Metal Oxide Sensors: Applications and Responses

The usage of SGX metal oxide sensors and the interpretation of sensor responses are explained in detail in this article.

The focus is on the following areas:

  • General sensor principle and performance parameters to obtain basic qualitative response
  • Method to connect the sensors
  • Method to operate the sensors, which includes type of test circuit and measuring criteria for the output
  • Time taken for warming up
  • Sensor responses to gas needed for calibration of each sensor with target gas
  • Effects of temperature, environmental factors, drift, flow rates and others
  • Cross-sensitivity, response time, sensor poisons among other things are also looked into

Basic Sensor Parameters

SGX and other metal oxide sensors work on the basic principle that the resistance of a sensor’s detecting layer is altered when target gases are present. In the case of oxidising gases such as nitrogen dioxide and ozone, the resistance goes up while, for reducing gases like VOCs and carbon monoxide, the resistance comes down. Reducing gases have the tendency to eliminate certain ‘insulative’ oxygen species in grain boundaries that cause the resistance to go up. Oxidising gases act in exactly the opposite manner.

One of the key benefits of using the SGX sensor is that the power needed for heating and operating it is lesser than what is required for other sensors available commercially.

The variation in resistance when plotted against the variation in gas concentration is not a linear but a polynomial relationship. Hence sensors are ideally deployed in areas where a user is keen on detecting possibilities of occurrence of gas. These ‘event sensing’ devices do not require a high accuracy rate and do not have many safety issues.

How to Connect the Sensors

It is recommended that the sensors are connected using a reflow oven to a PCB. About 11% flux is required in the solder paste so as to create a suitable joint. Heraeus F640 Sn95.5Ag4Cu0.5 type 3 is a commonly used solder paste. A basic solder features can be viewed in figure 1. A neutral environment with good filtration and air extraction is ideal for the procedure.

Figure 1.

Sensor Operation

A 5V supply as well as a series resistor is required for the heater circuit to avoid excess heating of the sensor. Another series resistor is necessary in the measuring circuit in order to control voltage in the detecting layer. An example of very simple circuits used for providing power to SGX Sensortech sensors is can be viewed in figure 2.

A voltage divider with a heater resistance (Rh) is fitted with a resistor (Rd). The two resistors are series connected and supplied with 5 V. A sensing resistor (Rs) is measured using an AD converter or a microcontroller.

Figure 2.

How to Interpret the Sensor Signal

The resistance (Rs) is measured using an A/D converter. The microcontroller can switch between resistors in series. According to the accuracy of the ADC, the resistors can be switched. For example, a 10-bit ADC would be enough for two switching resistors to handle a sensing resistance range of 2kW to 1MW.

Figure 3.

Warm–Up Time

For the sensor to function at an optimal level, it must have a certain amount of warm up time for its chemical equilibrium to build up as chemical compounds tend to be absorbed or desorbed on the sensing surface. Stabilization occurs faster when the sensor functions at high temperatures. The pre-heating phase is conducted at a higher voltage, and then the normal operating voltage is used to realize stability, which is followed by a reduced voltage.

Sensor Responses to Gas

Each sensor has a different resistance in air, which in turn will alter the concentrations of the target gas. In order to convert from resistance readings to concentration, a calibration curve for each sensor has to be derived. This can be done by measuring the resistances in air and a number of gas concentrations over a preferred range.

When the concentrations are in oxygen-rich environment, the sensors work well and more points can be achieved, thereby ensuring accuracy. The response can be easily plotted on a polynomial.

The Effect of Environmental Factors, Temperature, Drifts

The resistance of sensing layer will be affected by ambient temperature when the sensor is heated. A change of 50% for a temperature increase of 25°C for MiCS-5521 CO/VOC sensor was observed. Sensor response in air as well as in concentrations of the target gas is affected by this. Pressure fluctuations too will affect the resistance although this will be lesser compared to temperature fluctuations. Similarly, thermal or chemical processes of humidity affect sensor resistance. This can be solved if humidity is measured.

Sensor temperature and resistance is also affected by gas flow rate within the sensor. SGX recommends that the sensor should be placed at right angles to enable flow by protecting by a fine mesh or porous PTFE filter (Donaldson part no. EN.07.01.586). In this case, the gas diffuses to the sensor. Calibration at the flow conditions should be performed and the flow has to be very steady.

The detecting layer resistance of metal oxide sensors tends to alter with time when there are changes in the internal framework. Although these changes happen over a long period of time, the sensors should be calibrated for maintaining precision. Auto-zeroing function is used to balance drift effects.

Other Performance Parameters

Cross-sensitivity

Metal oxide sensors are non-specific sensors, hence it is difficult to differentiate between oxidizing or reducing chemical species. Positive cross-sensitivity gases containing opposite oxidising properties tend to cause negative changes in displayed gas concentration. Dopants addition, filter fitting, and rise of the operating temperature help in decreasing the effects; however a cross-sensitivity degree will be present.

Response Time

The SGX sensors display quick response to changes in gas concentration. Usually T90 response times are less than 10s.

Poisoning

Sensor response can be affected by compounds present as they tend to chemically react with the detecting layer or may decompose thereby influencing the rate of gas reaching the layer. Chemicals that commonly cause problems are high concentrations of organic vapors and fumes from soldering and organic silicones.

About SGX Sensortech (IS)

SGX Sensortech is a market leader in innovative sensor and detector devices that offer unrivalled performance, robustness and cost- effectiveness.

SGX have been designing and manufacturing gas sensors for use in industrial applications for over 50 years, offering excellent applications support for an extensive range of gas sensors and the expert capability for custom design or own label.

As an independent OEM supplier of gas sensors, we pride ourselves on providing customers with unrivalled product reliability and personal product support via specialist engineers.

SGX gas sensors are built to the highest standards with all pellistor and infrared gas sensors achieving ATEX and IECEx certification, SGX gas sensors are also UL and CSA approved.

Our product portfolio has continued to expand in technology and detectable gases used in a wide range of applications including:-

  • Mining
  • Oil and gas
  • Confined space entry
  • Indoor air quality
  • Industrial area protection
  • Leak detection

This information has been sourced, reviewed and adapted from materials provided by SGX Sensortech (IS) Ltd.

For more information on this source, please visit SGX Sensortech (IS) Ltd.

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