Galvanic Cells Typed Oxygen Sensor

Published Date: 02 Nov 2017

(Jon S. Wilson, 2005) Sensors convert physical world into electrical signals. Due to this, sensors are part of the interface between the world of electrical devices and physical world, such as computers. In short, a sensor receives a stimulus and responds with an electrical signal. In this section, we will talk about the theory behind and the operating principles of each sensor involved in this project.

3.1.1. Galvanic Cell Typed Oxygen Sensor

The galvanic-cell oxygen sensor is a lead-oxygen battery and it incorporates an oxygen cathode made of gold, a weak acid electrolyte and a lead anode. The gold electrode is bonded to a non-porous Teflon FEP membrane. (GS Yuasa, 2012) Oxygen molecules penetrate into the electrochemical cell via a non-porous fluorine resin membrane and then are reduced with the acid electrolyte at the gold electrode. A thermistor and a resistance are connected in between the anode and the cathode and thus, the lead-oxygen battery is discharged always. The oxygen concentration in the gas mixture measured is proportional to the current flowing between the electrodes. The resistor and the terminal voltages across the thermistor which is for temperature compensation are read as a signal referring the change in output voltages indicating the change in oxygen concentration. Figure 3.1 shows the structure of Galvanic cell typed Oxygen sensor. The following chemical reactions that take place in oxygen sensors:

Anodic reaction : 2Pb + 2H2O → 2PbO + 4H+ + 4e-

Cathodic reaction : O2 + 4H+ + 4e- → 2H2O

Total reaction : O2 + 2Pb → 2PbO

http://www.gs-yuasa.com/gyid/us/products/ke_series/img/structure_s.jpg

Figure 3.1: Structure of Galvanic cell typed O2 sensor

(Figaro, 2005/06) A small volume air bubble is confined inside the sensor body to compensate for internal influence which is from pressure changes. The electrolyte of the sensor is composed of acetic acid with pH around 6. Fluorine resin membrane induces an effect on the speed of diffusion of oxygen molecules and indirectly affects the speed of response and the life span of the sensor.

3.1.2. Metal Oxide Semiconductor Ozone Sensor

For more than four decades, it has been known that the electrical conductivity of semiconducting metal oxides varies with the composition of the gas atmosphere surrounding them. Since 1980´s, there have been many efforts done to develop ozone sensors based on metal oxide materials. According to the working temperature, such ozone sensors can be classified as either high-temperature or room-temperature ozone sensors. As the ozone sensing materials, In2O3, ZnO, SnO2 and CeO2 have been used.

The gas sensor is made up of sensor base, sensor cap, and sensing element. The sensing element comprises of heater to heat up sensing element and sensing material. The sensing element will utilize different materials, depending on the target gas, such as Tin dioxide (SnO2). (Chunyu Wang, 2008) A heating element is integrated to regulate the sensor temperature for two purposes: to reactivate the gas sensor by gas desorption at high temperatures, and to detect different gas species as the sensors exhibit different gas response characteristics for different temperature ranges. Usually, at room temperature there is only a slow and not appreciable interaction of the surrounding gas with the surface.

(Chunyu Wang, 2008) The boundary layer theory is well suited and widely accepted for the qualitative description of the processes and the prediction of the sign of the change in conductivity of metal oxide in the presence of a reducing or oxidizing gas. As shown in Figure 3.2, as a n-type metal oxide surface or the grain surface such as SnO2 is exposed to an oxidizing (reducing) gas, chemisorbed particles cause a localized energy level within the band gap of the metal oxide, acting as electron acceptor (donor).

Figure 3.2: Classical gas sensor with integrated heating system

This results in a charged layer of electron depletion (accumulation) at the surface leading to a compensating boundary layer which prevents carriers from moving freely and to the formation of surface potential Vs. Electric current, inside the sensor, flows via the conjunction parts which is grain boundary of SnO2 micro crystals. The electrons must overcome the electronic barrier to go across the grain boundaries. Figure 3.3 (a) and (b) indicate the surface charge of n-type metal oxide in the presence of an oxidizing and reducing gas while (c) and (d) refer to the corresponding band bending on the n-type metal oxide surface.

Figure 3.3: N-type metal oxide surface and energy value of surface barrier

Then, the conduction of the sensing layer can be approximated with the help of the Schottky model by

[3.1]

where is the conductivity of the semiconductor material and is the bulk conductivity, which depends on the temperature and geometric properties of the layer.

The relationship between the concentration of deoxidizing gas and sensor resistance can be denoted over some range of gas concentration by the following equation:

Rs = A[C] –α [3.2]

where Rs is the electrical resistance of the sensor, A is constant, [C] is the target gas (Ozone) concentration and is the slope of Rs curve.

3.1.3. Capacitive Humidity Sensor

(Denes K. Roveti, 2001) Capacitive relative humidity (RH) sensors are broadly applied in commercial, weather telemetry and industrial applications. Humidity sensors based on capacitive effect consists of a hygroscopic dielectric material sandwiched between a pair of electrodes forming a small capacitor. Most capacitive sensors use a plastic or polymer as the dielectric material, with a typical dielectric constant ranging from 2 to 15. In absence of moisture, the dielectric constant of the hygroscopic dielectric material and the sensor geometry determine the value of capacitance.

At normal room temperature, the dielectric constant of water vapor has a value of about 80, a value much larger than the constant of the sensor dielectric material. Therefore, absorption of water vapor by the sensor results in an increase in sensor capacitance. At equilibrium conditions, the amount of moisture present in a hygroscopic material depends on both the ambient temperature and the ambient water vapor pressure. This is true also for the hygroscopic dielectric material used on the sensor. By definition, relative humidity is a function of both the ambient temperature and water vapor pressure. Therefore there is a relationship between relative humidity, the amount of moisture present in the sensor, and sensor capacitance. This relationship governs the operation of a capacitive humidity instrument. Figure 3.4 shows the basic structure of capacitive type humidity sensor.

A capacitive humidity sensor fluctuates its capacitance depending on the relative humidity (RH) of the surrounding air. The capacitance increases as the relative humidity increases. Relative humidity is given in the form of percentage of actual vapor pressure (P) and compared with saturated vapor pressure (Ps). [19]

[3.3]

Figure 3.4: Basic structure of capacitive type humidity sensor

3.1.4. Temperature Sensor

A Thermistor is widely used today as it is very economic with its precise heat sensing feature over a prescribed range of temperatures. The change in resistance of this metal-oxide sensor is inversely proportional to the change in temperature. An agitation current is flowed across the sensor; meanwhile the voltage, whose change is a direct function of the resistance, is measured and then converted to the desired units of heat calibration. Thermistors vary their resistance, like RTDs, as the ambient temperature fluctuates. The material used in a thermistor, unlike RTDs which use pure metals, is generally a polymer or ceramic.

(Kamal Siddique, 2010) When a thermistor is applied to sense the changes in temperature, the relationship between temperature and resistance is of principal concern. The general expression applying to almost all thermistors is

[3.4]

Where, R0 = resistance value at reference temperature T0 (K), Ω

RT = resistance at temperature T (K), Ω

= constant over temperature range, dependent on manufacturing process and construction characteristics (specified by supplier)

There are two common types of thermistors where the positive temperature coefficient (PTC) type is equipped with a resistance which increases with increasing temperature while the negative temperature coefficient (NTC) thermistors show a decrement in resistance as the temperature increases. Figure 3.5 shows the common types of thermistor sensors.

Figure 3.5: Common types of thermistor sensors

3.2. Data Acquisition Systems

Data acquisition is the process of opting signals which measure physical world and convert the physical samples into digital quantities that can be controlled and analysed by a computer. The data acquisition systems encompass sensors that convert physical quantities into electrical signals. Besides, it also consists of the signal conditioning system which is used to amplify the signals before they can be further converted into digital numeric values later on. Last but not least, the Analog-to-digital converters are equipped to convert the amplified or conditioned sensor signals to digital values. Data acquisition applications are manipulated by software developed with programming languages such as BASIC C, C++, Java.

3.2.1. Signal Conditioning

The signal from the sensor must first be transmitted to an interface device and then from there to the computer to retrieve information from a sensor into a computer. However, the signal from the sensor must be conditioned before going further down in order to be useful to the interface device. Almost all interface devices are designed to accept a voltage signal ranging 0 to 5 volts and digitize this. In other words, the signal conditioning circuit is to take whether voltage or resistance and then convert it to a 0 to 5 volts signal. This process generally involves converting a resistance to a voltage, dividing, amplifying and shifting a voltage sequentially.

(A) Voltage Divider

In sensors, a voltage divider or better known as potential divider is a linear circuit which yields an output voltage (Vout) that is fractional to its input voltage (Vin). Voltage division is defined as the voltage partitioning among the components of the divider. Most input transducers vary their resistance. Usually, a voltage divider is utilised to convert the resistance to a more useful voltage. The voltage signal can be transmitted to other parts of the circuit, such as the input to a transistor switch or an IC. Besides, a voltage divider with two resistors in series is applied to fraction the input voltage to the ratio of the resistances as shown in Figure 3.6. The voltage divider is expressed in the following formula:

Formula for dividing a voltage [3.5]

Figure 1: Basic Voltage Divider Circuit

Figure 3.6: Basic schematic voltage divider

(Kristin Wood, 2012) If one of the two resistors in the circuit is substituted with a variable resistance, the output voltage is proportional to the resistance change of the variable resistor. When a resistive sensor operating as a variable resistor, it can be used to replace one of the resistors, yielding a voltage output that is proportional to the resistance of the sensor.

(B) Signal Amplifying

(Om Prasad Patri, K Sanmukh Rao, 2012) Signal amplification performs two important functions, increasing the resolution of the input signal, and increasing its signal-to-noise ratio. Sometimes a sensor will generate a small output voltage with probably only a few millivolts of range. In this case there is a need to amplify the voltage so that it falls in the range of the analog-to-digital converter. To achieve this amplification, the easiest way is to use an operational amplifier (Op-amp) which is an amplifier circuit with very high gain in open loop and differential inputs which employs external feedback for control of its transfer function or gain. Figure 3.7 shows the circuit required to use an op-amp chip as a voltage amplifier. This circuit amplifies the voltage with the ratio of both resistors as follows:

http://www.sensorwiki.org/lib/exe/fetch.php/tutorials/voltage_amplifier_formula1.png?w=&h=&cache=cache [3.6]

Figure 2: Basic Voltage Amplifier Circuit

Figure 3.7: Basic schematic op-amp

3.2.2. Signal Digitization

Digitization converts information into a digital format. In this format, information is organized into bits that can be separately addressed. This is the binary data that computers and many devices with computing capacity can process. Digitizing information makes it easier to access, share, and preserve. Analog signals vary continuously, both in the number of points and possible values in the signal in a given period of time. However, digital signals are discrete in both of these respects and thus in practical terms, a digitization can only ever be an approximation of the signal it represents. In addition, digitization occurs in discretization and quantization. Discretization denotes that the reading of an analog signal and at regular period of time sampling the value of the signal at the point A. Such reading is called a sample and may be considered to have infinite precision at this stage.

Generally, the embedded system uses the Analog-Digital Converters (ADC) as its digitizer. An analog-to-digital converter is a device that converts the input continuous physical quantity to a digital numeric values. The conversion encompasses quantization of the input, so it gives out a small amount of error. (Nicholas Gray, 2006) An ADC has an analog reference current or voltage relying on the analog input. The digital output tells us what fraction of the reference current or voltage the input current or voltage is. So, basically, the ADC is a divider. An n-bit ADC has a resolution of one part in 2n. In other words, a 3-bit A/D converter has 23 = 8 possible output codes. The difference between each output code is VREF/23. The Resolution of an A/D converter is the number of output bits it has (3 bits, in this case).

3.2.3. Universal Serial Bus (USB)

Just about any computers that we buy today come with one or more Universal Serial Bus (USB) connectors. Compared to other ways of connecting devices to our computers including serial ports, parallel ports, and special cards, USB devices are far simpler. (Geoff Knagge, 2010) USB defines the communications protocols, connectors, and cables used in a bus for connection, communication and power supply between electronic devices and computers. Then how data is sent across the USB? This happens when the software requires data transfer to occur between itself and the USB, it sends a block of data called an I/O Request Packet (IRP) to the appropriate pipe, and the software is later notified when this request is completed successfully or terminated by error. The pipe has no interaction with the USB other than the presence of an IRP request. In the case of an error after three retry attempts, the IRP is cancelled and all outstanding and further IRPs to that pipe are ignored till the software responds to the error signal which is produced by trasmitting a suitable call to the USB. In addition, data transmission in the bus occurs in a serial form. Bytes of data, with the least significant bit first, are broken up and sent along the bus one bit at a time. As illustrated by Figure 3.8, it indicates the serial transmission of the binary number 11010010.

Figure 3.8: Serial transmission of the binary number 11010010

(Geoff Knagge, 2010) The actual data is sent across the bus in packets. Each packet is a bundle of data along with information concerning the destination, length and source of the data as well as error detection information. During configuration, since each endpoint sets a limit to the size of the packet it can handle, an IRP may ask several packets to be sent. Except for the final packet, each of these packets should be the maximum possible size. The USB host has a built in mechanism so that the software can tell it when to expect full sized packets.