Understanding the operation of SST’s Zirconia Oxygen Sensors can sometimes be difficult. SST has screeds of documentation that explains the background physics and the oxygen sensor function that would require a lot of time to read and comprehend what they mean. This article summarizes all of that information and structures in an easy and understandable manner.
Oxygen Sensor Function
Sensor Cell Construction
The sensing cell is at the core of SST’s zirconia oxygen sensors (Figure 1). The cell comprises of two zirconium dioxide (ZrO2) squares coated with a thin porous layer of platinum which function as electrodes and provide the required catalytic effect for the oxygen to dissociate, enabling the oxygen ions to be transported in and out of the ZrO2.
The two ZrO2 squares are separated by a platinum ring which forms a hermetically sealed sensing chamber. There are two additional platinum rings at the outer surfaces which along with center platinum ring provide the electrical connections to the cell.
A pair of outer alumina (Al2O3) discs filters and prevents any ambient particulate matter from entering the sensor and also removes any unburnt gases. This prevents contamination of the cell which may lead to unstable measurement readings. Shown in Figure 2 is a cross-section of the sensing cell with all the key components highlighted.
The cell assembly is surrounded by a heater coil which generates the necessary 700 °C required for operation. The heater and cell are then positioned inside a porous stainless steel cap in order to filter larger particles and dust and also to protect the sensor from mechanical damage. The complete sensor assembly is shown in Figure 3.
The first ZrO2 square functions as an electrochemical oxygen pump, which evacuates or re-pressurizes the hermetically sealed chamber. Based on the direction of the DC constant current source, the oxygen ions travel through the plate from one electrode to the other; this in turn alters the oxygen concentration and thus the oxygen pressure (P2) within the chamber. The pumping is controlled in such a way that the pressure inside the chamber is always below the ambient oxygen pressure outside the chamber. The electrical connections to the cell are shown in Figure 4.
A Nernst voltage, which is logarithmically proportional to the ratio of the oxygen ion concentrations, is generated by a difference in oxygen pressure across the second ZrO2 square. Due to the oxygen pressure inside the chamber (P1), the voltage at sense with respect to common is always positive.
This voltage is measured and compared with two reference voltages and each time one of these two references is reached, the direction of the constant current source is reversed. The higher the ppO2, the longer it takes to reach the pump reversal voltages than it does in a low ppO2 atmosphere. The reason for this is a greater number of oxygen ions have to be pumped to create the same ratiometric pressure difference across the sensing disc.
P1, which is the O2 pressure that has to be measured, is 10 mbar and the set reference voltage is achieved when P2 is 5 mbar. On changing P1 to 1 bar, P2 would have to be 0.5 bar in order to attain the same reference voltage. This would include evacuation of a greater number of oxygen ions and since the current source used to pump the ions is constant, would hence take much longer.
This information has been sourced, reviewed and adapted from materials provided by SST Sensing Ltd.
For more information on this source, please visit SST Sensing Ltd.