Thermal Flow Sensors and Thermal Conductivity Sensors for Fuel Cell Applications

Xiangzheng Tu

A fuel cell generates electricity by the chemical reaction of hydrogen and oxygen. It is different from a battery in that it requires a continuous source of hydrogen and oxygen to sustain the chemical reaction, whereas in a buttery the chemicals to be reacted for generating electricity are stored in the buttery. The fuel cell can produce electricity continuously for as long as these inputs are supplied.

Fuel cell cars have been on the road for many years. The first commercial fuel cell cars are being sold in California by Toyota. Fuel cells are being developed and tested in buses, boats, motorcycles and bicycles, among other kinds of vehicles. Fuel cells have many more applications including portable generators, battery replacements, residential back power, and so on.

As shown in the above picture, the heart of the fuel cell is the polymer electrolyte membrane. It is sandwiched by the anode layer and the cathode layer. At the anode layer, the input hydrogen is converted into the hydrogen ions and the electrons. The hydrogen ions are transferred to the cathode through the conductor and the electrons are transferred through the outer circuit. At the cathode the transferred hydrogen ions together with the transferred electrons and the oxygen of the input air react to produce the water vapor. The water vapor is carried away by the unused input air.

It can be seen that in order to regulate the chemical reaction of the fuel cell the amount of the reacted hydrogen, the reacted oxygen and the produced water vapor should be measured in real-time. The fuel cell converts chemical energy into electrical energy and also, as a by-product of this process, into heat energy. This makes capacitive and resistive humidity sensors no useful for measuring the water vapor. Fortunately, POSIFA Microsystems provide thermal conductivity humidity sensors with greater resolution at higher temperature. It measures the absolute humidity by quantifying the difference in thermal conductivity of the input air and the output air containing water vapor. The accuracy of the sensors can reach at least +3 g/m3; this converts to about ±5% RH at 40°C and ±0.5% RH at 100°C. It is worth to note that the unused hydrogen also contains water vapor, which moves through the polymer electrolyte membrane from cathode to anode by diffusion. Similarly, the absolute humidity in the unused hydrogen can be measure by quantifying the difference in thermal conductivity of the input hydrogen and the unused hydrogen containing water vapor.

The thermal flow sensors provided by POSIFA Microsystems have been used for fuel cells successfully. The sensor consists of a heater and two symmetric thermopiles positioned on a porous silicon membrane. The heater is made of polysilicon, whereas the thermopiles are made from a combination of polysilicon and aluminum. The sensor is operated by a constant power circuit which provides constant power to the heater during the measurement. Power consumption and response time of the sensor are 30mW and 5ms, respectively. The noise of the sensor is very low and has the same amount of the noise as the equivalent resistor of the thermopile.

Flow rate and humidity are critical parameters associated with the performance of fuel cells. The product of the chemical reaction that proceeds in a fuel cell is water, which lowers the working efficiency of the cell and affects the fuel flow rate. With POSIFA thermal conductivity sensors any water vapor flooding in the fuel cell can be diagnosed easily and correctly. When the chemical reaction is not balanced well a microcontroller can modify the hydrogen flow rate and air flow rate automatically, which is based on the data collected by the thermal flow sensors. It should be no doubt that POSIFA flow sensors and POSIFA thermal conductivity sensors is a powerful combine for promoting fuel cell applications in fuel cars, fuel boats, fuel smart phones, etc.

Wireless Drinking Water Flow Sensor Systems

A wireless drinking water flow sensor system has been developed by POSIFA Microsystems, as shown in the above picture. This system can be installed in the lid of a water bottle and measures the flow rate and the water volume taken by a user.

The working principle of the system is very simple. Suppose that a bottle is filled with water, it is at a pressure as atmosphere. When you suck water from the bottle, the pressure is lower. Once the pressure in the bottle is lower than atmosphere, air is forced into the bottle. Then the pressure in the bottle is back to atmosphere. Such cycle can be carried out until the water in the bottle has gone completely. Based on this principle, the system can be used to measure the water flow rate and the water volume from a water bottle taken by users.

In combine with any drinking water application software such as iDrated, Waterlogged, Daily Water, Eight Glass a day, and OasisPlaces, this system can work with your smart phone for tracking your water intake easier than ever. With such system a drinking water schedule can be designed to help you reach drinking water goals and control the volume of water you are drinking on a daily basis. It also allows for schedule reminders and offers a basic statistical analysis of the amount of what you have consumed over the course of a day.

The wireless drinking water flow sensor system includes an integrated thermal flow sensor, a preamplifier, a microcontroller, a RF transmitter, a RF receiver, and a lithium battery. The system performs flow measurement, A/D conversion, digital processing, and scaling. The flow single measured by the system is transmitted to your smart phone via the RF transmitter. The incorporated drinking water application software processes the flow signal received by the smart phone and converts it into video Information displaying on the screen of your smart phone.

An outstanding advantage of the system is that the measured water taken by a user can be precisely expressed in liters/second or in liters instead of in glasses. Glass is not a standard volume unit. A drinking glass actually could be range from 8-16 ounces. People may drink a part of water in a glass which is difficult to count in glasses correctly. If drinking water application software is equipped with the system all these confusions can be eliminated forever.

As well known, water is our body's principal chemical component and makes up about 60 percent of our body weight. Every cell in our body depends on water. Water flushes toxins out of vital organs, carries nutrients to our cells, and provides a moist environment for ear, nose and throat tissues. Lack of water can lead to dehydration, a condition that occurs when we don't have enough water in our body to carry out normal functions. Even mild dehydration can drain our energy and make us feel tired.

Drinking water is absolutely essential. How do you make drinking water part of your daily routine? No worries, the wireless drinking water flow sensor system has been created for you. It can help you to find drinking enough water according your drinking water schedule. You will get endless benefits. Among them are: helping with weight loss; natural wrinkle-buster; stopping headaches and dizziness; clearing your skin; fighting your infections; keeping you regular, making you exercise better; improving your concentration; boosting your energy; supporting your heart; and so on.

Wireless Spirometers Based on Integrated Thermal Flow Sensors

Reference to the above figure, in combination with a smart phone a spirometer is used to generate a flow-volume loop for a man who is required to measure his forced vital capacity.

As shown in the graph on the screen of the smart phone, a normal flow-volume loop begins on the x-axis (volume axis): at the start of the test both flow and volume are equal to zero. After the starting point the curve rapidly mounts to a peak: peak (expiratory) flow.

After the forced expiratory volume in one second (PEF) the curve descends as more air is expired. A normal, non-pathological F/V loop will descend in a straight or a convex line from top of PEF to bottom of forced vital capacity (FVC). The forced inspiration that follows the forced expiration has roughly the same morphology, but the peak inspiratory flow (PIF) is not as distinct as PEF.

The flow-volume loop can take on many distinguishable shapes that correspond to a certain type of pathology. Using the flow-volume loop displayed by the smart phone, the following pathologies can be easily and correctly diagnosed.

• normal spirometry,
• pathological spirometry,
• obstructive lung disease,
• restrictive lung disease,
• mixed lung disease,
• obstruction of the large airways,
• exercise induced asthma.

The spirometer in the above figure utilizes an integrated thermal flow sensor provided by POSIFA Microsystems, which is a famous MEMS company located in Silicon Valley in the United States. The integrated thermal flow sensors exhibit short response time (1ms), low power consumption (30mw), and high accuracy (0.5%). It represents an attractive solution for portable spirometers in home-care applications.

The spirometer further consists of a CC2650 ultra-low power wireless MCU, which can provide a wireless connectivity solution supporting multiple standards to enable faster internet designs. With this device the spirometer can be managed being small, inexpensive and up to years of battery life.

The smart phone is not only to visualize a flow-volume loop based on the data send by the spirometer, but also con­nect the spirometer to the cloud. The cloud is a great place to back things up, because the spirometer has lim­ited stor­age and unre­li­able flash mem­ory. Plus, the spirometer could get lost, dam­aged, or stolen. The cloud is great for data pro­cess­ing because pro­cess­ing in the cloud reduces the load on the micro­proces­sor. This saves power and might reduce the per­for­mance require­ments of the micro­proces­sor. Furthermore, data that is already in the cloud can be shared quickly and eas­ily by many users.

The flow-volume loop is a remarkably versatile and informative measurement, which can identify a range of diseases including chronic obstructive pulmonary disease (COPD). COLD is a type of obstructive lung disease characterized by chronically poor airflow. It typically worsens over time. The main symptoms include shortness of breath, cough, and sputum production. Most people with chronic bronchitis have COPD. Used together with other clinical features, the flow-volume loop can substantially improve assessment of the patient and their long-term management.

Worldwide, COPD affects 329 million people or nearly 5% of the population.^[6] In 2013, it resulted in 2.9 million deaths up from 2.4 million deaths in 1990. The number of deaths is projected to increase due to higher smoking rates and an aging population in many countries.^[8] It resulted in an estimated economic cost of $2.1 trillion in 2010.

Walking Distance Measurement Systems

POSIFA Microsystems developed walking distance measurement systems as shown above. These systems can measure the distance for many walking ways to take including:

• Walk with your spouse, child, or friend
• Walk the dog
• Use the stairs instead of the elevator
• Park farther from the store
• Better yet, walk to the store
• Get up to change the channel
• Window shop
• Plan a walking meeting
• Walk over to visit a neighbor
• Walk around the garden or do a little weeding 

There are countless physical activities out there, but walking is a good form of exercise for many people. Knowing walking distance will help people develop an exercise program that's effective for losing weight, increasing aerobic fitness, and improving heart health.

Many efforts have been made to measure the distance traveled by humans for a long time. There are several ways to measure the distance moved by walking or running in daily life, some of which already use commercial products, but none of them is ideal and perfect.

As well know, one way is based on radio frequency, such as GPS, or may use optical-based systems, such as video tracking. Such systems are not available indoors and degrade significantly in deep city canyons.

As an alternative, the measurement can be made by self-contained tracking systems. Such systems use commercially available MEMS inertial sensors and magnetic sensors. However, the performance of MEMS accelerometers and angular rate sensors is limited by random noise and calibration error. When they are used in a position tracking systems, it becomes the source of unbounded growth in position error.

A new walking distance measurement system is developed by POSUFA Microsystems, which is based on their MEMS thermal flow sensors. Human walking exhibits variability from step to step. As above listed walking ways, all most each step may be different in walking direction or in step length. It is impossible to do correct and precise measurement using available distance measurement systems. Only the POSIFA systems can do that because it measures the distance simply by adding all step length together. 

Each human walking step consists of stance phase, swing phase, and stance phase again. In any stance phases the foot traveling velocity is zero. In the swing phase the foot traveling velocity increases from zero to maximum and then decreases from maximum to zero like a pendulum curve. The thermal flow sensor can be managed to measure any instantaneous velocity of a particular foot traveling step. Then the instantaneous velocity is integrated over the traveling time of the particular step to produce the length of the particular step. All the lengths of each traveling steps can be accumulated to provide the distance of this human walking event.

In such systems, the thermal flow sensor only measures the instantaneous velocity.  The measurement signals are sent to a smart phone using Wi-Fi communication. The Smart phones are useful in this application because they are portable and have sufficient computational power to compute and analyze the measurement signals to provide the walking distance and other information people are interested. 

Thermal Flow Sensors for Man-Potable Vehicles Applications

Thermal Flow Sensors for Man-Potable Vehicles Applications

Man-portable air vehicles (UAVs) or micro air vehicles (MAVs) can be used in

many dangerous civil and military missions without any risk to human life, and they also have many potential industrial applications such as plant supervision, power line and construction site inspection, pollution and weather monitoring, forest fire and disaster control, etc.

All these vehicles operate exclusively within the atmospheric boundary layer,

which typically extends up to 5 km above ground level depending on surface heating, climatic conditions, and terrain. The flow in the atmospheric boundary layer is dominated by horizontal transport of atmospheric properties. As air travels over buildings and various obstacles, there will be a local increase in wind speed. It has been shown that mean wind speeds above a certain threshold can make the air speed vary substantially. When a steady 4.6 m/s magnitude wind is presented it is inability of an MAV to reach its pre-programmed waypoints. This degradation in performance puts the aircraft at risk and often leaves the MAV unable to complete its mission.

In order to improve the performance of the air vehicles a powerful onboard

sensing system is required. This sensing system can detect individual air speeds in a turbulent boundary layer. Once detected, the turbulent influence can be suppressed or eliminated by an action of micro electromechanical devices.

As shown in figure 1, POSIFA thermal flow sensors are ideal for the onboard

sensing system. These thermal flow sensors have the benefits of miniature size, low power dissipation and fast response time, which is essential for detecting turbulent flow. The miniature size allows for their installation in small spaces without significantly interfering with the flow and consequently altering its behaviour.                   

These thermal flow sensors can be used for detecting the surface velocity

distribution over critical locations of the wing area, such as the points of flow transition, reversal, or separation.

POSIFA thermal flow sensors can be used to determine three flight parameters: air

speed, angle of attack and angle of sideslip. As shown in figure 2, so as to do so an array of thermal flow sensors is installed over critical locations of the wing area, such as the points of flow transition, reversal, or separation. The signal of each thermal flow sensor is send to a microcontroller through wireless communication. The microcontroller will act the micro electromechanical device for suppressing or eliminating the turbulent influence.

 Figure 1. An assembled printed circuit board with a POSIFA thermal flow sensor and a lot of diced POSIFA thermal flow sensor chips surrounding the PCB.

These thermal flow sensors can be used for detecting the surface velocity

distribution over critical locations of the wing area, such as the points of flow transition, reversal, or separation.

POSIFA thermal flow sensors can be used to determine three flight parameters: air

speed, angle of attack and angle of sideslip. As shown in figure 2, in order to do so an array of thermal flow sensors is installed over critical locations of the wing area, such as the points of flow transition, reversal, or separation. The signal of each thermal flow sensor is send to a microcontroller through wireless communication. The microcontroller will act the micro electromechanical device for suppressing or eliminating the turbulent influence.

Figure 2. An Unmanned air vehicle equipped with a POSIFA thermal flow sensor array controlled by a microcontroller.

MEMS Pirani Sensors for Plasma Sterilization Applications

Low pressure plasma sterilization has been developed as secure sterilization

methods and has already been put to practical use for medical equipment. Although the mechanisms are not yet fully understood, the following cellular inactivation mechanism has been reported: ultraviolet radiation in the far-ultraviolet and mid-ultraviolet range is capable of damaging nucleic acids (DNA, RNA) and proteins.

In order to produce such ultraviolet radiation the pressure of the gas mixture used

as plasma source should be well controlled in the range of 13 to 130 Pa.

We have developed Pirani sensors which can be used to control the pressure of

the gas mixture. The amplified response of a Pirani sensor to pressure variation is shown in the following figure. It can be seen from the figure that the controlled pressure locates the sensitive range of the Pirani sensor.

Medical Liquid Thermal Flow Sensors

Recently, our thermal flow sensors have been developed

for medical liquid applications. These new applications require low noise, high reproducibility, short response time, battery powered, potable and wearable, and low-cost disposable. Our sensors meet all these requirements because they are fabricated using a CMOS comparable technology and are based on a structure integrating a heater and a thermopile in a single chip.

   The following figure shows a typical characteristic line of 1% NaCl in DI water (stroke-physiological saline solution) flow measured by a liquid thermal flow sensor.