JP4359769B2 - Sensor dirt detection method - Google Patents

Description

The present invention relates to a stain detection method for an optical sensor that is used by immersing a light projecting unit and a light receiving unit in measured water .

  2. Description of the Related Art Conventionally, there is known a sensor that detects the concentration of a suspension contained in water to be measured or the physical properties of the suspension using, for example, a particle state detection probe (see, for example, Patent Document 1). As shown in FIG. 7, this type of sensor includes a light projecting unit 2 that irradiates laser light into the measured water and a transmitted light that is provided near the light projecting unit and receives transmitted light that has passed through the measured water. At least one of a light receiving unit 3T, a scattered light receiving unit 3S that receives light scattered or reflected by a suspended matter contained in the water to be measured, and a reflected light receiving unit 3R is provided, and these light receiving units 3 (3T, 3T, 3S, 3R) is configured to include a measuring unit 10 that detects the concentration of the suspension contained in the water to be measured or the physical properties of the suspension from the level of the light received by 3S, 3R).

For example, the transmitted light type optical sensor is a light received by the transmitted light receiving unit 3T with respect to the light transmitted through the water M to be measured, that is, the degree of light blocked by the turbidity (particles) contained in the water M to be measured. Is measured by the measuring unit 10. For example, a sensor (concentration sensor) that measures the concentration of suspended matter contained in the water to be measured has a level (signal intensity) of the detection signal output from the transmitted light receiving unit 3T as shown in FIG. Measured by using the decrease in proportion to the quality concentration.
Japanese Patent Application No. 2002-337778

  In the above-described optical sensor using light, suspended matter contained in the water to be measured may adhere to the light projecting unit 2 and each light receiving unit 3 (3T, 3S, 3R) and become dirty. Therefore, the level of the light received by the light receiving unit 3 is reduced due to the suspended substance adhering to the sensor, and a concentration different from the turbidity concentration (true value) of the measured water or the physical property of the suspended matter is detected. There was a problem that. However, such an optical sensor also has a problem that it cannot be determined whether the turbidity concentration of the water to be measured is high (or low) or due to the contamination of the sensor as described above. Therefore, there is a problem that it is not possible to confirm whether or not the measurement has been performed, that is, the suspension concentration or physical property of the water to be measured is detected without knowing whether or not the measurement value is reliable.

The present invention has been made in response to such a conventional situation, and an object of the present invention is to provide a sensor dirt detection method for determining the presence or absence of dirt adhered to a sensor, particularly an optical sensor .

In order to achieve the above-described object, a sensor dirt detection method according to the present invention includes a light projecting unit and a light receiving unit immersed in measured water, and the measured water of light irradiated from the light projecting unit into the measured water. Receiving at least one of the transmitted light that has passed through, the scattered light scattered by the suspension contained in the measurement water, and the reflected light reflected by the suspension, and the concentration of the suspension in the measurement water In a sensor for determining the physical property of a suspended matter,
(1) A measurement value obtained by measuring the water to be measured in a state where the sensor is not contaminated in advance, and in this state, air bubbles are introduced between the light projecting unit and the light receiving unit to remove the water to be measured. The difference [ΔD] from the measured value obtained by measurement is obtained.
(2) Next, a measurement value obtained in a measurement process of measuring the water to be measured using the sensor, and a measurement value obtained when bubbles are introduced between the light projecting unit and the light receiving unit in this state The difference [Δd] is obtained.
(3) Then, [ΔD] and [Δd] are compared to determine whether the sensor is dirty.

  According to the sensor dirt detection method described above, for example, in the case of a sensor (concentration sensor) that measures the suspension concentration of water to be measured, if bubbles are introduced between the light projecting unit and the light receiving unit, Suspended substances existing between the light projecting part and the light receiving part are reduced. Accordingly, the suspension concentration detected by the concentration sensor is reduced accordingly. The change [Δd] in the suspension concentration when this bubble is introduced between the light projecting unit and the light receiving unit is compared with the change [ΔD] obtained when the concentration sensor is not contaminated in advance. [Delta] D] and [[Delta] d] are compared to determine the degree of contamination of the density sensor. Conventional methods (gas injection method, liquid injection method, gas-liquid mixing method, mechanical cleaning method using a brush, sponge, wiper, etc.) can be employed for cleaning the sensor unit.

Preferably, the introduction of the bubble between the light projecting unit and the light receiving unit is to wash the light projecting unit and the light receiving unit of the sensor,
(4) A measurement value obtained by measuring the water to be measured in a state where the sensor is not contaminated in advance, and in this state, air bubbles are introduced between the light projecting unit and the light receiving unit to remove the water to be measured. The difference [ΔD] from the measured value obtained by measurement is obtained.
(5) Next, while intermittently repeating the introduction of the bubble, the measurement value obtained when the bubble is not introduced between the light projecting unit and the light receiving unit, and the measurement obtained when the bubble is introduced The difference [Δd] from the value is obtained.
(6) It is desirable to compare the [ΔD] and [Δd] to determine the degree of dirt removal of the sensor.

  Note that [ΔD] varies depending on the measured suspension concentration and varies depending on the physical properties of the suspension to be measured, such as the particle diameter, particle shape, color tone, and density. It is preferable to create a relational expression between [ΔDs] and the suspension concentration [S] (or sensor output [S]) as shown. [ΔD] corresponding to the time of [Δd] measurement can be obtained from the suspension concentration at the time of [Δd] measurement and this relational expression.

  According to the sensor dirt detection method described above, for example, in the case of a sensor (concentration sensor) that measures the suspension concentration of water to be measured, if bubbles are introduced between the light projecting unit and the light receiving unit, Suspended substances existing between the light projecting part and the light receiving part are reduced. Accordingly, the suspension concentration detected by the concentration sensor is reduced accordingly. The change [Δd] in the suspension concentration obtained when this bubble is introduced between the light projecting part and the light receiving part is compared with the change [ΔD] obtained when the density sensor is not contaminated in advance. To determine the degree of dirt adhering to the concentration sensor.

More preferably, the measurement value [D1] obtained by measuring the water to be measured in a state where the sensor is not contaminated in advance, and in this state, air bubbles are introduced between the light projecting unit and the light receiving unit, The difference [ΔD = D1−D2] from the measured value [D2] obtained by measuring the water to be measured is preferably stored in the memory as a soil determination criterion and used for sensor soil determination .

  As described above, in the sensor dirt detection method of the present invention, the measured value obtained by measuring the water to be measured in a state in which the sensor is not soiled in advance, and in this state, between the light projecting unit and the light receiving unit. The difference [ΔD] between the measured value obtained by introducing air bubbles into the measured water and the measured value obtained in the process of measuring the measured water using the sensor, and the light projecting unit and the light receiving in this state The presence or absence of contamination of the sensor is determined by comparing the difference [Δd] with the measured value obtained by introducing air bubbles between the two parts and measuring the water to be measured. For this reason, it is possible to reliably detect the presence or absence of dirt on the sensor, for example, the suspension adhered to the concentration sensor.

  In the sensor dirt detection method of the present invention, the measurement value obtained by measuring the water to be measured in a state where the sensor is not soiled in advance, and bubbles are formed between the light projecting part and the light receiving part in this state. The difference between the measured value obtained by introducing and measuring the measured water, the measured value obtained by measuring the measured water while intermittently introducing air bubbles, and between the light projecting part and the light receiving part The degree of dirt removal of the sensor is determined by comparing the difference with the measured value obtained by introducing bubbles into the water and measuring the water to be measured. For this reason, for example, the state of the suspension adhering to the concentration sensor can be reliably detected.

Hereinafter, a sensor dirt detection method according to an embodiment of the present invention will be described with reference to the drawings. This embodiment relates to a method for detecting contamination of a sensor according to an embodiment of the present invention, and shows a concentration sensor that measures the concentration of suspended matter in the water to be measured as an example, and restricts the embodiment of the present invention. is not.
FIG. 1 is a block diagram showing a schematic configuration of an optical turbidity concentration meter to which a sensor dirt detection method according to the present invention is applied. In this figure, reference numeral 1 denotes a concentration sensor that detects the concentration of the suspension contained in the water M to be measured, and outputs a detection signal representing the detected suspension concentration to the concentration measuring unit 10.

  For example, in the case of a transmission type densitometer, the concentration sensor 1 placed in the water to be measured schematically shows a laser beam amplitude-modulated at a predetermined frequency by a light emitting unit 11 to be described later, as shown in FIG. The first optical fiber 4 that is guided from the light projecting unit 2) into the measured water and irradiated, and the laser light irradiated from the light projecting unit 2 is blocked by the suspended matter (particles) contained in the measured water M. The second optical fiber 5 is configured to receive transmitted light that has passed through the end surface (light receiving unit 3) and guide the detection unit 12 described later. The density sensor 1 is fixed to a predetermined pedestal (support member) 6 so that the light projecting unit 2 and the light receiving unit 3 are close to each other, and the optical axes of the respective end surfaces are made to coincide with each other.

  A nozzle 7 that introduces air or the like supplied from the outside by a pump P or the like into the water to be measured, and blows bubbles generated by the air or the like to the light projecting unit 2 and the light receiving unit 3 in the water tank 7 that stores the water M to be measured. 8 is attached. Air or the like introduced into the measurement water from the nozzle 8 is controlled by opening and closing a valve 9 controlled by a control unit 13 provided in a concentration measuring unit 10 described later.

  On the other hand, the concentration measuring unit 10 is provided with a light emitting unit 11 that emits laser light from the light projecting unit 2 and a detection unit 12 that detects the level of light received by the light receiving unit 3. Specifically, the laser light generated by the light emitting unit 11 is, for example, amplitude-modulated at a predetermined frequency and irradiated from the light projecting unit 2 at the end thereof into the measurement target water via the first optical fiber 4. The light emitted from the light projecting unit 2 into the measured water is received by the light receiving unit 3 and supplied to the detection unit 12 via the second optical fiber 5 in order to detect the received light level.

The detection unit 12 has a photoelectric conversion function for converting it to an electric signal level corresponding to the light reception level of the light received by the light receiving unit 3. The electric signal level obtained by the detection unit 12 is given to the calculation unit 14 as will be described in detail later, and is subjected to a predetermined calculation, and a storage unit 15 that holds the electric signal level converted by the detection unit 12. Is to be given to.
Further, the concentration measuring unit 10 compares the light reception level (electric signal level) obtained in the measurement process with the electric signal level held in the storage unit 15 to determine whether or not the concentration sensor 1 is contaminated and the degree of contamination. A determination unit 16 is provided.

  Schematically, the sensor dirt detection method according to the present invention described above is characterized in that a measured value obtained by measuring the water to be measured M in advance in a state in which the sensor is not dirty, and a light projecting unit in this state A point that holds a difference [ΔD] from a measurement value obtained by introducing bubbles between 2 and the light receiving unit 3 and measuring the measurement water M, a measurement value of the measurement water M using a sensor, In this state, the difference [Δd] from the measured value obtained by measuring the measured water M by introducing bubbles between the light projecting unit 2 and the light receiving unit 3 and the obtained difference [ΔD] The difference [Δd] is compared to determine whether the sensor is dirty or not, or the degree of sensor dirt.

The sensor dirt detection method according to the present invention having such characteristics will be described in more detail. A concentration sensor that measures the suspension concentration of the water to be measured will be described below as a sensor.
As shown in FIG. 3A, the light emitted from the light projecting unit 2 into the measurement target water reaches the light receiving unit 3. At this time, a part of the light irradiated from the light projecting unit 2 into the measurement target water is blocked by the suspension G contained in the measurement target water. Therefore, in the transmitted light type concentration sensor 1, the light blocked by the suspension G increases as the concentration of the suspension contained in the water M to be measured increases as described above. The light reception level of the received light is lowered. Here, it is assumed that the light reception level detected in this way, that is, the level of the electric signal (detection signal) converted by the detection unit 12 is [S1] as shown in FIG.

  Next, the control unit 13 of the concentration measuring unit 10 opens the valve 9 to detect bubbles B between the light projecting unit 2 and the light receiving unit 3 as shown in the detection region of the concentration sensor 1, that is, as shown in FIG. Is introduced. Then, a part of the suspension G contained in the water M to be measured is excluded by the introduced bubbles B. Accordingly, at this time, the suspended matter G existing between the light projecting unit 2 and the light receiving unit 3 is reduced as compared with that before the introduction of the bubbles B. That is, the suspension concentration in the detection region is lowered. Then, the light receiving level received by the light receiving unit 3 increases, and therefore the level of the detection signal output by the detecting unit 12 increases. This is as shown in the graph showing the detection signal level and the suspension concentration in FIG.

  Specifically, depending on the state of the bubble B existing between the light projecting unit 2 and the light receiving unit 3, the light reception level received by the light receiving unit 3, that is, the level of the detection signal output by the detection unit 12 varies. For example, when there is almost no bubble B between the light projecting unit 2 and the light receiving unit 3 (detection region), the original suspension concentration (detection signal level) is [S1], while the light projecting unit When there are many bubbles B between 2 and the light receiving unit 3 (detection region), the suspension concentration decreases. Here, if the suspended matter concentration (the level of the detection signal) when there are many bubbles B in the detection region is [S2], the level of the detection signal output by the detection unit 12 when the bubble B is introduced is as shown in FIG. As shown in FIG. 4 (b), it fluctuates between [S1] and [S2].

  The level minimum value [S1] and the maximum value [S2] of the detection signal obtained in this way are obtained as the difference [ΔD = S2−S1] by the calculation unit 14, and the detection signal detected by the detection unit 12 is detected. A level difference [ΔD] of the detection signal when the bubble B is introduced into the detection region is held in the storage unit 15 together with the level [S1]. Of course, the storage unit 15 stores all of the maximum value [S2] and the difference [ΔD] or the minimum value [S1] and the maximum value [S2] or the minimum value [S1], the maximum value [S2], and the difference [ΔD]. You can hold it.

  Thus, by introducing the bubble B into the water to be measured M, the light receiving level received by the light receiving unit 3, that is, the level of the detection signal output by the detecting unit 12 is changed. The sensor dirt detection method according to the present invention has been made paying attention to this point. The level of the detection signal at various concentrations of the water M to be measured and the detection level when the bubble B is introduced between the light projecting unit 2 and the light receiving unit 3 while the concentration sensor 1 is not contaminated. The difference is obtained by the calculation unit 14 and stored in the storage unit 15 in advance. Such initialization is previously performed for the optical turbidity concentration meter.

  [S1] varies depending on the concentration and physical properties of the suspension contained in the water to be measured, and [S2] also varies depending on the amount of bubbles introduced. When the amount of bubbles introduced is constant, the relationship between the suspension concentration [S] and the difference [ΔD] can be expressed as a linear function. That is, if the slope of this function is [α] and the intercept of the [ΔD] axis is [β], it can be expressed as [ΔD = α × S + β]. Since this [ΔD] is a function of [S], the relationship as shown in FIG. 9 is obtained in advance and stored in the storage unit 15, and [S1] measured by the density sensor and the stored relational expression are used. [ΔD] in the measurement water can be obtained and used for comparison with [Δd].

Next, a method for detecting the contamination of the concentration sensor 1 when measuring the turbidity concentration of the water M to be measured using the optical turbidity concentration meter that has been initialized as described above will be described.
For example, as shown in FIG. 5A, when the suspended matter G contained in the measured water M adheres to the light receiving unit 3, the light irradiated by the light projecting unit 2 into the measured water adheres to the light receiving unit 3. It is blocked by the suspended suspension G (dirt) and does not reach the light receiving unit 3. At this time, it is assumed that the light receiving level received by the light receiving unit 3, that is, the level of the detection signal output by the detecting unit 12 is [S1] as shown in FIG. At this time, the suspension concentration of the water to be measured M measured by the optical turbidity concentration meter shows a higher value than the actual suspension concentration due to the suspension G (dirt) adhering to the light receiving unit 3. Will be. In other words, this is equivalent to an increase in the suspended matter concentration of the water M to be measured due to the suspended matter G (dirt) adhering to the light receiving unit 3.

  Next, the control unit 13 opens the valve 9 and introduces the bubbles B between the light projecting unit 2 and the light receiving unit 3. Then, as described above, the suspended matter existing between the light projecting unit 2 and the light receiving unit 3 is pushed away by the bubbles, and the suspended matter G in the detection region is reduced (FIG. 6A), and the detecting unit 12. The level of the detection signal output from becomes higher. However, the detection level output by the detection unit 12 at this time is that when the suspension G is attached to the light receiving unit 3, the concentration sensor 1 is not contaminated (when the suspension G is not attached). The detection level is lower than when the bubble B is introduced between the light unit 2 and the light receiving unit 3 (detection region). In other words, if the level of the detection signal output from the detection unit 12 is [S3] when bubbles are introduced when the detection area is dirty, [S3] <[S2].

Specifically, as shown in FIG. 6B, when the light receiving unit 3 is contaminated, or when the bubble B is introduced between the light projecting unit 2 and the light receiving unit 3, the detection signal output by the detection unit 12 is as shown in FIG. It is assumed that there is a change between [S1] and [S3].
At this time, the calculation unit 14 obtains a difference [Δd = S3−S1] between the detection signals. Then, the determination unit 16 compares the difference [Δd] obtained as a result of the calculation of the calculation unit 14 with the difference [ΔD] when the density sensor 1 previously stored in the storage unit 15 is not contaminated. When the difference [Δd] and [ΔD] are different, it is determined that the density sensor 1 is dirty.

In the above-described stain determination method, the light receiving unit 3 is illustrated as being dirty. However, the light projecting unit 2 is dirty, or each of the light projecting unit 2 and the light receiving unit 3 is dirty. However, it can be detected in the same manner.
In addition, the concentration of the suspended matter obtained by measuring the water to be measured M in a state in which the concentration sensor 1 is not contaminated, and in this state, bubbles B are introduced between the light projecting unit 2 and the light receiving unit 3 to measure the water to be measured. The difference [ΔD] from the suspension concentration obtained by measuring M, the suspension concentration obtained in the measurement process, and the bubble B is introduced between the light projecting unit 2 and the light receiving unit 3 in this state. It is also possible to determine the degree of dirt attached to the concentration sensor 1 by comparing the difference [Δd] with the suspension concentration obtained by measuring the water to be measured M. That is, it is determined that the closer the values of [ΔD] and [Δd] are, the less the contamination of the density sensor 1 is, and the greater the values of [ΔD] and [Δd] are, the more dirty the concentration sensor 1 is. it can.

  Thus, the above-described sensor contamination detection method includes the measurement value obtained by measuring the water to be measured M in a state where the sensor is not contaminated in advance, and the bubble B between the light projecting unit 2 and the light receiving unit 3 in this state. The difference [ΔD] between the measured value obtained by introducing and measuring the measured water M, the measured value of the measured water M obtained by measuring using the sensor, and the light projecting unit 2 in this state Since the bubble [B] is introduced between the light receiving unit 3 and the measured water M is measured to compare the difference [Δd] with the measured value, the presence or absence of dirt on the sensor is determined. It is possible to reliably detect the contamination of the unit 2 and the light receiving unit 3.

  Further, the contamination of the sensor detected as described above often occurs when the suspended matter G contained in the water to be measured M adheres to the light projecting unit 2 or the light receiving unit 3. A gas-liquid injection method or a gas-liquid mixing method can be used to remove the dirt. Therefore, the control unit 13 intermittently opens and closes the valve 9 to introduce the bubbles B between the light projecting unit 2 and the light receiving unit 3, so that the suspended matter attached to the light projecting unit 2 or the light receiving unit 3. G (dirt) can be removed, and at the same time, it can be used to detect dirt. That is, the measured water M is measured by measuring the measured water M with no dirt on the sensor, and the measured water M is measured by introducing the bubbles B between the light projecting unit 2 and the light receiving unit 3 in this state. The difference [ΔD] from the measured value obtained in this way, the measured value obtained in the measurement process, and the object when the bubble B is introduced for cleaning between the light projecting unit 2 and the light receiving unit 3 in this state. By comparing the difference [Δd] with the measured value obtained by measuring the measured water M, it is also possible to determine the degree of dirt attached to the degree sensor.

When such a sensor dirt detection method is applied, it is possible to provide a sensor cleaning method for determining the cleaning time for introducing bubbles B into the sensor 1 and removing the dirt. Specifically, a sensor cleaning method according to the present invention will be described by taking as an example a concentration sensor that measures the suspension concentration of water to be measured.
First, the concentration of the suspended matter obtained by measuring the water to be measured M in a state where the concentration sensor 1 is not contaminated, and in this state, bubbles B are introduced between the light projecting unit 2 and the light receiving unit 3 to measure the water to be measured. The difference [ΔD] from the suspension concentration obtained by measuring M, the suspension concentration obtained in the measurement process, and air bubbles are introduced between the light projecting unit 2 and the light receiving unit 3 in this state. The difference [Δd] with the suspension concentration obtained by measuring the water to be measured M is compared.

  The difference ratio [Δd / ΔD], which is the ratio of these differences, serves as an index indicating the degree of dirt attached to the density sensor 1. That is, as the value of the difference ratio [Δd / ΔD] is closer to [1], the density sensor 1 is less contaminated. On the other hand, the value of the difference ratio [Δd / ΔD] is smaller ([1] or less). Can be determined. In general, as the cleaning time elapses, the concentration sensor is removed, and [Δd / ΔD] monotonically increases as shown in FIG. 10 and eventually approaches [1]. Therefore, when the bubble B is introduced between the light projecting unit 2 and the light receiving unit 3, as shown in FIG. 10, the time change in which the value of the difference ratio [Δd / ΔD] approaches [1] is captured. The correlation between the introduction time of B and the difference ratio makes it possible to predict or determine the sensor cleaning time.

Needless to say, the above-described method for detecting contamination of a sensor according to the present invention can be applied not only to the concentration sensor exemplified as the sensor but also to other sensors that detect physical properties of water to be measured.

It is a block diagram which shows schematic structure of the optical turbidity concentration meter to which the dirt detection method of the sensor which concerns on one Embodiment of this invention is applied. It is a figure which shows schematic structure of the transmission-type densitometer applied to the sensor shown in FIG. FIG. 3 is a diagram schematically showing a suspension contained in the sensor and the water to be measured shown in FIG. 2, and a graph showing a change over time of a detection signal output from a detection unit at that time. It is the figure which showed typically that the bubble was introduce | transduced between the light projection part and light-receiving part of the sensor shown in FIG. 3, and the graph which shows a time-dependent change of the detection signal output from a detection part at that time. FIG. 3 is a diagram schematically illustrating a suspension contained in water to be measured and a graph showing a change over time in a detection signal output from a detection unit when the sensor shown in FIG. 2 is contaminated. It is the figure which showed typically that the bubble was introduce | transduced between the light projection part of the sensor shown in FIG. 4, and the graph which shows the time-dependent change of the detection signal output from a detection part at that time. It is a block diagram which shows schematic structure of the sensor used for the conventional optical turbidity concentration meter. It is a figure which shows the relationship between the detection level which a transmitted-light type | mold densitometer outputs, and suspension concentration. It is a figure which shows the relationship between (DELTA) D which a transmitted-light type | mold densitometer outputs, and suspension concentration. It is a figure which shows the relationship between the time of washing | cleaning continuation T, the change of (DELTA) d, and washing | cleaning completion time.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Sensor 8 Nozzle 9 Valve 10 Concentration measurement part 11 Light emission part 12 Detection part 13 Control part 14 Calculation part 15 Storage part 16 Judgment part M Water to be measured

Claims (3)

The light projecting unit and the light receiving unit are immersed in the measured water, and the light irradiated from the light projecting unit into the measured water is transmitted through the measured water and scattered by the suspension contained in the measured water. In the sensor for detecting at least one of the scattered light and the reflected light reflected by the suspension by the light receiving unit, and detecting the suspension concentration in the measurement water or the physical properties of the suspension,
A measurement value obtained by measuring the water to be measured in a state where the sensor is not contaminated in advance, and in this state, bubbles are introduced between the light projecting unit and the light receiving unit to measure the water to be measured. The difference from the required measurement value,
The difference between the measured value obtained in the measurement process of measuring the water to be measured using the sensor and the measured value obtained when bubbles are introduced between the light projecting unit and the light receiving unit in this state is compared. And determining whether the sensor is dirty or not. The introduction of air bubbles between the light projecting unit and the light receiving unit is to wash the light projecting unit and the light receiving unit of the sensor,
A measurement value obtained by measuring the water to be measured in a state where the sensor is not contaminated in advance, and in this state, bubbles are introduced between the light projecting unit and the light receiving unit to measure the water to be measured. The difference from the required measurement value,
While intermittently repeating the introduction of the bubble, the difference between the measured value obtained when the bubble is not introduced between the light projecting unit and the light receiving unit, and the measured value obtained when the bubble is introduced 2. The sensor dirt detection method according to claim 1, wherein the degree of dirt removal of the sensor is determined by comparing the two.   A difference between a measured value obtained by measuring the water to be measured in a state where the sensor is not contaminated in advance and a measured value obtained when bubbles are introduced between the light projecting unit and the light receiving unit in this state. The method for detecting dirt on a sensor according to claim 1 or 2, wherein the dirt is stored in a memory as a dirt judgment criterion and used for dirt judgment.

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