Carbon monoxide concentration meters. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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As you know, carbon monoxide (carbon monoxide, CO) is very toxic and poisonous. Exceeding its permissible concentration in the air can lead to the death of a person in a gassed room. This gas is odorless and colorless, which makes it especially dangerous, making it difficult to detect in time without special instruments, which usually use semiconductor or electrochemical sensors.

Semiconductor carbon monoxide sensors are much cheaper than electrochemical ones, but they are used, as a rule, only for signaling the presence of carbon monoxide in the air, but not for accurately measuring its concentration, for which it is necessary to use electrochemical sensors.

If it is extremely simple to describe the operation of an electrochemical sensor, we can say that during its operation, the detected gas penetrates into the zone where a redox reaction occurs on the electrode, which leads to the appearance of a signal. An electrochemical gas sensor consists of two or three electrodes for an electrochemical catalytic reaction immersed in an electrolyte. The voltage at the working electrode of the sensor is directly proportional to the gas concentration, which can be determined by measuring this voltage.

A description of a carbon monoxide concentration analyzer using a two-electrode electrochemical sensor was published in [1]. It uses the TGS5042 sensor, which is relatively inexpensive, but has low sensitivity, which makes it impossible to measure low concentrations of CO with high accuracy. And the carbon monoxide concentration meter, according to regulatory documents, should determine precisely the small values ​​\u1b\uXNUMXbof its concentration, starting with units of milligrams per cubic meter (in Russia, the concentration of pollutants in the air is usually measured in such units, for carbon monoxide XNUMX mg / m³ = 0,86 ppm).

Documents [2, 3] require that the concentration of carbon monoxide in the open air does not exceed 3 mg/m³ (average daily) and 5 mg/m³ (peak). In indoor air, the concentration should not exceed 20 mg/m³ throughout the working day, 50 mg/m³ - within an hour, 100 mg/m³ - within 30 minutes or 200 mg/m³ within 15 min. The table shows the values ​​of sensitivity and the maximum measurable concentration of carbon monoxide for some two- and three-electrode electrochemical sensors.

Table

Among the two-electrode sensors presented in this table, the CO/SF-2E sensor has the highest sensitivity [4]. The scheme of the carbon monoxide concentration level meter with such a sensor is shown in fig. 1.

Rice. 1. Scheme of the carbon monoxide concentration level meter (click to enlarge)

Compared to the meter described in [1], only the element base has been changed. As DA1, a TSZ122IDT chip [5] was used, which consists of two precision op amps, which makes it possible to measure carbon monoxide concentration with higher accuracy. The typical value of the input bias voltage of these op amps is 1 µV, and the input current is 50 pA. OU DA1.1 converts the output current of the sensor into voltage (U_O=I_дR4). The resistance of the resistor R4 is chosen to give a conversion factor of 10 mV per 1 mg/m³. The indicator is a built-in digital voltmeter SM3D-DV2 (PV1) with a measurement limit of 1999 mV, which allows you to measure the concentration of carbon monoxide up to 199,9 mg/m³ with a resolution of 0,1 mg/m³.

Op-amp DA1.2 and transistor VT2 form a voltage comparator. Its threshold, set by resistors R5 and R6, is 200 mV, which corresponds to a carbon monoxide concentration of 20 mg/m³. Resistor R7 provides a small amount of hysteresis in the comparator's switching response, preventing the comparator's output voltage from bouncing when triggered. A triggered comparator turns on the HA1 piezo sound emitter (with built-in generator), which gives an audible alarm signal. Through optocoupler U1, the alarm signal enters the control device for the elements of the ventilation system of the room - fanlight window openers and exhaust fans.

To prevent polarization of sensor B1, it is necessary to keep its electrodes connected when the power is off. For this, a p-channel field-effect transistor VT1 is designed, which is open in the absence of power, but closes when a voltage of +5 V is applied to its gate relative to the source.

The sensitivity spread of CO/SF-2E sensors reaches ±20%. Therefore, it is necessary to calibrate the manufactured instrument against the indications of a reference carbon monoxide concentration meter, preferably verified in one of the many laboratories for the maintenance of gas control systems. During the calibration process, the sensitivity of the device is adjusted by selecting the resistance of the resistor R4. It is sufficient to set the comparator response threshold with an accuracy of ±5%.

Three-electrode sensors, compared to two-electrode sensors, have higher technical characteristics, which increases the accuracy of measurements. But the switching circuit of such a sensor is more complicated. If a Russian-made three-electrode electrochemical sensor 2FS-90L [6] is used, the carbon monoxide concentration meter can be assembled according to the circuit shown in Fig. 2.

Rice. 2. Diagram of a carbon monoxide concentration meter (click to enlarge)

This sensor has three electrodes: W - measuring or working electrode, C - reference electrode, R - auxiliary electrode. To power a three-electrode sensor, a special unit is usually used - a potentiostat, which must provide a zero offset of the potential of the measuring electrode relative to the reference electrode with high accuracy. As a rule, a potentiostat for a three-electrode sensor is assembled according to a standard scheme, which can be found in the manuals for the use of sensors published by their manufacturers [7-10].

The meter uses a TSZ124IPT chip containing four of the same op amps as in the TSZ122lDT. Transistor VT1 serves to prevent sensor polarization. Resistive voltage divider R1R2 and op-amp DA1.1 create an artificial "ground", the potential of which is equal to half the supply voltage of the device. Shelter DA1.2 and DA1.3 - elements of the potentiostat. Resistor R9 sets the coefficient for converting the W-sensor electrode current into voltage. As in the previous case, if R9=117kΩ, the concentration of carbon monoxide is 1mg/m³ corresponds to a voltage of 10 mV at the output of the potentiostat.

The manufacturer of the 2FS-90L sensor guarantees its sensitivity of 100 nA/ppm with a deviation of no more than 10%. If such measurement accuracy is sufficient, it is possible to do without instrument calibration, although its verification according to the readings of a standard meter will not hurt.

To measure the concentration of carbon monoxide in ppm (ppm), in both versions of the meter, it is enough to reduce the resistance of the resistor that sets the sensor current-to-voltage conversion factor to 100 kOhm (based on a ratio of 1 mg / m³ = 0,86 ppm). If necessary, two measurement scales can be provided by introducing a two-position resistor switch into the device.

To power both devices, you can use an uninterruptible power supply, assembled according to the circuit shown in Fig. 3. It works both from the mains ~ 230 V and from a galvanic cell with a voltage of 1,5 V. This allows you to use the meter not only in stationary, but also in the field.

Rice. 3. Diagram of an uninterruptible power supply (click to enlarge)

The AC-to-DC converter U1 (this can be a regular network adapter), being connected to the network, generates a constant voltage of 5 V at the output. The transistor VT1 and the Schottky diode VD1 form an automatic power switch from battery to mains and vice versa. When the converter U1 is operating and the voltage at its output is higher than the voltage of the galvanic cell G1, the field-effect transistor VT1 is closed, because the voltage between its gate and source has a polarity closing for the p-channel transistor. A voltage of 5 V is supplied further through an open diode VD1. When the converter U1 is disconnected from the network, the gate voltage of the transistor VT1 becomes zero relative to the common wire. After charging the capacitor C2 through the internal diode of the field effect transistor to a voltage exceeding the threshold voltage of the transistor VT1, its drain-source channel will open. From this point on, the load current of element G1 will flow through the extremely low resistance of the open channel.

Further, a voltage of 5 V to power the meter forms a step-up voltage converter DA1 (HT7750A). The presence of voltage at its output is signaled by the LED HL1.

In the power supply, an L1 inductor with low DC resistance and high Q factor should be installed. It must be designed for current up to 2 A, have a magnetic circuit in the form of a ferrite rod and be wound with copper wire with a diameter of at least 0,5 mm. Oxide capacitors C1 - C3 - tantalum, capacitor C4 - ceramic.

Instead of the TSZ12x series op amp, other precision op amps with possibly lower zero offset voltage and low input current can be used in carbon monoxide meters. Resistors R4-R6 (see Fig. 1) and R1-R5, R9-R11 (see Fig. 2) must have a deviation from the nominal value of not more than 1%.

Note. For reliable operation of the automatic power switch, the gate of the transistor VT1 must be connected to a common wire (minus of the G1 element) with a resistor of 10 ... 100 kOhm. The threshold gate-source voltage of the KP507A transistor can lie in the range of 0,8 ... 2 V. If for the applied instance of the transistor it is greater in absolute value than the voltage of the G1 element, then when working from the latter, the transistor channel will not open and the voltage will be supplied to load only through the built-in protective diode of the transistor. A voltage drop across this diode (about 0,6 V) will significantly degrade the efficiency of the converter. In such a situation, it is better to replace the transistor with a Schottky diode similar to VD1, connecting it with the anode to the plus of G1, and with the cathode to the connection point of the capacitor C2, the cathode of the VD1 diode and the inductor L1.

Literature

1. Kornev A. Analyzer of carbon monoxide concentration. - Radio, 2014, No. 5, p. 36, 37.
2. Maximum Permissible Concentrations (MPC) of Pollutants in the Atmospheric Air of Populated Areas. Hygienic standards GN 2.1.6.1338-03 (approved by the Chief State Sanitary Doctor of the Russian Federation on June 25, 2003). - URL: ohranatruda.ru/ot_biblio/normative/data_normativ/42/4-2030/index/php.
3. Maximum allowable concentrations (MAC) of harmful substances in the air of the working area. Hygienic standards GN 2.2.5. 1313-03 (approved by the Chief State Sanitary Doctor of the Russian Federation on April 27, 2003). - URL: norm-load.ru/SNiP/Data1/42/42033/index.htm.
4. MEMBRAPOR Electrochemical gas sensors. Specification sheet for CO sensor type CO/SF-2E-S. - URL: membrapor.ch/sheet/CO-SF-2E-S.pdf.
5. TSZ121, TSZ122, TSZ124 Very high accuracy (5 pV) zero drift micropower 5 V operational amplifiers. - URL: mouser.com/ds/2/389/tsz1 21-957398.pdf.
6. Electrochemical cells 2FS-90L. - URL: deltainfo.ru/content/elektrokhimicheskie-yacheiki-2fs-90l.
7. Electrochemical Sensors Application Note 2 Design of Electronics for Electrochemical Gas Sensors. - URL: sgxsensortech. com/content/uploads/2014/08/AN2-Design-of-Electronics-for-Electrochemical-Cells.pdf.
8. Application Note MEM1 Electrochemical Gas Sensor. - URL: membrane. ch/sheet/Application_Note_MEM1.pdf.
9. Circuit Note CN-0357. - URL: analog. com/media/en/reference-design-documentation/reference-designs/CN0357.pdf.
10. AN4348 Application note. Signal conditioning for electrochemical sensors. - URL: st.com/content/ccc/resource/technical/document/application_note/b7-/3a/2b/63/6c/10/46/27/DM00093722.pdf.

Author: A. Kornev

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