Smart Phone Compatible Thermal Conductivity

Carbon Dioxide and Humidity Sensors

              

                        

A smart phone sensor has built-in more and more sensors to gather all kinds of data on who we are, what we are doing, and the world around us. Any sensors used in smart phones should be miniaturization, low power consumption, fast response, high accuracy, high reliability, and low cost. Micromachined thermal conductivity sensors provided by POSIFA can meet all these requirements. It has been used for hydrogen, vacuum, and methane measurement, and exhibited very high performance.

The device is fabricated with CMOS compatible micromachining processes. The sensor structure consists of a hot plate suspending over a cavity. A platinum resistor is located on the hot plate, which acts as both a heater and a temperature sensor. The sensor chip is installed in a metal casing using a high thermal conductivity epoxy adhesive. A metal mesh disposed on the metal casing provides gas exchange with the surrounding atmosphere. A photo graph of the encapsulated sensor can be seen in the above figure.

A thermal conductivity sensor measurement circuit is shown in the following figure. The circuit consists of two thermal conductivity sensors: a measurement sensor and a reference sensor, in which the measurement sensor is used for measuring the mixture gas containing air, carbon dioxide, and water vapor (humidity), and the reference sensor is sealed and used for canceling the signal generated by the pure air and the temperature of the surrounding atmosphere. The heaters of the sensors are heated by two DAC respectively, which are provided by a precision analog microcontroller such as ADuC7019/20 or the like. The differential voltage of the two sensor output is amplified by a chopper stabilized zero-drift operational amplifier, such as TP5554-TR. When zeroing, the measurement sensor is immersed in a reference air, the output of the two sensors will be the same and the output of the amplifier will be zero. When the measurement sensor is immersed in the mixture air containing carbon dioxide and water vapor, the output of the amplifier will correlate to the concentrations of the carbon dioxide and water vapor.

     

The working principle of the thermal conductivity sensor is easy to understand. It senses change of the thermal conductivity of a mixture air consisting of air, carbon dioxide, and water vapor, and compares it to the thermal conductivity of reference pure air. Since both carbon dioxide and water vapor have a thermal conductivity less than the thermal conductivity of a pure air, the thermal conductivity of the mixture air will be low containing carbon dioxide and a detectable signal will be produced.

According to Wassiljewa equation and empirical measurements, at fixed temperature t₁, a linear equation can be used for calibration and calculation of the concentration of each composition gas of a mixture air, which can shown as the follows:

y(t₁) = a(t₁)_air+a(t₁)_carN_car+a(t₁)_waterN_water.           (1)

In this equation (1), y is the output of the sensor response; a(t₁)_air is a bias term, N_car and N_water are the carbon dioxide and water vapor concentrations, and a(t₁)_car and a(t₁)_water are the coefficients sensitive to carbon dioxide and water vapor concentrations. The output is expressed in the unit of volt, and so are the coefficients, a(t₁)_car and a(t₁)_water, while the concentrations N_car and N_water are expressed as molar fractions. The equation (1) has two degrees of freedom, so N_car + N_water + N_air = 1, where N_air is the air concentration.

The coefficients a(t₁)_air, a(t₁)_car, a(t₁)_water can be obtained by measurement of several precision calibration sample mixtures. The data collected by the sensors can be processed using partial least squares (PLS). It has been shown that PLS can produce proper coefficient.

It should be noticed that N_car and N_water are two independent variables. It is impossible to find two variables by solving a single linear equation. Another similar linear equation (2) at fixed temperature t₂ needs to be built as follows:

y(t₂) = a(t₂)_air+a(t₂)_carN_car+a(t₂)_waterN_water.        (2)

In this equation the coefficients a(t₂)_air, a(t₂)_car, a(t₂)_water are different from the above similar coefficients, while N_car and N_water are the same. Based on the measured data collected by the thermal conductivity sensors, both N_car and N_water can be calculated by solving the equation (1) and equation (2).