of the measuring object or the pressure to the connection 7 may be provided by a separate flow pump.

In one embodiment of the invention, the measuring chamber is about 10 millimeters long. The light source is a thermal radiation source (miniature incandescent lamp) and the detector is infrared selective so that the detector receives radiation at the absorption wavelength of the substance to be measured. The measurement also typically includes so-called. a reference measurement at a reference wavelength other than the absorption wavelength. This measures the intensity of incoming light and can be used to compensate for eg. radiation source aging and detector temperature sensitivity.

A preferred embodiment of the arrangement according to the invention is the so-called non-dispersive infrared measuring device (NDER). These devices are commonly used for gas concentration measurements. This method is very selective for the gas to be measured by limiting the wavelength range used for the measurement to the characteristic wavelength range of the gas being measured. The wavelength range in the NDIR-20 method is usually selected by means of a bandpass filter. For example, an adjustable interferometer may be used as a bandpass filter in this technique. The wavelength of the bandwidth of the interferometer is voltage controlled and can be used for a sweeping measurement where the measurement is made at two or more wavelengths. In this case, it is useful to perform a measurement at the absorption band of the gas to be measured and a reference measurement adjacent to it. Reference measurement can compensate for the aging and temperature dependence of the measuring device. It is also possible to determine the concentration of several different gases by measuring at several wavelengths determined by the absorption of different gases.

The NDIR instrumentation typically uses a modulated IR source as a light source because the signal from the detector is easier to process in AC mode. The IR radiation can be modulated by interrupting the power supplied to the IR source. However, a sufficiently low thermal time constant is required from the ER source to allow a sufficiently high modulation frequency. A suitable IR source is, for example, a miniature incandescent lamp which can be modulated up to about 10 Hz. However, modulation of the temperature of the filament filament causes additional stress, which reduces the filament 5 lifetime. In order to achieve a higher radiation power, a heating element with a higher radiant surface area and slow heating must be used. In this case, a separate mechanical interrupter, located on the optical path of the radiation, must be used to modulate the radiation. However, the life of a mechanical chopper is short.

The alternatives of the solution according to the invention are presented below:

Typical light sources: - eg incandescent lamp, microwave, LED, laser, gas discharge tube Typical detector techniques:

15 - eg thermopile, bolometer, photon detector, LDR

Typical measuring objects: - eg C02, CO, hydrocarbons, 02, H20, fine gas Characteristics of measuring chamber (eg channel length) in different applications: - The length of the light path of the measuring chamber depends on the concentration of the substance to be measured. The minimum length of the measuring chamber is determined by the required measurement sensitivity and the required linearity determines the maximum length. There are several options for positioning the light source and the light detector in the measuring chamber. For example, a mirror can be used to position the light source and the detector at the same end of the measuring chamber.

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The optical measurement bands (wavelengths) used in accordance with the invention may be in the visible light range, but very typically also in the infrared range. Of course, the use of UV bands is also possible within the scope of the invention.

According to the invention, the diffusion coupling 9 is shaped such that its connection to the measuring chamber 1 does not cause a substantial flow therein. Typically, the diffusion coupling 9 7 is connected to the measuring chamber 1 by a single aperture and may have a filter. Within the scope of the invention, the aperture may be divided into a plurality of parts or the diffusion connection 9 may be formed from separate bundles of tubes immediately adjacent to one another. These alternative methods of the invention mentioned above may be referred to as a diffusion result arrangement. A technical object of the above is that a number of openings are not formed in the measuring chamber 1, between which a substantial pressure difference is formed to form a flow.

According to the invention, the diffusion inlet assembly 9 is implemented such that the average mass flow in the measuring chamber 10 is approximately zero. In this case, the molecular shift caused by diffusion represents at least 90% of the total molecular flux of the substance being measured. Hereby, according to the invention, it is meant that no substantial flow takes place in the measuring chamber 1.

Figure 5 illustrates a general solution according to the invention, wherein the measuring chamber 1 includes a sensor for measuring the concentration of any gas 10. Such a sensor may be, for example, a capacitive relative humidity sensor.

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Claims (12)

A gas or liquid content measuring device comprising a measuring device - a measuring chamber (1), 5. measuring devices (2, 3, 10) arranged in the measuring chamber (1) for measuring gas or liquid content, and other means (7, 6, 8) for retrieving the gas or liquid for measuring to be measured in the measuring chamber (1) by means of a diffusion device (9) in such a way that no substantial flow through the measuring chamber (1) can occur, characterized in that - the measuring device comprises a by-pass path (6) connected to the measuring chamber (1) with an inlet connection (7) and a discharge connection (8) arranged from this outlet connection, by means of which by-pass path (6) the gas or liquid intended for measurement is collected in the vicinity of the diffuser (9). Gas or liquid content measuring device according to claim 1. characterized in that it has only one diffusion device (9). Gas or liquid content measuring device according to claim 1 or 2, characterized in that the device comprises a light source (2) arranged in the measuring chamber (1) and a light detector (3). Gas or liquid content measuring device according to claim 1 or 2, characterized in that the device comprises a humidity measurement sensor. Device according to any of the preceding claims, characterized in that the measuring chamber (1) is connected by means of the diffusion device (9) in the vicinity of the flow input (4) or discharge (5) of the by-pass path (6). Device according to any of the preceding claims, characterized in that the device comprises a flow pump for forming a flow in the by-flow path (6). Device according to any of the preceding claims, characterized in that the measuring device is an NDIR measuring device. 5 A method for measuring gas or liquid content, in which method - the gas or liquid intended for measurement is allowed to diffuse in the measuring chamber (1) in such a way that no substantial flow through the measuring chamber can occur, and - gas or the liquid content of the measuring chamber (1) is measured, characterized in that - the gas or liquid intended for measurement is allowed to diffuse in the measuring chamber (1) via the diffusion device (9) from a bypass tube (6) fed by the inlet connection (7). the gas is discharged from the by-pass tube (6) via a discharge connection (8) arranged at a distance from the feed connection. Method according to claim 8, characterized in that radiation is directed towards the measuring chamber (1) and the radiation passed through the measuring chamber (1) is detected. Method according to claim 8 or 9, characterized in that a mass flow in the measuring chamber (1) is prevented. 20 Method according to any of the preceding claims, characterized in that the measuring chamber (1) is connected to the bypass path (6) in the vicinity of its inlet (4) or output (5). Method according to any of the preceding claims, characterized in that the gas or liquid intended for measurement is brought in the form of a mass flow to the by-pass path (6) by means of the pressure difference between its inlet (4) and outlet (5), e.g. by means of a flow pump.