Zolo Technologies FAQ

What is TDLAS?
What can be measured?
How fast is it?
How accurate is it?
Does it have to be calibrated
Is it patented?
How can I learn more?

What is TDLAS?

Tunable diode-laser absorption spectroscopy is a proven technique that uses lasers to measure combustion constituents. An industry standard diode-laser is tuned in wavelength across a tiny portion of the optical spectrum. Light at the chosen wavelength is absorbed by a given combustion constituent and the relative amount of absorption is proportional to the concentration. Zolo Technologies instruments transmit multiple laser wavelengths, simultaneously along a single path and measures an average of each concentration.

More information on TDLAS is available on our website. Further details are available at wikipedia's entry on TDLAS.
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What can be measured?

Path-averaged gas temperature, pressure, and concentration. The TDLAS instrument unambiguously derives these gas properties from measurement of optical absorption features along the measurement path (reference TDLAS tutorial section). Unlike in-situ measurement probes, such as thermocouples and pressure transducers, and surface temperature measurement devices, such as optical pyrometers, TDLAS instruments directly measure gas-phase properties, and they do so non-invasively. This aspect of TDLAS allows accurate measurements in ultra-hot, highly corrosive environments, such as turbine engine combustors and augmentors.

Zolo Technologies instruments have measured temperatures, pressures, and concentrations in a wide range of combustion applications, from coal-fired boilers to jet engine afterburner flames to rocket engine combustors.
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How fast is it?

Zolo Technologies has made temperature measurements in combusting environments at updates rates as high as 25 kHz and with precision as good as 10°C. Higher update rates are possible, and in fact, Zolo’s embedded flight sensor instrument (ZoloEFS) is capable of making measurements at greater than 200 kHz. Physical limitations to update rate are set by how fast one can scan laser wavelength over the desired measurement range and how many scan averages are needed to achieve the desired measurement precision.
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How accurate is it?

It is important to differentiate between instrument accuracy and instrument precision. Instrument accuracy is a measure of how closely the instrument measurement will be to the true gas property of interest (temperature, pressure, concentration, etc.), if taken to the limit of essentially infinite measurement averages. Accuracy is thus a measure of systematic error in the measurement. Instrument precision is a measure of how closely any given measurement will be to the true gas property. Measurement precision is intricately related to measurement rate or speed, as discussed in How fast is it?. For a given measurement application and instrument, the faster the update rate, the lower the measurement precision, and vice-versa.
TDLAS instrument accuracy and precision can vary depending on the measurement environment and measurement requirements. The physical parameters of gas absorption spectra are fixed by nature but can be affected by such environmental factors as temperature, pressure, and gas mixture. Zolo Technologies carefully calibrates its instruments in controlled laboratory conditions prior to releasing changes to hardware or software that would affect accuracy and precision. Additionally, Zolo Technologies has many person-years of experience developing software algorithms to precisely and accurately extract measurement parameters from optical absorption spectra. To see actual field data and associated measurement accuracy, see the Example Data.
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Does it have to be calibrated?

TDLAS measurements are ratiometric so that measurement accuracy does not depend on signal strength, at least until signal level dips below the system noise level. Most other optical measurement techniques, such as optical pyrometry, depend on signal level to make a measurement. Thus, TDLAS measurements are highly tolerant of reduced (or increased) signal levels due to dirty windows, partial obscurations, dust and other aerosols, etc.

The instrument is calibrated internally at the factory, and once that calibration is complete, the instrument should maintain its calibration for many years. Zolo Technologies TDLAS instruments take advantage of lasers and photodetectors developed for telecommunications markets. These devices must pass strict Telcordia performance and reliability standards. The end result is a highly reliable TDLAS instrument.
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Is it patented?

Yes. Zolo Technologies has a number of very key patents, both issued and pending, covering various aspects of TDLAS instrument and application technology. Additionally, Zolo Technologies has a wealth of application know-how related to making practical measurements in ultra-harsh environments. It is this engineering know-how that makes the difference between being able to make TDLAS instruments that work in the lab and TDLAS instruments that work reliably in the field.
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How do I learn more?

For further information on Zolo Technologies’ products and capabilities, don't hesitate to contact us.
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