Portable Gas Safety Sensor

Final Report

Project:

sddec3-11

Client:

Senior Design

Advisors:

Dr. Ratnesh Kumar

Dr. Nicola Elia

Team Members:

Miao Qin

Hwa-Sung Ryu

Pek-Yee Teoh

Sven Soell

November 18, 2003

Table of Content

List of Figures

List of Tables

Definition of Terms

Executive Summary

Background

Need for the Project

The Actual Project

Project Results

Acknowledgements

1. Introduction

1.1 General Background

1.1.1General Problem Statement

1.1.2General Solution

1.2Operating Environment

1.3Intended User(s) and Intended Use(s)

1.3.1 Intended Users

1.3.2Intended Uses

1.4.Assumptions and Limitations

1.4.1Assumptions

1.4.2Limitations

1.5Expected End Product and Other Deliverables

2. Project Approach and Results

2.1End-Product Functional Requirements

2.2Resultant Design Constraint

2.3 Approaches Considered and One Used

2.3.1Technology considerations

2.3.2 Selected Approach or Technology

2.3.3Reason(s) for the Particular Selection

2.4Detailed Design

2.5Implementation Process Description

2.6 End-Product Testing Description

2.7 Project End Results

3. Resources and Schedules

3.1Resource Requirements

3.2 Schedules

3.2.1Project Schedules

3.2.2Deliverable Schedules

4 Closure Materials

4.1 Project Evaluation

4.2Commercialization

4.3 Recommendations for Additional Work

4.4 Lesson Learned

4.5Risk and Risk Management

4.6 Project Team Information

4.7 Closing Summary

4.8References

4.9Appendices

List of Figures

Figure 2.3.2.1 Model of Potential Barrier

Figure 2.4.1 Carbon Monoxide Gas Sensor Circuit

Figure 2.4.2 Micro controller and LCD Circuit

Figure 3.2.1: Gantt Chart

Figure 3.2.2: Gantt Chart (Deliverable)

List of Tables

Table 2.4.1: Parts needed

Table 3.1: Personal Effort Budget

Table 3.2: Other required resources

Table 3.3: Estimated project costs

Table 3.4: Personal Effort Budget

Table 3.5: Actual project costs

Table 4.1 Milestones and their success ratings as of Nov 18, 2003

Definition of Terms

  • CO - Carbon monoxide
  • LCD - Liquid crystal display
  • PCB- Printed circuit board
  • PIC - Peripheral integrated controller
  • PPM - Parts per million

1

Executive Summary

Background

The portable gas safety sensor can be a useful, valuable, and affordable tool to every household. When an environment is too dangerous or unstable due to the leakage of toxic or combustible gases from home appliances, the portable gas safety sensor will detect this leakage and therefore provide a safer environment.

Combustion gases can contain toxic elements that are often hard to detect with human senses. As a result this project shall implement a domestic gas detector which will detect and monitor gases such as carbon monoxide and natural gas and warn the user when a certain gas concentration is exceeded.

The gas safety sensor can be used in different workplaces such as garage, kitchen, and small business shops due to its portability.

Need for the Project

The objective of this project is to produce and test a gas safety sensor. The gas safety sensor has to be small, portable, battery-powered and economical. The portable gas sensor will detect gases such as carbon monoxide and natural gas. When certain gas concentration is over the safety level, the device will warn the user of hazardous situations due to dangerous gases. The device also has to be self-contained, wearable and should not interfere with the wearer’s work activities.

The Actual Project

A significant amount of time was spent to define the design of the project. Different options available were weighed against the available time to implement the project. One of the options was to use different kinds of gas sensors. At first a multi-purpose toxic gas detector was considered, but later changed to a single gas type sensor. The reason for this was that if the target gas is unknown the user does not know which gas the detector is reacting to. Furthermore, the single gas type sensor provides more accuracy especially in the lower ppm range. This gas concentration will be displayed on the LCD screen that is controlled by a microcontroller. The concentration level is displayed in the units parts per million. When a certain ppm level is exceeded, the microcontroller will send a signal and activate an audible and visual alarm. The programming language C was used to program the microcontroller.

Project Results

At first the entire project was implemented using the programming language PIC-Basic and a multi-purpose toxic gas detector. It was then tested with a commercial gas sensor. However, it was found the multi-purpose toxic gas sensor is not suitable as it can react to the target gas as well as other gases. This may alter the sensor’s readings. Therefore, a single gas type sensor was considered next. To implement this new sensor, alterations in the circuit were necessary as well as a change in programming language. To date the new circuit has been implemented and the PIC has been partially programmed. However, not all of the components of the circuit are working properly and therefore, need more work.

Acknowledgements

Special thanks go to the faculty advisors for this project, Professor Ratnesh Kumar and Professor Nicola Elia. They have provided valuable advice and insight, and played an active role in enabling the development of this project to its maximum potential. Thanks are also due to Professor John W. Lamont and Professor Ralph E. Patterson.

1. Introduction

1.1 General Background

In most households, combustion gases are produced by appliances such as gas-fired furnace, boiler, or water heater or other fuel-burning devices. These combustion products give out visible smoke, and invisible gases that should be vented to the outdoors. Unfortunately, these gases may instead escape into your home, where it could raise a variety of health and other concerns.

Toxic elements can be presented in combustion gases, sharing your home with these gases can lead to problems ranging from nuisance headaches to serious illness like carbon monoxide poisoning. Thus, the purpose of this project is to design and implementation of a gas detector for combustion spillage in residential domains.

1.1.1General Problem Statement

The design and implementation of the portable gas detector is to detect gas carbon monoxide. The gas concentration of the target gas is displayed on a LCD in the unit ppm and an alarm will sound if a certain concentration is exceeded.

The detector will consist of three parts:

1. Sensory for detection of the target gas

2. Data acquisition via a micro controller

3. Display of target gas concentrations

1.1.2General Solution

The implemented sensors are a type of thick film metal oxide semiconductor, which offer low cost, long life, and good sensitivity to target gases while utilizing a simple electrical circuit. The sensor will indicate the concentration of the gas via a unique relationship between the concentration of the gas and the internal sensor resistance. This signal will be processed using an analog to digital converter and a micro controller. It then shall be displayed on a display. The concentration of the target gas will be displayed in parts per million. To alert the user if a certain concentration is exceeded, an alarm will sound. The alarm will be triggered by means of a comparator.

1.2Operating Environment

The gas detector will mainly be used in residential domains to detect the concentration of poisonous gas. Thus, it will not have to withstand any extreme environmental conditions. In order to protect the detector from corrosion and unwanted humidity, which may occur in kitchen or basement, it should be water-resistant. As being a portable device, it should be shock resistant.

1.3Intended User(s) and Intended Use(s)

Since the gas detector will mainly be used in residual domains, a great variety of user(s) and use(s) arise which are described below.

1.3.1 Intended Users

The intended users are those people who live in the houses using appliances such as gas-fired furnace, boiler, or water heater or other fuel-burning devices. They can be any age and background since the gas detector is easy to operate and easy to use. However, the user should know what to do in the case of combustion leakage of poisonous gas.

1.3.2Intended Uses

The gas detector has to be used domestically to detect and monitor carbon monoxide and natural gas. It will display the concentration of the gas in parts per million with up to three significant digits. Since it is a portable device it can be carried around and can be monitored different areas in the house.

Since the detector is targeted for a domestic use only, it must not be used in extreme conditions. Also, it is not suited for large area monitoring as this would require multiple sensors positioned at different places.

1.4.Assumptions and Limitations

Assumptions and limitations are crucial for a successful end product and have been identified as outlined below.

1.4.1Assumptions

  • Need to be positioned according to the nature of the target gas.
  • For raising gases, it should position above the danger point
  • For falling gases, it should position below the danger point.
  • The temperature does not exceed 40C° and dip below -5C°
  • The concentration of CO does not exceed 1000 ppm
  • The concentration of methane does not exceed 10,000 ppm
  • Environmental condition should be dry.
  • All parts are compatible.
  • Accuracy up to 3 significant numbers.
  • No high concentration of other gases is present
  • The gas detector will only display one gas at a time and the unit will be in ppm

1.4.2Limitations

  • The cost can not exceed one hundred dollars.
  • Can not exposure to silicone vapors
  • Exposure to high density corrosive materials will cause corrosion or breakage of the materials.
  • Not water proof.
  • Long term exposure in adverse environment.
  • Excessive vibration will resonate and break the gas sensor.

1.5Expected End Product and Other Deliverables

  • The end product has to be a portable device to detect and to monitor CO and displays the target gas concentration on a LCD display.
  • Batteries for independent supply and an adapter for an alternating current supply.
  • Operating manual.

2. Project Approach and Results

2.1End-Product Functional Requirements

The end products functional requirements should be:

  • The portable gas detector is to detect carbon monoxide
  • The gas detector has to be portable and has a LCD display, which shows the concentration of the gas in ppm
  • An on/off switch will be provided
  • The detector can work on batteries or power supply
  • The detector is set to take the measurement in a certain period of time since the concentration of the gas leaking is changing instantaneously
  • If certain gas concentration is exceeded, an alarm will sound to notify the user

2.2Resultant Design Constraint

This subsection defines the constraints that were considered during the design and implementation.

  • Cost – Care must be taken to limit the cost of the project components as well as the complexity of the design, both to meet the budget of the team.
  • Environment – The gas detector is to be used in a domestic environment only. Therefore, it will not have to operate in extreme conditions. The typical operating range will be at room temperature but can be as low as 0˚C and as high as 40˚C.
  • Size – The end-product must be small enough to be portable. Therefore it will be in a range of a pocket calculator.
  • Simplicity – The end-product must be designed in a way so that it easy to operate.

2.3 Approaches Considered and One Used

This section talks about different approaches which were considered before and during the implementation process.

2.3.1Technology considerations

For the completion of the project, different kinds of sensors were looked at. This includes infra-red sensors, electrochemical sensors, catalytic sensors and semiconductor sensors. For the implementation a semiconductor sensor was used as it offers low cost, long life, and good sensitivity to the target gas. Infra-red sensors and catalytic gas sensors are better suited for detecting combustible gases but less desirable for the detection of toxic gases.

For the display of the gas concentration different kinds of displays were considered. These included LCDs and LEDs. While LEDs are cheaper and easier to implement, LCDs offer better functionality and user-friendliness.

For the alarm audible as well as visual solutions were considered. These include a speaker for the audible alarm and diode for the visual alarm. To be thoroughly both approaches were implemented.

PIC-Basic and C were looked at and compared for their advantages and drawbacks.

2.3.2 Selected Approach or Technology

As mentioned above semiconductor sensors were used to detect the target gas. These sensors are especially suitable for the detection of CO. The sensing material in these gas sensors is metal oxide. When a metal oxide crystal such as SnO2 is heated at a certain high temperature in air, oxygen is absorbed on the crystal surface with a negative charge. Then, donor electrons in the crystal surface are transferred to the absorbed oxygen, resulting in leaving positive charges in a space charge layer. Thus, surface potential is formed to serve as a potential barrier against electron flow. Inside the sensor, electric current flows through the conjunction parts of SnO2 micro crystals.

At grain boundaries, adsorbed oxygen forms a potential barrier which prevents carriers from moving freely. The electrical resistance of the sensor is attributed to this potential barrier. In the presence of a deoxidizing gas, the surface density of the negatively charged oxygen decreases, so the barrier height in the grain boundary is reduced. The reduced barrier height decreases sensor resistance. The relationship between sensor resistance and the concentration of deoxidizing gas can be expressed by the following equation over a certain range of gas concentration:

Rs = A[C] -a

where:

Rs = electrical resistance of the sensor A = constant

[C] = gas concentration a = slope of Rs curve

Figure 2.3.2.1 Model of Potential Barrier

A single type gas sensor was used as it features a more precise reading of the parts per million when compared to a multi-purpose gas sensor.

For the programming language C was chosen.

For the visualization a LCD display was chosen.

2.3.3Reason(s) for the Particular Selection

The quality of the gas detector heavily depends on the sensor which is to detect the target gas. To obtain best results a single type gas sensor was used. It offers good resolution in the lower ppm range it is not influenced by other gases than the target gas.

For the programming language C was preferred over PIC basic as it offers more functionality.

The selected LCD display offers more user-friendliness and functionality than LEDs. Also, a LCD can be used to communicate with the user and provide information or directions.

2.4Detailed Design

To implement the gas sensor, the circuit shown in Figure 2.4.1 below was used.

Figure 2.4.1 Carbon Monoxide Gas Sensor Circuit

The sensor requires two voltage inputs: heater voltage (VH) and circuit voltage (VC). The heater voltage (VH) is applied to the integrated heater in order to maintain the sensing element at a specific temperature which is optimal for sensing. Circuit voltage (VC) is applied to allow measurement of voltage (VRL) across a load resistor (RL) which is connected in series with the sensor.

A common power supply circuit will be used for both VC and VH to fulfill the sensor's electrical requirements. The value of the load resistor (RL) is chosen to optimize the alarm threshold value, keeping power dissipation of the semiconductor below a limit of 15mW. Power dissipation will be highest when the value of Rs is equal to RL on exposure to gas.

For the A/D conversion and signal processing a micro controller was used. The PIC16F84 series is suitable since they are cheap and reliable. They also provide LCD functions to control and operate a display.

Figure 2.4.2 below shows the microcontroller with the LCD connections.

The signal from the potentiometer (gas sensor) is fed into the micro controller, which uses an A/D converter to digitalize the signal. The converted signal will then be fed into the display.

Figure 2.4.2 Micro controller and LCD Circuit

The programming code for the PIC micro controller is shown in the appendix. This is, however, not the complete code as it is still in the development.

To be complete all circuit parts are summarized in the table below.

Table 2.4.1: Parts needed

Part Name / Description/Value
PIC 16F84 / Microchip Micro Controller
TGS 844 / Toxic Gas Sensor
Hitatchi 44780 / Display
R1, R2 / Resistors 4.7KOhm, 47KOhm
TLC548 / A/D converter
74HC164 / Shift Register
Socket / 6-pin-socket
C1 / Capacitor 0.1uF
X1 / 4Mhz crystal

2.5Implementation Process Description

After all the parts had been ordered and had arrived the basic circuit was assembled with the multi-purpose gas sensor. Then the PIC micro controller was programmed using Pic-Basic and the circuit was tested for functionality. A commercial gas safety sensor was obtained form the Ames Safety Labs to further test and/or calibrate the senior design safety sensor. It was found that the senior design implementation was not working properly. It was learned that the multi-purpose gas sensor would react to many other gases than the target gas and therefore would not give accurate readings compared to the commercial product which uses a singe gas sensor. It was then decided to exchange the multi-purpose sensor for a single type gas sensor. As a result the implementation circuit had to be changed as this sensor required a more complex operation. It was learned that the programming language Pic-Basic was not suitable anymore to program the micro controller and therefore C was adopted instead. To date the new circuit has been implemented and is working correctly. However, the programming of the PIC has not been completed and therefore a testing of the sensor with the commercial safety sensor has not been done either.

2.6 End-Product Testing Description

Once the PIC has been programmed completely the portable gas detector can be tested using the commercial safety sensor from the Ames Safety Labs. As it has been done before with the multi-purpose gas sensor both detectors will be sealed in a box and the target gas will be introduced into the container. If the portable gas detector works correctly, its reading will correspond to the readings of the commercial product. If the readings are off we can use the commercial gas detector to further calibrate the senior design gas detector.