Design of the hottest low power toxic gas detector

Low power toxic gas detector design

safety first! Many industrial processes involve toxic compounds, such as chlorine used in the manufacture of plastics, agricultural chemicals and pharmaceutical products; The production of semiconductors requires the use of phosphine and hydrogen arsenide; Burning consumer packaging materials will release hydrogen cyanide. Therefore, it is very important to know whether the concentration of toxic gases reaches a dangerous level

in the United States, the National Institute for occupational safety and health (NIOSH) and the American Conference of governmental industrial hygienists (ACGIH) have set short-term and long-term exposure limits for many toxic industrial gases. "Threshold limit value time weighted average" (TLV-TWA) refers to the time weighted average concentration that most workers can repeatedly contact within normal 8-hour working days without being adversely affected. "Threshold limit value - short term exposure limit value" (TLV-STEL) refers to the concentration that most workers can be exposed to for a short time without stimulation or injury. "Immediately life-threatening or health-threatening concentration" (idlhc) is a limiting concentration, which will pose an immediate or slow threat to life, cause irreversible health damage, or affect the ability of workers to escape independently. Table 1 lists the limits of several common gases

for instruments that detect or measure the concentration of toxic gases, electrochemical sensors can provide many advantages. Most sensors are designed for specific gases. The available resolution is less than one millionth of the gas concentration (1 ppm), and the required working current is very small. It is very suitable for portable battery powered instruments. An important characteristic of electrochemical sensor is its slow response: after the first power on, the sensor may take several minutes to establish the final output value; When exposed to the gas concentration in the middle range, it may take 25 to 40 seconds for the sensor to reach 90% of the final output value

this paper describes a portable carbon monoxide (CO) detector using electrochemical sensors. The IDLH concentration of carbon monoxide is much higher than that of most other toxic gases, which is relatively safer to handle. However, carbon monoxide is still a fatal gas, so extreme care and appropriate ventilation measures should be taken when testing the circuits described herein

Figure 1 Co-ax carbon monoxide sensor

Figure 1 shows the co-ax sensor of alphasense company. Table 2 is a summary of the technical specifications of the co-ax sensor

Table 2 Technical specifications of co-ax sensor

for portable instruments in this application, achieving the longest battery life is the most important goal. Therefore, it is essential to minimize power consumption. In a typical low-power system, the measurement circuit performs a measurement after it is powered on, and then turns off to enter a long-time standby state. However, in this application, due to the long time constant of the electrochemical sensor, the measuring circuit must always be powered on. Fortunately, because of the slow response, we can use micro power amplifiers, high-value resistors and low-frequency filters to minimize Johnson noise and 1/f noise. In addition, single power supply can avoid the power waste of bipolar power supply

Figure 2 shows the circuit of the portable gas detector. Dual channel micro power amplifier ADA is used in constant potential configuration (u2-a) and transconductance configuration (u2-b). The power consumption and input bias current of the amplifier are very low, which is a good choice for the constant potential part and transconductance part. The power consumption of each amplifier is only 10 μ A. Therefore, the battery life is very long

Figure 2 Portable gas detector using electrochemical sensor

in the three electrode electrochemical sensor, the target gas expands and improves the core basic industrial strength, spreads to the sensor, and acts on the working electrode (we) after passing through a thin film. The constant potential circuit detects the voltage of the reference electrode (RE) and supplies current to the auxiliary electrode (CE) to keep the voltage between the re terminal and the we terminal constant. There is no current flowing in or out of the re end, so the current flowing out of the CE end extrusion blow molding is usually the current flowing into the we end of hard or inelastic resins such as HDPE, PP and PVC, which is proportional to the concentration of the target gas. The current flowing through the we terminal may be positive or negative, depending on whether the reduction reaction or oxidation reaction occurs in the sensor. For carbon monoxide, when oxidation occurs, the current at the CE end is negative (the current flows into the output end of the constant potential operational amplifier). Resistance R4 is usually very small, so the voltage at we is about equal to vref

the current flowing into the we terminal will cause the output terminal of u2-a to generate a negative voltage relative to the we terminal. For carbon monoxide sensors, this voltage is usually hundreds of millivolts, but for other types of sensors, this voltage may be as high as 1 v. In order to use a single power supply, the micro power reference voltage source adr291 (U1) raises the whole circuit to 2.5 V above ground. The power consumption of adr291 is only 12 μ A； It can also provide a reference voltage so that the analog-to-digital converter can digitize the output of this circuit

where:

IWE is the current flowing into the we end

rf is the transconductance resistance (shown as U4 in Figure 2)

the maximum response of the sensor is 90 na/ppm, as shown in Table 2, and its maximum input range is 2000 ppm. Therefore, the maximum output current is 180 μ A. The maximum output voltage is determined by the transconductance resistance, as shown in formula 2

sensors from different manufacturers have different current output ranges for different gases. If U4 uses programmable rheostat ad5271 instead of fixed resistance, it can adopt the same structure and material for different gas sensors. In addition, such products also support the exchange of sensors, because the microcontroller can set the ad5271 to an appropriate resistance value for different gas sensors. The temperature coefficient of ad5271 is 5 ppm/° C, which is better than most discrete resistors; Its power supply current is 1 μ A. The impact on system power consumption is minimal

when using 5 V single power supply, according to Formula 1, the output range of transconductance amplifier u2-b is 2.5 v. If the ad5271 is set to 12.5 K Ω, the range under the worst sensitivity of the sensor can be used, and it can provide about 10% over range capability

with a typical sensor response of 65 na/ppm, the output voltage can be converted into ppm of carbon monoxide by the following formula:

when using differential input ADC, it is only necessary to connect the 2.5 V reference voltage output end to the AIN end of ADC, so as to eliminate the 2.5 V term in Formula 3

resistor R4 keeps the noise gain of transconductance amplifier at a reasonable level. The value of R4 needs to weigh two factors: the amplitude of noise gain and the deformation of the sensor when exposed to high concentration gas. Overview of measurement: vertical time error. For this circuit, R4 = 33 Ω, so the noise gain can be calculated to be equal to 380, as shown in formula 4

the output noise is equivalent to the gas concentration above 1.3 ppm P-P, and this low-frequency noise is difficult to filter out. Fortunately, the sensor response is very slow, so the low-pass filter composed of R5 and C6 can have a cut-off frequency of 0.16 Hz. The time constant of this filter is 1 second, which is negligible compared with the 30 second response time of the sensor

q1 is a p-channel JFET. When the circuit starts, the gate voltage is VCC and the transistor is disconnected. When the system is turned off, the grid voltage drops to 0 V, and the JFET is turned on, so that the re terminal and we terminal maintain the same potential. When the circuit is started again, this can greatly improve the start-up time of the sensor

this circuit is powered by two AAA batteries. Using diode to provide reverse voltage protection will waste valuable electric energy, so this circuit uses p-channel MOSFET (Q2). The MOSFET protects the circuit by blocking the reverse voltage and turns on when a positive voltage is applied. The on resistance of MOSFET is less than 100 m Ω, so the voltage drop caused by MOSFET is much smaller than that of diode. In addition to AAA batteries, the buck boost regulator adp2503 also allows the use of external power supplies up to 5.5 v. When working in power saving mode, the power consumption of adp2503 is only 38 μ A。 The filter composed of L2, C12 and C13 can eliminate any switching noise generated by the analog power rail. When connecting the external power supply, the instrument does not disconnect the battery through a circuit, but uses a jack to disconnect the battery mechanically, so as to avoid the waste of electric energy

when using AAA battery, the total power consumption under normal conditions (no gas detected) is about 100 μ A. The total power consumption under the worst case (2000 ppm co detected) is about 428 μ A。 If the instrument is connected with a microcontroller, it can enter the low-power standby mode without measurement, and the battery life can reach more than 1 year

references

niosh Pocket Guide to chemical hazards

the selected electromechanical power is only 46osh/npg/

alphasense co-ax data manual

brief introduction to the author

luis Orozco [ozco@] is a system application engineer in the industrial and instrumentation Department of ADI company, mainly involved in precision instrumentation, chemical analysis and environmental monitoring applications. He joined ADI in February, 2011. (end)