Photoacoustic spectroscopy has passed through several distinct eras in the fifty years it has been applied to dissolved gas analysis, and the story is usually told as a ladder: laboratory chromatography, then online chromatography, then broadband photoacoustic, and now laser photoacoustic. The history is real, but the ladder is a simplification. What separates the present era from the last is the light source — and the light source is an architecture choice with its own cost and its own failure mode, not a single measure of quality.
This article sets out the four eras, then looks closely at the current one: laser photoacoustic spectroscopy (L-PAS). What it genuinely changes, and what it leaves untouched. A companion article compares the broadband infrared and laser light sources axis by axis.
1st era: laboratory gas chromatography (1970s–present)
Laboratory GC remains the reference method for DGA accuracy. An oil sample is transported to a lab, dissolved gases are extracted by headspace or vacuum degassing, and the gas mixture is injected into a chromatographic column. Different gases travel through the column at different speeds and are separated for sequential detection by a thermal conductivity detector (TCD) or flame ionization detector (FID).
Strengths: the highest accuracy recognised in the international standards (IEC 60599, IEEE C57.104, ASTM D3612). Limitations: a 2–4 week turnaround from sampling to result, a quarterly measurement frequency at best, sample-handling error, and gas escaping during transport.
2nd era: online gas chromatography (1990s–present)
Online GC miniaturises the laboratory instrument into a field-deployable unit. The Serveron TM8 (now Qualitrol) brought GC to the substation. Strengths: lab-comparable accuracy, automated measurement every few hours. Limitations: carrier gas (helium) must be replaced every 2–4 months, columns degrade and need replacement every 3–5 years, and regular maintenance visits are required.
3rd era: broadband photoacoustic spectroscopy (2002–present)
Introduced commercially in the early 2000s, PAS was a paradigm shift: optical gas detection without consumables. A broadband thermal infrared emitter is modulated — classically by a mechanical chopper — and optical band-pass filters select a wavelength band for each target gas. Strengths: no consumables, measurement on an hourly cycle, and a long expected sensor life. Limitations: optical filters have finite spectral selectivity, so gases with overlapping infrared absorption bands can interfere with one another.
4th era: laser photoacoustic spectroscopy (present day)
L-PAS replaces the broadband emitter and its filter wheel with tunable semiconductor laser diodes, typically one per target gas. The laser is tuned onto that gas’s own rotational-vibrational absorption line, and its line width is far narrower than the band a filter passes, so the wavelength is chosen with much finer resolution. The practical consequence is structurally lower cross-interference between gases whose absorption bands overlap.
That advantage is real, and it is not absolute. A laser still has a finite tuning range and can sit near a neighbouring line, and water vapour is avoided by line choice rather than removed. Broadband instruments address the same problem with filter selection, a reference cell and spectral deconvolution. Neither architecture is free of interference; they pay for it in different places (cross-sensitivity and selectivity in DGA sensors).
The other half of the trade is cost shape. A broadband emitter is a commodity component, so adding a target gas is largely a matter of adding a filter. A laser instrument needs a laser per gas, so its bill of materials grows with the gas count. That is why laser multi-gas instruments are dearer at nine gases than at two, while broadband instruments scale more gently — and why some suppliers pair laser detection for a few critical gases with a broadband or non-photoacoustic channel for the rest.
What the light source changes — and what it does not
| Parameter | Broadband photoacoustic | Laser photoacoustic |
|---|---|---|
| Light source | Thermal emitter — blackbody, filament or MEMS membrane — plus band-pass filters | Tunable laser diodes: DFB/DBR near-infrared, ICL/QCL mid-infrared |
| Wavelength selection | Filter band, typically 50–200 nm wide | Tuned onto the gas’s own absorption line |
| Cross-interference | Controlled by filter choice, reference cell and spectral deconvolution | Structurally lower, set by how far the lines are spaced |
| Adding a gas | Largely a matter of adding a filter | Requires a laser, or a broadly tunable source covering several gases |
| Modulation | Mechanical chopper in the classic form; electronic pulsing in modern emitters | Direct current modulation — no chopper |
| Moving parts in the optical path | Chopper and/or filter wheel, a scheduled wear item | None; the trade is thermal control and wavelength-locking electronics |
| Cost shape | Lower capital, more routine maintenance | Higher capital up front, a costly replaceable laser module later |
Notice what is absent from that table: any row on accuracy. The light source governs selectivity, gas scaling, moving parts and cost. It does not govern the accuracy of an installed monitor, which is decided mainly by the oil-to-gas stage and the calibration chain — the subject of the next section.
Where field accuracy is actually decided
A 2024 peer-reviewed review by M. Grisaru compared PAS monitors with heated-headspace gas chromatography on 100 ppm and 500 ppm gas-in-oil standards and reported a mean deviation of about 24% for the PAS instruments against about 3% for the chromatograph, with the widest per-gas gaps on carbon dioxide and acetylene. The same work, comparing online PAS with online chromatography in service, found the largest divergence on hydrogen — about 59% — a channel the photoacoustic cell does not measure directly at all. Its own conclusion was that PAS required improvement to match standard GC performance, and it named two parallel routes: the light source and the acoustic transducer (research comparison, not a product claim).
To our knowledge there is no published, controlled head-to-head study that isolates laser PAS from broadband PAS on dissolved-gas samples and demonstrates a cross-generational accuracy gap. A claim that one light source is a generation ahead of another is therefore an architectural statement, not a measured one, and it is worth reading any such claim in that light.
The lifecycle picture points the same way. CIGRE TB 409 (2010) reported that online gas-monitoring instruments have a service life of at most half the transformer’s, and typically about a quarter. An independent 15-year total-cost model presented by A. Hilgers at the Stuttgarter Hochspannungssymposium (2021) found that the instrument with the highest purchase price delivered the lowest total cost of ownership, because its running costs were lowest across three five-year segments. Both findings argue for scoring a monitor on its measurement uncertainty budget and whole-life cost rather than on its light source alone.
The channel no photoacoustic cell can reach
Hydrogen has no infrared absorption, so no photoacoustic cell — laser or broadband — can measure it directly. Every photoacoustic DGA instrument that reports hydrogen does so through a separate channel: thermal conductivity, electrochemical, or a solid-state sensor such as a palladium-alloy element. Oxygen and nitrogen behave the same way, and nitrogen is frequently a calculated rather than a measured value. This is molecular physics, not a limitation of one light source, and it means the gas count on a datasheet should always be read together with the delivery method behind each gas (why PAS cannot measure hydrogen).
What this means for a specification
The useful move is not to pick a light source, but to make the supplier state the consequence of theirs. Ask for the C₂H₂ detection limit measured on transformer-oil matrix rather than on dry standard gas; for a quantified drift figure over 6–12 months; for a written T63 degassing response; for the delivery method behind H₂, O₂ and N₂; for a list of moving parts and their service interval; and for a segmented consumables schedule covering years 0–5, 5–10 and 10–15. Two monitors built on different light sources can both pass that test, and one built on the “newer” source can fail it. A weighted version of the checklist is available as our DGA monitor selection tool, with the reasoning set out in the selection scorecard.
PAS DGA for online transformer gas monitoring
At the multi-gas end, DGA-900 reports nine gases plus moisture for units where a fault type must be named rather than merely detected. Where the need is a continuous hydrogen trend, the DGA-500 valve-mount monitor and the DGA-300 probe place a palladium-alloy element directly in the oil, and the hydrogen sensor family covers standalone and OEM duty, with the sensing-element basis set out in dissolved hydrogen sensor compliance. Because the reading is decided at the oil-to-gas stage, every PAS DGA instrument is supplied with that handling matched to the site. Send us the transformer ratings, the oil-circuit drawing and the gases you need to trend, and we will return a specification and an installation proposal — contact PAS DGA.
Sources
- M. Grisaru, “Photoacoustic method: The contemporary premier method for DGA,” Transformers Magazine 11(3):50–61 (2024) — PAS vs GC-HS mean deviation ≈24% vs ≈3% on 100/500 ppm standards; online PAS vs online GC hydrogen divergence ≈59% (research comparison).
- CIGRE TB 783, DGA Monitoring Systems (2019) — monitor classes, detection-limit and accuracy tables; Annex C (infrared types: PAS, NDIR, FTIR) and Annex D (gas chromatography).
- CIGRE TB 409, Report on Gas Monitors for Oil-Filled Electrical Equipment (2010) — online instrument service life at most half, typically about a quarter, of the transformer’s.
- A. Hilgers, “Überlegungen bei der Spezifizierung eines DGA Online-Monitoring-Geräts,” Stuttgarter Hochspannungssymposium 2021, pp. 109–127 — independent 15-year segmented cost model.
- M. W. Sigrist, “Trace gas monitoring by laser photoacoustic spectroscopy,” Infrared Physics & Technology (1995); D. C. Dumitras et al., “Laser photoacoustic spectroscopy,” J. Optoelectronics Adv. Mater. (2007) — the laser-PAS method.
- IEC 60599:2022 (fault types); IEC 60567 (sampling and gas extraction); IEEE Std C57.104-2019 (Condition 1–4 ratings); ASTM D3612.
- Manufacturer specification data for broadband and laser photoacoustic DGA instruments (vendor data), accessed 3 October 2026.