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.