Innovating in NDIR technology

Innovative thinking is Vaisala’s core strength. Throughout our history we have applied it to developing technological expertise and new products to help clients’ assets and businesses perform better. This attitude is clear to see in the step-by-step approach the product team took to developing and refining enhanced non-dispersive infrared (NDIR) technology, and introducing it to new applications.

Vaisala’s NDIR journey began already in 1992, when we were intensively researching surface-micromechanical sensors. This led to the ground-breaking idea of creating a miniaturised tunable Fabry-Perot Interferometer (FPI), a technology that filters light at different wavelengths, and applying this to measuring CO₂ concentrations.

Over the years, Vaisala has evolved its NDIR technology for use in different disciplines, for instance to measure gas levels in commercial greenhouses and incubators used for cell cultivation. Vaisala’s development team was the first to work out how this tunable FPI technology could also effectively monitor gases in power transformer insulating oil.

Reengineering for versatility and reliability

Over time, the team’s commitment to engineering and manufacturing excellence produced a well-developed product capable of thriving in a variety of environments. Humidity and heat are typically the enemy of gas sensors. Operators, however, need their sensors to be accurate, easy to use, reliable and, usually, small. Vaisala’s development of NDIR technology has helped in achieving this. By simplifying the technology, we have cut the number of necessary components, and made the equipment more robust. For example, Vaisala’s long-lasting low power IR emitter, Microglow was first released commercially in 2013. It replaced traditional IR sources, which have relatively short lifetime and require a lot of power to run.

When Vaisala set out to develop the FPI technology, it didn’t have a product in the CO2 measurement market. But the re-developed tunable FPI filter became the key component in CARBOCAP®, a high-quality optical gas sensor, and later, the backbone of our Optimus^TM OPT100 DGA Monitor, used in the power industry to accurately and reliably assess the health of power transformers.

Using NDIR to measure the presence of gases in transformer oil for example, relies on the on the fundamental absorption physics that different gases absorb light at different wavelengths. That is to say, the gases have unique “fingerprints” of wavelengths they absorb. Conventional NDIR technology uses a separate band-pass filter for every gas the operator wants to measure. Each filter is fixed to transmit only a specific wavelength of light to a detector that identifies how much light has been absorbed, and therefore how much gas is present. For this to work, the fixed filters must be very stable and accurate, but other factors, such as cross-sensitivity of gases, contamination of optics and detector drift can also affect accuracy (Fig.1).

Tuning for stability and accuracy

Vaisala’s version of the FPI is tunable, meaning it can be adjusted to allow different wavelengths to pass. The distance between two mirrors, separated by an air gap, can be tuned by passing a voltage over the air gap. This adjusts the optimized wavelength the filter transmits and therefore allowing a range of gases to be accurately detected.

Together with an infrared detector, the tunable FPI filter can indicate how much light is absorbed at a given wavelength, and therefore detect the type of gas and its concentration. In doing so, it cuts the need for multiple detectors. Because one FPI filter can produce measurements for several optimized wavelengths, cross-sensitivity from other gases is minimised.

The most significant advantage of the tunable sensor is its ability to take reference measurements from points in the spectrum where no absorption occurs. These reference measurements enable auto-calibration of the detector signal in order to compensate for changes in the intensity of the IR emitter over time, for example.

These reference measurements mean Vaisala’s NDIR technology is stable over long term and also in challenging environments. Testing Vaisala sensors in different conditions has produced consistent readings of CO₂ levels at both low and high concentrations. For example, over a five year period, 22 units measuring CO₂ concentration of 1000 ppm at room temperature showed excellent stability (Fig. 2.) Also, Vaisala sensors tolerance to high temperatures has proved superior (Fig. 3.)

Vaisala now has over 25 years of experience refining the production of these relatively complex chips, and today all components are manufactured by Vaisala in-house, giving us total control over the quality of the entire process.

Cost savings for a lifetime

There are also cost benefits to using a single tunable filter and detector. Competitor technologies require multiple filters and detectors with the failure of any of them leading to maintenance costs and a loss of monitoring data. A single filter allows Vaisala to reduce the overall cost of manufacturing and promise greater reliability for customers, producing reliable monitoring components with lifetimes even up to 50 years.

By eliminating the problem of cross-sensitivity and ensuring the technology is robust enough to work in even the hottest and most humid conditions, customers can rely on the accuracy and reliability of the sensors. For owners of power transformers, reliable and accurate monitoring data means lower lifetime operating and maintenance costs across the lifespan of their transformers.

Our inventive approach to product development and manufacturing has created a world-beating sensor and NDIR technology that customers can use in multiple environments across many different industries.