JPH11241977A - Fluid concentration measurement device - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To provide a fluid concentration measurement device capable of performing long-term measurement without the influence of drift. SOLUTION: A sensor unit 1 has a flow path 5 in contact with a measurement substance.
And a control unit 8 for picking up and processing the control and output of the sensor unit, and through the three-way valve 4, the reference substance inlet 2 and the test substance inlet 3 are connected to either the reference substance or the test substance. Is supplied to the sensor unit 1. As soon as it is determined that the sensor has reached the steady state, the three-way valve 4 is switched to switch the supply from the test substance to the reference substance or from the reference substance to the test substance. The measurement is performed by repeating this operation, and the difference between the output when the test substance is introduced and the output when the reference substance is introduced is output.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a measuring device for measuring the concentration of a specific component in a fluid such as a gas or a liquid.

[0002]

2. Description of the Related Art A sensor used in a fluid concentration measuring device is an element for mainly detecting and quantifying a specific component in a fluid. It is used for such purposes.

This sensor uses current, voltage,
It outputs resistance, luminous intensity, wavelength, frequency, etc., and outputs a certain signal value when the substance to be measured does not exist, but when the substance to be measured is introduced, a value corresponding to the concentration is output Is output, and after a lapse of a predetermined time, a steady state is set.

For example, FIG. 7 shows a typical response waveform of the above sensor. The sensor outputs a constant signal even in the presence of a zero-point calibration substance (hereinafter referred to as a reference substance). This value is generally 0 with no signal or generates a very small output, and the output is identified by +/- depending on the polarity. When a substance to be measured (test substance) is introduced into this sensor, the output of the sensor changes to a value corresponding to the concentration of the substance to be measured, and a steady state is reached at that value.

[0005]

When measurement is performed continuously for a long time using such a sensor, when the base zero point drifts, the output peak fluctuates accordingly. For example, when a test substance having a constant concentration is supplied for a long time using a sensor having a baseline drift characteristic, a response waveform as shown in FIG. 8 is obtained.
That is, the output 32 at the zero point fluctuates as time passes, and the output 33 of the test substance fluctuates accordingly, so that a correct value cannot be obtained. If the concentration of the test substance is high, the fluctuation is negligible because the output peak is high, but especially when the concentration of the test substance is low, the current generated is very small. The effect of the drift on the output curve becomes remarkable,
There has been a problem that the output signal fluctuates and an accurate value cannot be measured.

The present invention has been made in order to solve the above-mentioned problem. By alternately supplying a zero-point calibration substance and a test substance in a short time, the measured value can be made such that the influence of drift can be ignored. An object of the present invention is to provide a fluid concentration measuring device capable of performing a long-term measurement without the influence of drift by repeating such a short-time measurement over a long period.

[0007]

Means for Solving the Problems To solve the above problems, a fluid concentration measuring device of the present invention is a concentration measuring device equipped with a sensor for measuring the concentration of a specific component in a fluid. Introducing means for introducing a test substance and a reference substance for zero-point calibration to the sensor unit, and switching these introduction means to alternately supply the reference substance for zero-point calibration and the test substance to the sensor unit in a short time And means for processing the supply, wherein the difference between the output of the test substance and the reference substance obtained from the sensor at that time is used as the output of the apparatus.

When the fluid is a gas, a zero gas purifier equipped with a filter in which a test gas removing agent is sealed is provided at the test substance introduction section, and the test gas is used to remove the test components to remove the zero gas. It is characterized by supplying.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to FIGS.

FIG. 1 shows a basic configuration of a fluid concentration measuring apparatus according to the present invention, wherein 1 is a sensor section, 2 is a reference substance inlet, 3 is a test substance inlet, 4 is a three-way valve, and 5 is a flow path. , 6 is a pump for discharging fluid, 7 is a three-way valve control unit, and 8 is a device control unit.

The sensor section 1 is connected to a flow path 5 in contact with a substance to be measured and an apparatus control section 8 for controlling the sensor section and for picking up and processing the output. The introduction of the measurement substance into the apparatus is performed from the reference substance introduction port 2 and the test substance introduction port 3 via the three-way valve 4. The three-way valve 4 is electrically driven, is connected to the three-way valve control unit 7, and supplies either a reference substance or a test substance to the sensor according to an instruction from the device control unit 8. These fluids may be supplied under pressure from a cylinder or the like, or may be sucked using a pump 6 or the like.

Next, the operation of this device will be described. First, the three-way valve controller 7 switches the three-way valve 4, and a reference substance (zero calibration substance) flows through the flow path 5 from the reference substance inlet 2 to introduce the reference substance into the sensor unit 1. The signal S1 at that time
Is stored. The three-way valve 4 is switched by the three-way valve control unit 7, and the test substance is supplied to the sensor unit 1 so that the test substance introduced from the test substance introduction port 3 flows through the flow path 5. Wait until a steady state is reached with a value specific to the substance concentration.

The substance to be introduced into the sensor section may be switched by simple time control using a built-in clock or the like, but at this time, the switching is carried out taking into account the steady state of rising and falling of the sensor. You need to set the time. The determination as to whether or not a steady state has occurred may be made by actually measuring the response time of the sensor in a characteristic test in advance and controlling it with a time trigger or the like based on the measured response time.

In software terms, the sensor output It at a certain time _t and the output It _{+ Δt} after the lapse of the time _{Δt have been calculated.}
By comparing the magnitude of the output difference ΔI and the default parameters I _i between, no variation below a certain value, i.e. the three-way valve control unit 7 to output the switching signal is judged to be a steady state three-way valve May be controlled.

Alternatively, a differentiating circuit may be mounted as hardware, and the supply may be switched when the differential value becomes zero, that is, when the change rate of the signal becomes zero, as a means for determining a steady state. In consideration of the measurement of the transition state in which the concentration of the test substance fluctuates, it is desirable that the supply type be forcibly switched after a certain period of time, with the use of time control. Further, when the transition state does not converge in a short time, the measurement mode may be switched to the normal measurement mode.

Next, when it is determined that the sensor has reached the steady state, the signal S2 at that time is recorded, the difference S3 between S2 and S1 is output as a measurement signal, and the signal is guided to the device control unit 8. Immediately thereafter, the three-way valve 4 is switched to switch the supply from the test substance to the reference substance, and the sensor is returned to the zero point state. The steady state at the zero point is determined using the same means as for the test substance, and the above operation is repeated.

FIG. 2 (a) shows the actual signal measurement state by the above operation. As can be seen from FIG. 2 (a), the supply of the reference substance and the test substance to the sensor is performed by each of the substances. If the output signal is a difference S3 between the signal S2 of the test substance and the signal S1 of the reference substance in each interval, which is alternately repeated in a short time within the interval at which the state can be maintained at a minimum, and the output difference is displayed, FIG. 2 (b)
Thus, an output that is not affected by a long-term drift can be obtained in a pseudo manner.

If necessary, based on the difference between the sensor output at the time of introduction of the reference substance and the sensor output at the time of introduction of the test substance in one measurement cycle, concentration conversion or the like is performed in the apparatus and output or display is performed.

By the way, the operating principle of a potentiostatic gas sensor, which is one of the electrochemical gas sensors, is to perform electrolysis while maintaining a constant potential difference at the interface between the electrode and the electrolyte solution and to identify the gas in the atmosphere. The oxidation-reduction reaction of the component is selectively advanced, and measurement is performed based on the oxidation-reduction current output at that time. In particular, if a porous film called a diffusion film is arranged in front of an electrode (working electrode) in which the reaction of the target substance proceeds, supply of the reactant to the electrode is restricted, so-called a limited current is generated, and the concentration of the reactant Is proportional to the current. FIG. 3 shows a typical transient response characteristic of this constant-potential electrolytic NOx gas sensor.
3 shows a response waveform of m to NO ₂ .

Explaining this response waveform, a sensor in the air, which is zero gas, initially generates only a weak current. However, when the sensor is placed under air containing NO ₂ as the test gas, a current starts to flow and about 20 μm is generated. Transition to steady state in seconds.
The current value in this steady state is 5 ppm of NO ₂ concentration.
1 μA is obtained in proportion to

However, when continuous measurement is performed for a long time, FIG.
A baseline drift similar to that shown in FIG. Therefore, FIG. 4 shows the result of measurement using the configuration of FIG. 1 in order to eliminate the influence of the drift. 39 ppb of NO ₂ was measured over a period of one hour with a rise of 30 seconds and a fall of 30 seconds, and the absolute value of the output signal showed a substantially constant value.

FIG. 5 shows a gas purifier 1 when the fluid is gas.
An example in which zero gas is purified from the test gas using 0 and used for measurement is shown. The three-way valve 4 is controlled by the three-way valve control unit 7. The test gas is taken into the apparatus through the reference substance inlet 2 and the test substance inlet 3. These inlets may be integrated. The reference substance inlet 2 is connected to a gas purifier 10, where a sensor sensing component in the test gas is removed. For example, the configuration of the purifier for removing NO ₂ includes KMn as the NO oxidizing agent 12.
Purafil and N, O ₄ coated porous ceramics
Silica gel as O ₂ absorbent 14 is used as separator 16
What is divided and filled with polypropylene cotton is available.

Further, the supply control of the zero gas and the test gas is as follows.
It may be performed by providing a stop valve that can electrically control opening and closing in addition to the three-way valve. FIG. 6 shows an example of this, in which stop valves 18 are provided before and after the gas purifier 10 and in the test gas flow path. To eliminate the effects of residual gas,
It is desirable to arrange the valves 18a to 18c and to sequentially open and close the stop valves, but it is possible to omit 18a or 18b according to the specification. Gas inlet 17
May be divided only for each route as in the above example.

[0024]

As described above, according to the fluid concentration measuring apparatus of the present invention, it is possible to obtain an accurate measurement value which is not affected by the baseline drift even if measurement is performed for a long time.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of an embodiment of the present invention.

FIG. 2 is a diagram showing a sensor output state of the fluid concentration measuring device of the present invention.

FIG. 3 is a diagram showing an example of a response waveform of a constant potential electrolytic sensor.

FIG. 4 is an output example when measurement is performed using the sensor of FIG. 3 according to the configuration of the present invention.

FIG. 5 is a diagram showing an embodiment in which the present invention is applied to a gas.

FIG. 6 is a view showing another embodiment in which the present invention is applied to a gas.

FIG. 7 is a diagram showing a general response waveform of a sensor used in the fluid concentration measuring device.

FIG. 8 is a diagram showing a response waveform when the sensor has a baseline drift.

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Claims (2)

[Claims] 1. A concentration measuring device equipped with a sensor for measuring the concentration of a specific component in a fluid, an introduction means for introducing a test substance to be measured and a reference substance for zero-point calibration to a sensor section; Control means for alternately supplying the reference substance for zero-point calibration and the test substance to the sensor unit in a short time by switching the introduction means of the apparatus, and this supply measures the output difference between the test substance and the reference substance obtained from the sensor at that time. A fluid concentration measuring device, comprising: a processing unit for outputting. 2. The fluid concentration measuring device according to claim 1, wherein a zero gas purifier provided with a filter in which the test gas removing agent is sealed is provided in the test substance introduction section.