O2 SENSORS – Zirconium Dioxide (ZrO2) Oxygen Sensor Operating Principle Guide This document describes the physics and concepts behind SST Sensing’s range of dynamic and highly accurate oxygen sensors. When reading this document, keep in mind the following key differentiators between SST’s range of sensors and other zirconium dioxide oxygen sensors: SST’s sensors measure partial pressure of oxygen in a gas or mixture of gases, NOT oxygen concentration %. SST’s sensors do NOT require a reference gas. Contents 1 INTRODUCTION ...................................................................................................................... 1-1 2 BACKGROUND PHYSICS ........................................................................................................... 2-1 2.1 Partial pressure ............................................................................................................... 2-1 2.2 Zirconium Dioxide (ZrO2) ................................................................................................. 2-4 2.3 Nernst Voltage ................................................................................................................ 2-4 3 SENSOR FUNCTION ................................................................................................................. 3-1 3.1 Sensor Cell Construction ................................................................................................. 3-1 3.2 Pumping Plate ................................................................................................................. 3-2 3.3 Sensing Plate ................................................................................................................... 3-2 4 MEASUREMENT ...................................................................................................................... 4-1 4.1 Recommended Values for Use in Normal Atmospheric Pressures .................................... 4-3 4.2 Initial Sensor Drift and Active Burn-In .............................................................................. 4-4 4.3 Calibration Processes – Converting td to ppO2 and O2% ................................................... 4-5 APPENDIX A – WATER VAPOUR PRESSURE LOOKUP TABLE ............................................................. A-1 APPENDIX B – SPECIAL NOTES AND APPLICATION HINTS ................................................................. B-1 APPENDIX C – DESIGNING INTERFACE ELECTRONICS ....................................................................... C-1 1 INTRODUCTION The zirconium dioxide oxygen sensor does NOT measure oxygen concentration %, but rather it measures partial pressure of oxygen in a gas or mixture of gases. The sensor employs a well proven, small zirconium dioxide based element at its heart and due to its innovative design does NOT require a reference gas. This removes limitations in the environments in which the sensor can be operated with high temperatures, humidity and oxygen pressures all possible. SST Sensing’s range of oxygen sensors are therefore ideal for use in the following applications: Laboratory measurements Combustion control of systems using natural gas, oil, biomass, etc. Automotive emissions testing Oxygen generation in medical and aerospace markets Aerospace fuel tank inerting applications Agricultural applications including composting and cultivation Bakery ovens and heat treatment furnaces Key to understanding the fundamentals of the sensor operation is the physics that govern it. For information on the correct use and implementation which is key to getting the most from the sensor in a wide range of applications, refer to AN-0050, Zirconia O2 Sensor Operation and Compatibility Guide. Page | 1-1 2 BACKGROUND PHYSICS 2.1 Partial pressure 2.1.1 Definition The partial pressure is defined as the pressure of a single gas component in a mixture of gases. It corresponds to the total pressure which the single gas component would exert if it alone occupied the whole volume. 2.1.2 Dalton’s law The total pressure (Ptotal) of a mixture of ideal gases is equal to the sum of the partial pressures (Pi) of the individual gases in that mixture. 푘 푃푡표푡푎푙 = ∑푖=1 푃푖 (1) From Equation 1 it can be derived that the ratio of the number of particles (ni) of an individual gas component to the total number of particles (ntotal) of the gas mixture equals the ratio of the partial pressure (Pi) of the individual gas component to the total pressure (Ptotal) of the gas mixture. 푛푖 푃푖 = (2) 푛푡표푡푎푙 푃푡표푡푎푙 ni Number of particles in gas i ntotal Total number of particles pi Partial pressure of gas i Ptotal Total pressure Figure 2-1 Ptotal = P1 + P2 + P3 (Constant Volume & Temperature) P a g e | 2-1 Example 1: The atmospheric pressure at sea level (under standard atmospheric conditions) is 1013.25mbar. Here, the main components of dry air are nitrogen (78.08% Vol.), oxygen (20.95% Vol.), argon (0.93% Vol.) and carbon dioxide (0.040% Vol.). The volumetric content (%) can be equated to the number of particles (n) since the above gases can be approximated as ideal gases. Equation 2 can be solved for the partial pressure of an individual gas (i) to get: 푛푖 푃푖 = ×푃푡표푡푎푙 (3) 푛푡표푡푎푙 The oxygen partial pressure then equates to: 20.95% 푃 = ×1013.25푚푏푎푟 = 212.28푚푏푎푟 푖 100% Figure 2-2 Partial Pressure at 0% Humidity Of course, this value is only relevant when the atmosphere is dry (0% humidity). If moisture is present a proportion of the total pressure is taken up by water vapour pressure. Therefore, the partial oxygen pressure (ppO2) can be calculated more accurately when relative humidity and ambient temperature are measured along with the total barometric pressure. Figure 2-3 Liquid Vapour Pressure Firstly, water vapour pressure is calculated: 퐻푅푒푙 푊푉푃 = ( ) ×푊푉푃 (4) 100 푚푎푥 WVP Water Vapour Pressure (mbar) HRel Relative Humidity (%) WVPmax Maximum Water Vapour Pressure (mbar) Page | 2-2 For a known ambient temperature, maximum water vapour pressure (WVPmax) can be determined from the lookup table in APPENDIX A. The maximum water vapour pressure is also referred to as the dew point. Warmer air can hold more water vapour and so has a higher WVPmax. Partial oxygen pressure then equates to: 20.95 푝푝푂 = (퐵푃 − 푊푉푃)×( ) (5) 2 100 ppO2 Partial Pressure O2 (mbar) BP Barometric Pressure (mbar) WVP Water Vapour Pressure (mbar) Example 2 below describes the effect of humidity reducing the partial oxygen pressure and therefore the volumetric content of oxygen. Example 2: On a typical day, the following information is recorded from a calibrated weather station: Temperature: 22°C Humidity: 32% Barometric Pressure: 986mbar Using the lookup table in APPENDIX A, WVPMAX = 26.43mbar. 32 푊푉푃 = ( ) ×26.43 = 8.458푚푏푎푟 100 Partial oxygen pressure then equates to: 20.95 푝푝푂 = (986 − 8.458)× ( ) = 204.795푚푏푎푟 2 100 As we now know the oxygen partial pressure and the total barometric pressure we can work out the volumetric content of oxygen. 204.8 푂 % = ( ) ×100 = 20.77% 2 986 Page | 2-3 2.2 Zirconium Dioxide (ZrO2) At high temperatures (> 650°C), stabilised zirconium dioxide (ZrO2) exhibits two mechanisms: 1. ZrO2 partly dissociates producing mobile oxygen ions and therefore becomes a solid electrolyte for oxygen. A ZrO2 disc coated with porous electrodes connected to a constant DC current source allows ambient oxygen ions to be transported through the material. This liberates an amount of oxygen at the anode proportional to the charge transported (electrochemical pumping) which according to Faraday’s First Law of Electrolysis is: 푖푡 푁 = (6) 푧퐹 N Number of Moles of Oxygen Transported i Constant Current t Time (seconds) z Ionic Valence of Oxygen a F Faraday Constant = 96487 C/mol 2. ZrO2 behaves like an electrolyte. If two different oxygen pressures exist on either side of a piece of ZrO2, a voltage (Nernst voltage) is generated across it. 2.3 Nernst Voltage Two different ion concentrations on either side of an electrolyte generate an electrical potential known as the Nernst Voltage. This voltage is proportional to the natural logarithm of the ratio of the two different ion concentrations. Electrolysis is: 푘퐵푇 푐1 ∆푉 = × ln ( ) (7) 푒0 푐2 -23 kB Boltzmann constant (kB = 1.38x10 J/K) T Temperature in K -19 e0 Elementary charge (e0 = 1.602x10 C) ci Ion concentration in mol/kg Either of these properties are used in many variants of oxygen sensors, however SST’s oxygen sensors employ both principals simultaneously. This removes the need for a sealed reference gas making the sensor more versatile for use in a range different oxygen pressures. a Where C is units of charge in coulombs, and mol is mole, a unit of substance. Page | 2-4 3 SENSOR FUNCTION 3.1 Sensor Cell Construction At the core of the oxygen sensor is the sensing cell (Figure 3-1). The cell consists of two zirconium dioxide (ZrO2) squares coated with a thin porous layer of platinum which serve as electrodes. The platinum electrodes provide the necessary catalytic effect for the oxygen to dissociate, allowing the oxygen ions to be transported in and out of the ZrO2. Figure 3-1 The Sensing Cell The two ZrO2 squares are separated by a platinum ring which forms a hermetically sealed sensing chamber. At the outer surfaces, there are two further platinum rings which along with centre platinum ring provide the electrical connections to the cell. Two outer alumina (Al2O3) discs filter and prevent any ambient particulate matter from entering the sensor and also remove any unburnt gases. This prevents contamination of the cell which