Advantages and Drawbacks of Metal Oxide Sensors for Continuous Emissions Monitoring
Author: Brad Roger, Product Manager at Qube Technologies
Introduction
When choosing a technology for your emissions monitoring program, you must carefully consider several key criteria including cost efficiency, accuracy, and reliability of the system.
Metal oxide (MOS) sensors are a viable solution for methane detection. They offer a low-cost, effective, and reliable method for monitoring low-level methane concentrations with minimal maintenance requirements. However, metal oxide sensors also have disadvantages which we must consider for the technology to be useful for continuous emissions tracking.
In this post, we will cover the accuracy, reliability, and cost advantages that MOS sensors provide, while also revealing some of the techniques we use to offset the technology’s potential limitations.
A Qube device monitors for emissions on a fence line near a field
How do metal oxide sensors work?
MOS sensors can detect a variety of gases by measuring the change in electrical resistance caused when metal oxide is exposed to the air. Oxygen particles in the atmosphere naturally stick to the surface of a MOS sensor, but when certain gases are around, some of those oxygen particles are pushed away, reducing the amount of electrical resistance they cause, and producing a measurable conduction property which is unique to that gas.
This remarkable ability to accurately detect low concentrations of specific gases makes properly calibrated MOS sensors ideal for continuous monitoring systems.
Advantages and Disadvantages of Metal Oxide Sensors
The main advantage of metal oxide sensors is the cost. MOS sensors are inexpensive, which allows operators to deploy accurate continuous monitoring at more facilities, at a far lower cost than other systems. However, these sensors do come with some drawbacks:
They are sensitive to environmental conditions, such as temperature and humidity
Sensor readings can drift off their baseline over time
Sensors have a limited useful lifespan (5-10 years)
To make out-of-the-box MOS sensors suitable for continuous monitoring, Qube invests significant R&D resources to address these potential drawbacks.
Testing and Calibration of Metal Oxide Sensors
All Qube sensors are individually tested under a wide range of conditions using a high precision environmental chamber. The chamber allows us to inject varying concentrations of methane and then test the sensors readings against a range of temperatures and humidity.
Measurements from the temperature, humidity, and methane sensors are recorded and compared against high accuracy reference values. These outputs are analyzed to produce calibration coefficients that are unique to each sensor. These coefficients allow the system to adjust readings for environmental conditions and reduce or eliminate errors across our full operating range.
Read More: How Qube validates readings from MOS sensors against a high-quality reference.
The Qube team rejects all sensors that produce significant errors under any combination of conditions and concentrations.
Compensating for Drift
Although every Qube device is tested and calibrated before deployment, environmental conditions in the field can still affect our sensor outputs, leading to incorrect concentration values. Qube ensures that the live voltage readings taken from sensors stay accurately mapped to gas concentrations by using the calibration coefficients and applying them against our real time sensor readings for temperature, pressure, and humidity.
This calibration also helps us deal with sensor drift - a natural warping of sensor readings usually caused by age or decay. During deployment, we use a proprietary baselining technique to match our methane detection readings with actual background concentrations on the site. Over time, when an individual sensor drifts from its baseline, Qube’s system can recognize the problem, and the device can be re-baselined remotely without the need to send a technician to the site. This remote feature is helpful for monitoring individual facilities, and is vital for operators planning larger program-wide deployments.
Lifespan of Qube Axon Devices
Every technology has a lifespan, including Qube Axon devices. Standard metal oxide sensors have an effective lifespan of approximately 10 years, but since Qube requires a detection accuracy that is higher than most applications, we limit the effective lifespan of our sensors to 5 years.
To help operators keep maintenance and support costs down, all Qube devices are built completely modular. If maintenance is required, or an upgrade is available, each individual sensor or component in an Axon device is field replaceable. Qube customers can monitor the live status of each device using device health dashboard, and in the rare case where a component or sensor fails, operators can quickly identify and allay the issue with minimal cost.
Future
Qube Technologies is constantly expanding our research to build cost-effective solutions for detecting, localizing, and quantifying emissions. Our goal is to be the lowest cost continuous monitoring provider in the industry and we believe that metal oxide sensors represent the optimal balance of cost, accuracy, and reliability for operators.
Learn more at www.qubeiot.com.