Sample gas which contains oxygen enters the electrochemical sensor through diffusion via the barrier membrane. The rate of diffusion depends on both the temperature and pressure of the sample gas, this article will focus on the particular effects of temperature on the sensor. Decreases or increases in temperature alter the rate of diffusion through the membrane which results in changes in the response time and life span, as detailed below.
The diffusion is faster when the temperature of the sensor increases, speeding up the response to changes in levels of oxygen. Conversely, the colder the sensor gets , the slower the rate of diffusion, meaning cold sensors cells respond much more slowly than warm sensors.
The sensor life span is also affected by temperature. The warmer the sensor is, the quicker the electrolyte is consumed, this is because more O2 crosses the diffusion barrier to react with the electrolyte. The life expectancy for electrochemical sensors will decrease by roughly 2.5% for every degree over 25 °C. The inverse of this is also true: the sensor will last longer if the temperature is consistently below 25°C
A GPR-12-333 sensor will last for 24 months in <1,000 ppm O2 at 25 °C . If the sensor is operated at 20 °C it will last around 27 months, or at 30 °C it will last approximately 21 months.
The signal output grows around 2-3% per degree Celsius rise in temperature. To mitigate this effect, Analytical Industries’ analyzers are fitted with a negative temperature coefficient resistor (NTC) and is at the same temperature as the sensor cell. This is important for this resistor to be effective.
Sample Gas with changing temperature
Run the sample gas through a piece of 1/8 ” stainless steel coiled tube (0.5 – 1 m long) before the analyzer if the gas temperature is likely to change, this will stabilize the sample gas temperature and bring it up or down to ambient.
The temperature of the sensor should be kept above the dew point of the sample gas by 5 to 10 degrees or water could condense on the membrane and hinder the diffusion, as is the case with any sensing technology. Use coolers or coalescing filters to mitigate this risk, rather than heating the sensor. As described above, this will avoid decreasing the sensor life-span.
When utilizing electrochemical oxygen sensors, both ambient and sample gas temperatures are crucial factors. AII has over 30 years’ experience in creating solutions for measuring oxygen in numerous applications.
This information has been sourced, reviewed and adapted from materials provided by Analytical Industries Inc.
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