Image Sensing Using Micromachined Ultrasonic Sensor Arrays

Tu Xiang Zheng 

Ultrasonic sensors convert ultrasound waves to electrical signals or vice versa. They detect a wide range of materials, are not influenced by problematic surfaces, and are largely immune to environmental influences. They have many uses in medicine as well as in other various advanced technologies including electronics, chemicals and construction. As well known, an ultrasonic sensor applied to the abdomen of a pregnant woman sends ultrasonic waves into the body and receives the echoes back from the inside, which are used for making visual images. These real time images showing the appearance and movement of the fetus allow observation of the development of the fetus.

Silicon based capacitive ultrasonic sensor arrays bring revolutionary improvement in performance and represent a major advance in ultrasonic sensor technology.

These sensors benefit from the economies of scale found in semiconductor manufacturing and are well suited for high-volume applications that demand high-performance sensors at low costs. A similar sensor array can be found in the US Patent 6,359,276 B1, which is used for sensing infrared image.

As shown in the above figure, each sensor of the array comprises two electrodes facing each other, one of which is fixed and the other is movable. The two electrodes are separated by an insulating layer and an air gap. It can operate on transmit and receive mode, by converting electrical energy into acoustic energy or vice versa through the displacement of the movable electrode.

When a voltage is applied between both electrodes and the membrane is pulled down to the bottom electrode by electrostatic forces. The membrane moves until the electrostatic force has equilibrium with internal force of the membrane. AC signals cause vibrations of the thin diaphragm and generate ultrasonic waves. Furthermore, the receiver can detect an ultrasonic wave using the change of capacitance when displacement of the membrane is caused by the pressure of an arriving ultrasonic wave.

In according to this patent, the sensor array is disposed in a silicon substrate in which there already exists a CMOS circuit with readout electronics. Each sensor includes a silicon nitride membrane bridging a cavity recessed into the substrate. The membrane has four beams. The distal ends of the beams are anchored to the substrate, so that the membrane is supported by the substrate and the surface of the beams is aligned with the plane surface of the substrate. The surface of the membrane and beams is coated with silicon dioxide film. A metal layer is disposed on the surface of the silicon dioxide film. The end portions of the metal layer are disposed on the beams and keep in contact with the proximal end portions of electrical conductors which already exist on the surface of the substrate. The cavity is a narrow gap, so that the membrane can touch the bottom of the cavity without damage as it is forced to bend downward. The trenches between the membrane and the beams and between the beams and the edges of the substrate are also narrow, so that the membrane and beams can touch the edge of the substrate, as they are forced to bend in the lateral directions. 

It was reported by Andrew et al. that MEMS technology makes it possible to produce air and gas ultrasonic sensors that can operate at higher frequencies (200 kHz to 5 MHz). The ability to make microscopic structures with MEMS technology permits the fabrication of very small sensors that emit high-frequency ultrasound. The smaller the sensor implies the higher the frequency of the ultrasonic signal. This was realized by Lemmerhirt et al. They developed a CMOS based ultrasonic 32 X 32 sensor array. The array was built with CMOS process for 3D image acquisition. Each sensor has 100 µm diameter membrane with 60 µm diameter top electrode and 0.6 µm gap. The center frequency of each element is 1.8 MHz.

Wireless Meter of Methane Number and Mass Flow of Natural Gas

Tu Xiang Zheng

  

Natural gas is used in an amazing number of ways. Although it is widely seen as a cooking and heating fuel in most households, natural gas has many other energy and raw material uses that are a surprise to most people who learn about them. In 2012 about 30% of the energy consumed across the United States was obtained from natural gas. It was used to generate electricity, heat buildings, fuel vehicles, heat water, bake foods, power industrial furnaces, and even run air conditioners.

Natural gas is a gaseous mixture chemically composed by methane, smaller fractions of higher molecular weight hydrocarbons and inert gases (mainly N₂ and CO₂). The different components ratio in the gas mixture determines its physical and chemical properties and consequently, its quality. Concretely, composition fluctuations affect to properties such as methane number.

The methane number is the parameter used to quantify the quality of the natural gas.  A 100 methane composition is given 100 methane number and as the higher hydrocarbons and inert gases percentage increases the methane number decreases. It is assigned that a 100 methane composition is used as the knock resistant reference fuel. Every natural gas engine has a higher than a 65 methane number to prevent engine knocking.

For methane number measurement many different sensor techniques are available in a variety of classes. Sensor principles include electrical techniques, like electrochemical detection, or electrical detection of adsorption by induced capacitance changes, optical techniques; for instance infrared (IR) adsorption or Raman spectroscopy, chromatography, calorimetry and acoustic analyses. What most sensor techniques have in common is that their applicability into real time monitoring systems is limited, either because sensors are hard to integrate based on practical considerations like, size, cost or response time, or because the sensors rely on principles that generally do not apply to all gasses.

The present paper proposes a new method to determine on line the methane number of natural gas. The method is based on the measurement of the gas density and the correlation between the density and the methane number of natural gas. The gas density can be measured with a thermopile flow sensor combining a differential pressure sensor. These two sensors are installed in an orifice plate. When natural gas flows through the orifice plate the mass flow rate and the pressure drop of the gas flow can be measured simultaneously. Then the density of the natural gas can be calculated based on the Bernoulli equation which states that there is a relationship between the pressure drop and velocity of the natural gas flow.

The correlation between the specific gravity and the methane number of natural gases is shown as the following table. The table gives methane number: 48.1, 66.2, 76.4, 80.8, 91.4 and 100 and the volume percent of their corresponding compositions. Using the specific gravity of each composition the specific gravity of each methane number can be calculated which is also given in the table. The specific gravy of each composition of the natural gas is shown in another table. It can be seen that the methane number increases and the specific gravity of the different methane number gases decreases. It is not surprising because the specific gravity of methane is lower than all other composition of the natural gas. 

A proposed wireless natural gas meter is shown in the above figure. The meter can measure both the methane number and wirelessly send the data to a smart phone for the user to monitor the consumption and the quality of the natural gas precisely. In order to do so a key component of the meter is a thermopile flow sensor developed based on a mix of integrated circuit manufacturing and micro-machining process. Some of the advantages of the thermopile flow sensors can be listed as

• Direct mass flow sensing;
• Large dynamic range;
• Fast response;
• Excellent low flow sensitivity;
• Low power consumption;
• Small size, mass, volume;
• low cost; and
• Easy to integrate in gas or fluid transport networks.

Thermopile Flow Sensors with Differential pressure sensors for Measurements of Mass Flow Rate, Density and Void Fraction of Gas-Liquid Two-Phase Flow Fluids

Tu Xiang Zheng

Gas-liquid two-phase flow exists broadly in chemical, petroleum and metallurgical industries. The measurements of gas-liquid two-phase flow parameters in real time without separating the phases is desirable in order to reduce costs, increase production and reach excellence in oil and gas transport. Although many measurement techniques have been developed, it is yet difficult to measure some flow parameters because of the complexity of the two-phase flow. It is necessary to explore new measurement techniques.

This paper proposes a measurement technique of gas-liquid two-phase flow parameters which has the advantages of low cost, simple structure and non-intrusiveness. This technique is based on the combined use of a thermopile flow sensor and a differential pressure sensor. The setup is shown in the above figure which consists of a venture, a bypass tube, a thermopile flow sensor, and a differential pressure sensor. The thermopile flow sensor is installed at the central point of the bypass flow tube and the differential pressure sensor is used to measure the pressure difference between the inlet and the outlet pressure of the bypass tube. The mass flow rate measured by the thermopile flow sensor can be used to derive the main mass flow rate passing through the venture according to a ratio dependent on the setup structure. Knowing the mass flow rate and pressure difference the density of the gas-liquid two-phase fluid can be obtained. The void fraction of the gas-liquid two-phase fluid can be further calculated using know pure liquid density and pure gas density.  

As can be seen from the above figure, there are two flow loops: the venture loop and the bypass loop. In accordance with the nature of these two parallel loops, it is reasonable to have their pressure drop to be equal.

According to the homogeneous model of two-phase flow two phases travel at equal velocities and mix well; therefore, they can be treated as if there is only one phase.

Using Bernoulli's equation in the special case of incompressible flows (such as the flow of water or other liquid, or low speed flow of gas), the theoretical mass flow rate through the venture can be given by:

m_v = C_vA₂{2(p_up-p_down)/ρ[1-(A₂/A₁)²]}^1/2   (1)

where m_v is the mass flow rate of the fluid, C_v is the discharge coefficient = (actual flow rate) / (theoretical flow rate), A₁ and A₂ is the cross-sectional area of the venture in the indicated section of the venture, p_up, p_down are the fluid's static pressure in the indicated section of the venture, and ρ₁ is the fluid's density passing through the venture.

Similarly, the theoretical mass flow rate through the bypass tube can be given by:

m_b = C_bA₃[2(p_up-p_down)/ρ₂]^1/2                       (2)

where m_b is the mass flow rate of the fluid, C_b is the discharge, A₃ is the cross-sectional area of the bypass tube, p_up, p_down are the fluid's static pressure in the indicated section of the bypass tube, and ρ₂ is the fluid's density passing through the bypass tube.

In arriving at the homogeneous model for two-phase flow, area averaging is performed for both phases and the density ρ₁ and ρ₂ must be equal a similar ρ. Forever the void fraction α of the two-phase fluid must satisfied the relation:

  ρ = (1-α)ρ_l + αρ_v                                           (3)

where ρ_l and ρ_v are the densities of the pure liquid and the pure gas, respectively.

It is clear that a thermopile flow sensor with a differential pressure sensor can be used to measure the mass flow rate, density and void fraction of a gas-liquid two-phase flow. A proposed setup comprises a thermopile, a differential pressure sensor, a venture tube and a bypass tube. Using the venture equation and the homogeneous model form the mass flow rate measured by the thermopile flow sensor and the pressure difference measured by the differential pressure sensor the density and the void fraction of the two-phase fluid can be derived respectively. This technique requires the gas and the liquid mixes so well that the mixture can be seen as one single phase approximately. The mixing degree of the gas and the liquid affects the measuring accuracy. For un-complete mixing two-phase fluid correction is needed to improve the accuracy.

Thermopile Flow Sensors and Differential Pressure Flow Meters

Xiang Zheng Tu

 

Air conditioning can refer to any form of technology that modifies the condition of air including heating, cooling, (de-)humidification, cleaning, and ventilation. In order to do so air movement needs to be created and air flow needs to be measured. This is hwy differential pressure flow meters are popular for a long time. But in recent years things have changed. Differential pressure flow meters have been gradually and irreversibility replaced by thermopile flow sensors.

It is not surprising in view on the working mechanism of the differential pressure meters and related limitations to the flow measurements. The working mechanism is based on Bermoulli’s Equation. Bernoulli’s equation states that the pressure drop across the constriction is proportional to the square of the flow rate, as shown in the following figure.

 

It can be seen from the above figure that using this relationship, 10 percent of full scale flow produces only 1 percent of the full scale differential pressure. At 10 percent of full scale flow, the differential pressure flow meter accuracy is dependent upon the meters being accurate over a 100:1 range of differential pressure. The meters accuracy is typically degraded at low differential pressures in its range, so flow meter accuracy can be similarly degraded. Therefore, this non-linear relationship can have a detrimental effect on the accuracy and turndown of differential pressure flow meters. Remember that our interest is the accuracy of the flow measurements instead of the differential pressure measurements.

In addition, the flow rate measured by the differential pressure flow meters is not mass flow rate that is required by many applications. According to ideal gas law, gas pressure changes with its temperature and volume. To obtain a mass flow rate, it is necessary to measure additional parameters: differential pressure; absolute pressure; and absolute temperature. These measurements with the differential pressure measurement then sent to a computer for calculating the mass flow rate.

All these limitations with the differential pressure flow meters can be eliminated by the thermopile mass flow sensors. The thermopile flow sensors use the thermal properties of the fluid to measure the flow rate. A measured amount of heat is applied to the heater of the sensor. Some of this heat is lost to the flowing fluid. As flow rate increases, more heat is lost. The amount of heat lost is measured using the thermopile(s) in the sensor. The output of the thermopile(s) represents the fluid flow velocity or flow rate.

The thermopile flow sensors are fabricated using micromachining techniques in a CMOS production line. They offer many advantages over the differential pressure meters, including but not limited to:

• Large dynamic range
• High accuracy
• Excellent low flow sensitivity
• Direct mass flow sensing
• Low pressure drop
• Very low power consumption
• Miniaturization and small device footprint
• Manufactured in CMOS production line and low cost

The thermopile flow sensors are not only used for air flow measurement in air conditioning, but also for monitoring flow in clean room, in fan/filter units, controlling flow in production facilities in the pharmaceutical, food processing and semiconductor industries; and monitoring flow in glove boxes, insulators, medical equipment such as anaesthetic machines and respirators in order to maximize energy efficiency, and also increase the accuracy of gas flow control.

Low Detectable Range Thermopile Flow Sensors

Xiang Zheng Tu

Thermal flow sensors are inevitably influenced by natural convection. An operating thermal flow sensor is a hot source surrounding by air. The air receives heat from the sensor, becomes less dense and rises. The cooler air then moves to replace it. This cooler air is then heated and the process continues, forming the convection current around the sensor. The temperature of the sensor will changes due to a part heat of the sensor is carried away by the convection current. Therefore the temperature change will add to the signal of sensor as a part of offset of the sensor.

But for thermopile flow sensors the natural convection can be reduced as low as being ignore. The installation of the thermopile flow sensors in a tube is commonly required to have their hot surface facing down to the Earth’s surface. In this case the heat transferred from the sensor chip by natural convection is the lowest. This is not surprising, since the hot air is “trapped” under the sensor chip and can not move away from the sensor chip easily. As a result, the cooler air in the vicinity of the sensor chip will have difficulty reaching the sensor chip, which results in a reduced rate of heat transfer.

Even the hot surface of the sensor intersects the Earth’s surface at a specific angle, the formed natural convection current is small and the influence still can be ignore. The reason can be explained as follows. Natural convection is characterized by Grashof number G_r which expresses the ratio between buoyancy forces due to spatial variation in air density to viscous forces acting on air. It is given as:

Gr = gβ(T_sensor - T_air )L³ /  ע² ,                   (1)

where g is the acceleration due to gravity; β is the volumetric thermal expansion coefficient; T_s and T_air are temperature of the heater of the sensor and the surrounding air, respectively; L is the characteristic length and the ע is the kinematic viscosity of the surrounding air. With g = 9.8 m/s², β = 3.25x10^-3 /k,   ע = 1.65 x 10^-5, T_s - T_air = 21.9 ^oC, and assuming the characteristic dimension L = 1x10^-3 m, we have

Gr = gβ(T_s - T_air )L³ /  ע² = 2.56    (2)

As well know, the ratio Gr/Re₂ defines the importance of natural convection in respect to a forced convection. The R_e is Rayleigh Number which represents forced convection and can be expressed as R_e = vL/ ע. The thermopile flow sensors provided by POSIFA Microsystems have been widely used for air mass flow meters. The characteristic length of the house tube of the air mass flow meter is 6mm. If assuming the velocity of air flow is 0.1mm/s, the Re is calculated to be 3.64 and the ratio Gr/Re₂ to be 0.19. This is lower than the lower bound of Gr/Re2 =0.3, which means that compare with the forced convection the contribution of the natural convection to the heat transfer can be negligible. (E. M. Sparrow, R. Eichhorn, and J. L. Gregg, “Combined forced and free convection in a boundary layer flow,” Phys. Fluids, vol. 2, no. 3, pp. 319–328, 1959.)

In addition to natural convection, the noises of the thermopile flow sensors are also limiting the detectable flow velocity. The noises of the thermopile flow sensors are basically the temperature noise and the thermal noise. The temperature noise is caused by temperature fluctuations in the surrounding atmosphere. This noise can be cancelled because the thermopile flow sensors are responsive to the temperature difference between a hot place and a cold place instead of a temperature. The thermal noise or the Johnson noise is an electrical noise source caused by random motion of electrical charges in the material. It is determined by the following equation:

V_noise = (4k_bT_eR∆f)^1/2               (3)

where k_B is the Boltzmann’s constant; T_e is the absolute temperature in kelvin ; R is the electrical serial resistance and ∆f is the frequency bandwidth.

Thermal noise calculation can be carried out online. The following results are given by http://www.sengpielaudio.com/calculator-noise.htm.

In the calculation T = 45.9 ⁰c representing the operating temperature of the thermopile flow sensor, R = 150 kΩ representing the serial resistance of a thermopile of the sensor. The bandwidth is calculated using equation ∆f = f_cutoff – 20 Hz, where f_cutoff is determined by measuring the response time of the sensor, as shown in the following figure.

 

It can be obtained from Fig.1 that the response time of the sensor is 0.74ms corresponding cutoff frequency 1351 Hz. With these parameters the thermal noise of the sensor is 1.88µV.

In conclusion, the thermopile flow sensors provided by POSIFA Microsystems can detect as low as 0.1mm/s velocity for air mass flow meter applications. This is resulted by the inherence low Grashof number of the sensors, which is down to 2.56. The temperature noise of the sensor can be canceled since the sensor is responsive to the temperature difference instead of temperature. The thermal noise or the Johnson noise is calculated to be 1.88µV, which is very low and is suitable for medical applications.  Furthermore the cutoff frequency of the sensors is up to 1331Hz, which is required by several medical instruments such as spirometers.

Low Detectable Range Thermopile Flow Sensors

 Xiang Zheng Tu

Thermal flow sensors are inevitably influenced by natural convection. An operating thermal flow sensor is a hot source surrounding by air. The air receives heat from the sensor, becomes less dense and rises. The cooler air then moves to replace it. This cooler air is then heated and the process continues, forming the convection current around the sensor. The temperature of the sensor will changes due to a part heat of the sensor is carried away by the convection current. Therefore the temperature change will add to the signal of sensor as a part of offset of the sensor.

But for thermopile flow sensors the natural convection can be reduced as low as being ignore. The installation of the thermopile flow sensors in a tube is commonly required to have their hot surface facing down to the Earth’s surface. In this case the heat transferred from the sensor chip by natural convection is the lowest. This is not surprising, since the hot air is “trapped” under the sensor chip and can not move away from the sensor chip easily. As a result, the cooler air in the vicinity of the sensor chip will have difficulty reaching the sensor chip, which results in a reduced rate of heat transfer.

Even the hot surface of the sensor intersects the Earth’s surface at a specific angle, the formed natural convection current is small and the influence still can be ignore. The reason can be explained as follows. Natural convection is characterized by Grashof number G_r which expresses the ratio between buoyancy forces due to spatial variation in air density to viscous forces acting on air. It is given as:

Gr = gβ(T_sensor - T_air )L³ /  ע² ,     (1)

where g is the acceleration due to gravity; β is the volumetric thermal expansion coefficient; T_s and T_air are temperature of the heater of the sensor and the surrounding air, respectively; L is the characteristic length and the ע is the kinematic viscosity of the surrounding air. With g = 9.8 m/s², β = 3.25x10^-3 /k,   ע = 1.65 x 10^-5, T_s - T_air = 21.9 ^oC, and assuming the characteristic dimension L = 1x10^-3 m, we have

Gr = gβ(T_s - T_air )L³ /  ע² = 2.56     (2)

As well know, the ratio Gr/Re₂ defines the importance of natural convection in respect to a forced convection. The R_e is Rayleigh Number which represents forced convection and can be expressed as R_e = vL/ ע. The thermopile flow sensors provided by POSIFA Microsystems have been widely used for air mass flow meters. The characteristic length of the house tube of the air mass flow meter is 6mm. If assuming the velocity of air flow is 0.1mm/s, the Re is calculated to be 3.64 and the ratio Gr/Re₂ to be 0.19. This is lower than the lower bound of Gr/Re2 =0.3, which means that compare with the forced convection the contribution of the natural convection to the heat transfer can be negligible. (E. M. Sparrow, R. Eichhorn, and J. L. Gregg, “Combined forced and free convection in a boundary layer flow,” Phys. Fluids, vol. 2, no. 3, pp. 319–328, 1959.)

In addition to natural convection, the noises of the thermopile flow sensors are also limiting the detectable flow velocity. The noises of the thermopile flow sensors are basically the temperature noise and the thermal noise. The temperature noise is caused by temperature fluctuations in the surrounding atmosphere. This noise can be cancelled because the thermopile flow sensors are responsive to the temperature difference between a hot place and a cold place instead of a temperature. The thermal noise or the Johnson noise is an electrical noise source caused by random motion of electrical charges in the material. It is determined by the following equation:

V_noise = (4k_bT_eR∆f)^1/2       (3)

where k_B is the Boltzmann’s constant; T_e is the absolute temperature in kelvin ; R is the electrical serial resistance and ∆f is the frequency bandwidth.

Thermal noise calculation can be carried out online. The following results are given by http://www.sengpielaudio.com/calculator-noise.htm.

  

In the calculation T = 45.9 ⁰c representing the operating temperature of the thermopile flow sensor, R = 150 kΩ representing the serial resistance of a thermopile of the sensor. The bandwidth is calculated using equation ∆f = f_cutoff – 20 Hz, where f_cutoff is determined by measuring the response time of the sensor, as shown in the following figure.

It can be obtained from Fig.1 that the response time of the sensor is 0.74ms corresponding cutoff frequency 1351 Hz. With these parameters the thermal noise of the sensor is 1.88µV.

In conclusion, the thermopile flow sensors provided by POSIFA Microsystems can detect as low as 0.1mm/s velocity for air mass flow meter applications. This is resulted by the inherence low Grashof number of the sensors, which is down to 2.56. The temperature noise of the sensor can be canceled since the sensor is responsive to the temperature difference instead of temperature. The thermal noise or the Johnson noise is calculated to be 1.88µV, which is very low and is suitable for medical applications.  Furthermore the cutoff frequency of the sensors is up to 1331Hz, which is required by several medical instruments such as spirometers.

Thermopile Flow Sensors Operating at as low as 45.9⁰C

Tu Xiang Zheng

There are two types of popular micromachined thermal flow sensors: resistive flow sensors and thermopile flow sensors. Both the thermal flow sensors work by heat convection transfer away from a heated resistor. As the resistor cool, the corresponding change in voltage or current can be calculated to fluid flow. The major difference between the resistive flow sensors and the thermopile flow sensors is the heat sensing element. Their heat sensing elements are unheated resistors and thermopiles, respectively.

An infrared camera is used to take the temperature image of a thermopile flow sensor. To do this, a DC voltage is applied to the resistor of the sensor. A typical temperature image is shown in Fig.1.The brightest (warmest) parts of the image are customarily colored white, intermediate temperatures reds and yellows, and the dimmest (coolest) parts black.

As can be seen, the highest temperature is 45.9⁰c at the central region of the sensor chip and the lowest temperature is 24.4⁰c at the surrounding region of the sensor chip. It can be seen from this image that the thermopile flow sensor can be operated at 45.9⁰c. This operating temperature is much lower than the operating temperature of any resistive flow sensors, which is usually over 100⁰c.

The circuit module of the thermopile flow sensor is also shown in Fig.1. The module includes a thermopile flow sensor, a microcontroller, a regulator, and an amplifier. The regulator may input a buttery voltage and output a regulated voltage to the microcontroller. The microcontroller may create a pulse width modulation (PWM) voltage to the heated resistor of the thermopile flow sensor. The thermopile of the thermopile flow sensor may provide a static output voltage to the amplifier. The microcontroller may process the static output voltage for adjusting the PWM voltage so as to set an original offset of the amplifier to be as close to zero as possible.

In order to operate the thermopile flow sensor at 45.9⁰c, the following settings should be made. The buttery voltage is 5V and the regulated voltage is 3V. The resistance of the resistive heater is 240Ω. The PWM applied to the resistive heater is 2.54V. These settings result in a heating power of 26.9mW, an operating temperature of 45.9⁰c, and a 20mV static output of a thermopile. The 20mV static output is the original offset of the thermopile, which may drift over time. It is necessary to be able to maintain the original offset by adjusting the PWM output accordingly. For this reason, the PWM is set a duty cycle of 60% yielding 1.8V and 90% yielding 2.7V so that a duty cycle is 84.7% can yield 2.54V PWM output.

The low temperature operating thermopile flow sensors can provide several advantages over high temperature operating resistive flow sensors. An outstanding advantage is that oil droplets can be avoided to form around the sensor chips when they use for automobile air mass flow meters. The reason for this can be explained using Fig. 2 that is a graph showing the relationship between partial pressure and temperature of gasoline vapor in a gaseous mixture of air. 

Reference to Fig.2, P_ms represents the saturated vapor pressure curve and P_m represents the un-saturated vapor pressure curve. The yellow star with 24.4⁰c indicates the temperature and un-saturated vapor pressure of the gaseous mixture and the green star with 45.9⁰c represents the temperature and saturated vapor pressure of the gaseous mixture in the temperature boundary layer over the heated sensor chip. As can be seen, when the gaseous mixture in the temperature boundary layer over enters its surrounding space it still keeps un-saturated. But if the sensor chip is heated up to higher than 60⁰c as indicated by the red star, the gaseous mixture entered the surrounding space will become saturated and condense to be gasoline droplets.

It should be noted that a laminar flow is supposed to form over the heated sensor chip with a temperature boundary layer built up thereon. Since the gaseous mixture of air in the temperature boundary layer is heated, its volume increases and its partial vapor pressure decreases correspondingly. This will result in a partial vapor pressure difference between the temperature boundary layer and the surrounding space which drives vapors diffuse from the surrounding space to the temperature layer until reach the balance between these two spaces. This is why the vapor pressure in the temperature boundary layer on the heated sensor chip is indicated by the green star vapor pressure instead of the corresponding yellow star vapor pressure.