The experiment study of the sensing mechanism of thermal magnetic type oxygen sensor

Abstract

The oxygen sensor is widely used in daily life. Thermal magnetic type oxygen sensor has some advantages such as long service life and high accuracy of measurement. It skillfully uses the change of the wall temperature that caused by the thermal magnetic convection phenomenon to sense the change of oxygen concentration. For a paramagnetic gas, the thermal megnetic convection is caused by the interaction between the temperature field and the magnetic field as the gradient of the magnet field and the temperature gradient both existing at the same time. The intensity of thermal megnetic convection is determined by the magnetic susceptibility of a gas and the gradient of H 2 (where H is the induced magnet field). Oxygen is a common paramagnetic gas, the magnetic susceptibility of oxygen has a much higher than other gas, thus, the oxygen concentration of a gas can be detected by the thermal magnetic convection phenomenon. However, the thermal magnetic convection phenomenon is complicated, The sensing mechanism of this type sensor is not clear. Therefore, it is necessary to study the sensing mechanism in-depth to improve the accuracy and efficencey of thermal magnetic type oxygen sensor. This paper, by means of experiments, studies the wall temperature change characteristics of the thin-wall circular tube in a non-uniform magnetic field generated by two pieces of rectangular permanent magnets. In the experiment, a quartz glass tube with a thin wall thickness was used as the channel through which the measured gas flows. Two permanent magnets are arranged outside the quartz tube, which are used to provide the magnetic field required for the thermal magnetic convection. A kind of the sensitive element with double helically wounded resistance wires is designed. In which two resistance wires with different temperature coefficient are helically wounded on the wall of outside the quartz tube, one is the Manganin resistance wire for heating, and one is the platinum wire for measuring the temperature of tube wall. The loops of the heating and the temperature measurement are separated, which is favorable for study of the thermal magnetic type oxygen sensor. The permanent magnets used in the experiment have the remanence of 1.32 T. The ambient temperature around the sensitive element is measured by the thermocouple. The experimental device, which includes the sensitive element and some appropriately insulating elements, is placed in a rectangular cavity, and the size of the square cavity is much larger than the sensitive element. By comparing temperature difference of the tube wall, the increment of the convective heat transfer coefficient and the increment of velocity under conditions with and without the magnetic field to illustrate the relationship of the thermal magnetic convection phenomenon and heating power, oxygen concentration and other parameters. It is found that when magnetic field is pesent, the surface temperature of the tube wall is significantly lower than that of the case without magnetic field, and with the increase of heat flux, the surface temperature of tube wall decreases significantly. The relationship between the heat flux density and the change of the wall temperature is nearly liner.With the increase of heat flux density, the increasments of average velocity and convective heat transfer coefficient caused by the thermal magnetic convection phenomenon increase. When oxygen content increases by 10%, the temperature of the tube wall decreases by 0.12°C. The resolution of the studied oxygen concentration senser is about 0.08%.