Pro-Longing the Life of Your Zirconia Oxygen Sensor

Table of Contents

Introduction
Fail Safe Operation and Sensor Asymmetry
Operating in Aggressive Humid Environments
Protecting from Excessive Moisture
Using the Sensor with Silicone

Introduction

This article provides helpful hints on how to extend the service life of SST Sensing Ltd’s zirconia oxygen sensors. The application wherein the zirconia oxygen sensor being used affects the life of the sensor. The following hints and tips need to be followed to prevent the premature failure of the sensor.

Fail Safe Operation and Sensor Asymmetry

One of the key benefits of using the dynamic and active cell within the oxygen sensor is that it is fail safe by nature. The continual cycling and measuring of the generated Nernst voltage is effectively the sensor’s heartbeat and if this heartbeat stops, then something fatal has happened within the cell. The interface electronics can detect this event very quickly.

Operating in Aggressive Humid Environments

When the zirconia oxygen sensors are operated in warm, humid environments, the sensor has to be kept at a higher temperature compared to its surroundings, in particular when the measurement gas contains corrosive components. This is not an issue during operation as the heater operates at a temperature of 700 °C. However, this means when the application or senor is being powered down, the sensor heater should be the last thing to be switched off after the surrounding temperature has suitably been cooled. Preferably, the sensor should be left powered on or at a lower standby voltage (typically 2 V) at all times in very humid environments.

Failure to follow these rules will cause condensation forming on the sensing element and heater. When the sensor is powered on again, the condensation will evaporate, forming corrosive salts which rapidly deteriorate the sensing element and heater as shown in the image given below. As can be noted from the image, the external metalwork of the sensor looks completely normal.

Protecting from Excessive Moisture

In environments likely to contain falling water droplets or excessive moisture, it is necessary to protect the sensor from water entering or falling directly onto the very hot sensor cap due to the possibility of causing massive temperature shocks to the heater and cell. Popular methods include having a hood over the sensor cap or mounting the sensor in a larger diameter cylinder.

At a very minimum the sensor cap should be angled downwards in the application so that falling moisture will get deflected and the sensor cap is prevented from filling with water.

Using the Sensor with Silicone

The presence of silicone in the measurement gas causes damage to zirconia oxygen sensors. Vapors (organic silicone compounds) of RTV sealants and rubbers are the major culprits and are commonly used in many applications. Cheaper silicones are often used to make such materials, which release silicone vapors into the surrounding environment upon heating. When these vapors make contact with the sensor, the sensor’s organic part will be burned at hot sensor parts, forming a very finely divided silicone dioxide (SiO2). This SiO2 blocks the active parts and pores on the electrodes completely. It is recommended to use high quality, well cured materials if RTV rubbers are used.

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.

Tell Us What You Think

Do you have a review, update or anything you would like to add to this article?

Leave your feedback
Submit