JPH0442770Y2 - - Google Patents

Description

[Detailed explanation of the idea] [Industrial application field] The present invention relates to a dissolved oxygen meter.

[Conventional technology]

One type of dissolved oxygen meter uses a diaphragm type electrode. This type of dissolved oxygen meter utilizes the principle that the electromotive force or interelectrode resistance generated between electrodes changes depending on the dissolved oxygen partial pressure of the electrolytic solution.

When displaying the amount of oxygen in a gas with the above dissolved oxygen meter, it is displayed as a partial pressure ratio, but when displaying the dissolved oxygen concentration in a liquid as ppm or mg/1, it is expressed as a partial pressure ratio. The temperature of the sample (liquid) at that time must be measured from the data and converted using a table or graph showing the relationship between partial pressure ratio and dissolved oxygen concentration, but the sensitivity of the measurement electrode and the saturated dissolved oxygen concentration Since it changes depending on the temperature, it is necessary to perform temperature compensation to fit the characteristic curve.

For this reason, recent dissolved oxygen meters not only function as DO meters to display dissolved oxygen concentration in liquids, but also function as DO meters.
It functions as an O ₂ meter to display the amount of oxygen in the gas and a thermometer to display the temperature.

Here, the measurement principle of a dissolved oxygen meter using a diaphragm electrode will be briefly explained. If the oxygen partial pressure ratio of the sample is P, the electrode current I is I=K・α(t)・P (1). Here, K is a proportionality constant, and α(t) is the oxygen permeability coefficient of the diaphragm, which is a function of temperature.

When displaying the amount of oxygen in the gas, if α(t) is subjected to temperature compensation, the oxygen partial pressure ratio P can be displayed in proportion to the current I.

If the dissolved oxygen concentration is C (ppm or mg/1), there is a relationship between it and the oxygen partial pressure ratio P mentioned above, from the so-called Henry's law, as follows: P=β(t)・C...(2) . Here, β(t) is a function of temperature.

Substituting equation (2) into equation (1) yields I=K・α(t)・β(t)・C ……(3). Therefore, α(t) and β(t) are functions of temperature.
By performing a functional calculation, the electrode current I can be expressed as the dissolved oxygen concentration C.

Figure 4 shows the circuit configuration of a conventional dissolved oxygen meter having three functions.
Reference numeral 3 designates temperature sensing elements provided in the feedback circuits of the operational amplifiers 4, 5, and 6, respectively. Then, the above (1) is achieved by the temperature sensing element 1 provided in the feedback circuit of the operational amplifier 4.
After performing temperature compensation for α(t) in the equation, O ₂ (oxygen concentration in the gas) is displayed, and β in the above equation (2) is displayed using the temperature sensing element 2 installed in the feedback circuit of the operational amplifier 5.
Performs temperature compensation for (t) and displays DO (dissolved oxygen concentration in the liquid). The temperature is displayed based on the detection output of the temperature sensing element 3 provided in the feedback circuit of the operational amplifier 6. In addition, 7, 8,
9 is a display section.

[Problem that the invention attempts to solve]

As mentioned above, conventional dissolved oxygen meters with three functions had a temperature sensing element for each function, which led to problems such as the complicated configuration of the detection part that was immersed in the test liquid. Ta.

The present invention has been made with the above-mentioned considerations in mind, and its purpose is to provide a dissolved oxygen meter that satisfies the above three functions with a single temperature sensing element.

[Means for solving problems]

In order to achieve the above object, the dissolved oxygen meter according to the present invention includes a feedback circuit including a temperature sensing element, a first operational amplifier that outputs a temperature signal, and a feedback circuit in which a resistor and an analog switch are connected in series. A second operational amplifier consists of a parallel circuit with a resistor and outputs a signal corresponding to the amount of oxygen in the gas based on the measurement signal from the measurement electrode, and a feedback circuit consists of a resistor and an analog switch connected in series, and a resistor. a third operational amplifier that outputs a signal corresponding to the dissolved oxygen concentration in the liquid based on the output signal of the second operational amplifier; and a pulse having a pulse width determined by the temperature signal. A pulse width modulation circuit is provided to output an output, and the analog switch is turned on and off by the pulse output, thereby adjusting the amount of feedback of the second and third operational amplifiers.

[Effect]

A signal proportional to temperature is obtained by a first operational amplifier having a temperature sensing element, and based on this signal, a pulse output having a pulse width (duty) determined depending on the temperature is obtained from the pulse width modulation circuit. This pulse output turns on/off the analog switches provided in the feedback circuits of the second and third operational amplifiers, thereby changing the on/off time of the analog switches. The amount of feedback in the operational amplifier No. 3 is adjusted appropriately depending on the temperature.

〔Example〕

Hereinafter, one embodiment of the present invention will be described based on the drawings.

FIG. 1 shows an example of a dissolved oxygen meter according to the present invention. In this figure, 10 is a temperature measuring circuit, a first operational amplifier 11, and a temperature sensing circuit provided in a feedback circuit 11K of this first operational amplifier 11. Element 12, resistor 1
3, 14 and an input resistor 15. A DC signal representing the temperature is output to the output point 16. Note that 17 is a display unit that displays temperature.

20 is a pulse width modulation circuit, which is provided on the output side of the temperature measurement circuit 10. This pulse width modulation circuit 20
For example, as shown in FIG. 2, the circuit includes a known triangular wave generating circuit 21 and a DC comparator 22. That is, the triangular wave output s from the triangular wave generating circuit 21 and the temperature measuring circuit 1 appearing at the output point 16
Based on the DC output d of 0, the pulse width modulation circuit 2
0, a pulse output having a pulse width determined by the level of the DC output d is output.

Therefore, this pulse width modulation circuit 20 provides a pulse output D(t) whose pulse width (duty) changes depending on the temperature.

Now, when changing the pulse width from 10 to 90% at 0 to 50℃, the pulse output D(t) is D(t) = (0.016t + 0.1) × 100 (%) ...(4) The waveform is shown in FIG.

Further, 23 and 24 are output terminals, and 25 is an inverter. Therefore, the output D is connected to the terminal 23.
(t), and an output D(t) which is an inversion of this D(t) is output to the terminal 24.

Reference numerals 31 and 41 are second and third operational amplifiers, respectively, and as explained below, these are constructed in the same way. The signal I is input, and the output of the second operational amplifier 31 (described later) is input to the third operational amplifier 41.

That is, the feedback circuit 3 of the second operational amplifier 31
1K consists of a parallel circuit of a resistor 32 and an analog switch 34 connected in series and a resistor 33, and a switch 35 is connected to a connection point 36 between the resistor 32 and the analog switch 34. Further, the feedback circuit 41K of the third operational amplifier 41 is
It consists of a parallel circuit of a resistor 42 and an analog switch 44 connected in series, and a resistor 43, and
Connection point 46 between resistor 42 and analog switch 44
A switch 45 is connected to the switch 45.

The analog switches 34, 44, switch 3
5 and 45 are each configured to turn on when there is an H (high) level input, and turn off when there is an L (low) level input. The analog switches 34 and 44 receive the output D(t), while the switches 35 and 45 receive the inverted output D(t).

When the output D(t) as shown in FIG. 3 is input to the analog switches 34, 44, the ON time of the analog switches 34, 44 changes according to the duty time. , the resistance values of the feedback circuits 31K and 41K change. For example, when the analog switch 34 is on, the resistance value of the entire feedback circuit 31K becomes small, and the amount of feedback from the operational amplifier 31 becomes large. At this time, the gain becomes small. Further, when the analog switch 34 is off, the amount of feedback of the operational amplifier 31 becomes small and the gain becomes large.

Note that the switches 35 and 45 are turned on when the analog switches 34 and 44 are in the off state, and serve to stabilize the potentials at the connection points 36 and 46. And 37 and 47 are respectively O ₂ display,
A display section 50 that performs DO display is a resistor provided between the second and third operational amplifiers 31 and 41.

Next, the voltage E ₀ at the display sections 37, 47,
Find E ₁ . In the following description, for example, R ₃₃ indicates the value of the resistor 33, and the same applies to other symbols.

First, the voltage E ₀ is expressed as E ₀ = (R ₃₂ · R ₃₃ / R ₃₂ + D / 100 · R ₃₃ ) · I ... (5), and by substituting formula (1) into this formula (5), , E ₀ = (R ₃₂ · R ₃₃ / R ₃₂ + D / 100 · R ₃₃ ) ・K · α (t) · P E ₀ = (α (t) / R ₃₂ + D / 100 · R ₃₃ ) · R ₃₂・R ₃₃・K・P ...(6) is obtained.

K ₀ = α(t)/(R ₃₂ +D/100・R ₃₃ ) ……(7) Then, the above E ₀ becomes E ₀ =K・K ₀・R ₃₃・R ₃₂・P ……(8 ), and if R ₃₃ and R ₃₂ are selected so that K ₀ is constant, an output E ₀ proportional to the oxygen partial pressure ratio P can be obtained regardless of the temperature.

Next, the voltage E ₁ is: E ₁ = {R ₄₂・R ₄₃ /R ₅₀・(R ₄₃ + D/100・R ₄₂ )}・E ₀
......(9), and by substituting equations (2), (6), and (7) into equation (9), E ₁ = {R ₄₂・R ₄₃ /R ₅₀・(R ₄₃ + D/100・_R42 )}
・K ₀・R ₃₂・R ₃₃・K・β(t)・C = (β(t)/R ₄₃ +D/100・R ₄₂ )・(K・K
₀・R ₃₂・R ₃₃・R ₄₂・R ₄₃ /R ₅₀ )・C...(10) is obtained.

K ₁ = β (t) / (R ₄₃ + D / 100 · R ₄₂ ) ... (11) If R ₄₃ and R ₄₂ are chosen such that the above K ₁ is constant, then they are proportional to the dissolved oxygen concentration C. The output _E1 is obtained.

[Effect of idea]

As described above, in the present invention, a signal proportional to temperature is obtained by the first operational amplifier having a temperature sensing element, and based on this signal, a pulse width ( This pulse output causes a second operational amplifier to output a signal corresponding to the amount of oxygen in the gas, and a third operational amplifier to output a signal corresponding to the dissolved oxygen concentration in the liquid. Since the analog switch provided in each feedback circuit with the operational amplifier is turned on and off, the on/off time of the analog switch in each operational amplifier changes, and this changes the amount of feedback in the second and third operational amplifiers. changes appropriately depending on the temperature.

Therefore, by appropriately selecting the pulse width of the pulse output and the value of the resistance in the feedback circuit, the amount of feedback from the second and third operational amplifiers changes appropriately depending on the temperature, and a predetermined temperature compensation is performed.

In addition, in this invention, instead of providing a temperature sensing element for each function as in the past, by providing a single temperature sensing element, it is possible to display not only the temperature but also the amount of oxygen in the gas. and temperature compensation in the dissolved oxygen concentration in the liquid.
Therefore, the configuration of the detection section immersed in the test liquid can be simplified, and the circuit configuration can also be simplified.

[Brief explanation of the drawing]

FIG. 1 is an electrical circuit diagram of a dissolved oxygen meter according to the present invention. FIG. 2 is an electrical circuit diagram showing a configuration example of a pulse width modulation circuit. Figure 3 A, B, and C are
FIG. 3 is a diagram showing an example of an output waveform of a pulse width modulation circuit. FIG. 4 is a circuit diagram showing a conventional example. 11...First operational amplifier, 11K...Feedback circuit, 12...Temperature sensing element, 20...Pulse width modulation circuit, 31...Second operational amplifier, 31K...Feedback circuit, 32, 33...Resistor, 34...Analog switch, 41...Third operational amplifier, 41K...
Feedback circuit, 42, 43...Resistor, 44...Analog switch, I...Measurement signal, _E0 ...Signal corresponding to the amount of oxygen in the gas, _E1 ...Signal corresponding to the dissolved oxygen concentration in the liquid , D(t)...Pulse output.

Claims (1)

[Scope of utility model registration request] The feedback circuit includes a first operational amplifier that has a temperature sensing element and outputs a temperature signal, and the feedback circuit consists of a series connection of a resistor and an analog switch, and a parallel circuit of the resistor. The second output signal corresponds to the amount of oxygen in the gas.
an operational amplifier, a feedback circuit consisting of a series connection of a resistor and an analog switch, and a parallel circuit of the resistor, and outputs a signal corresponding to the dissolved oxygen concentration in the liquid based on the output signal of the second operational amplifier. A third operational amplifier and a pulse width modulation circuit that outputs a pulse output having a pulse width determined by the temperature signal are provided, and the analog switch is turned on and off by the pulse output. and a dissolved oxygen meter, characterized in that the feedback amount of the third operational amplifier is adjusted.