DGA Calculator: interpret one transformer’s gases and test the Duval zone against lab uncertainty
The Seetalabs® DGA Calculator is a free tool for maintenance engineers, oil laboratories and asset managers. Enter the dissolved gases of one transformer and it returns the IEEE C57.104-2019 status, the IEC 60599:2022 assessment, every Duval method that fits the insulating fluid, and whether the Duval zone holds within laboratory uncertainty. Nothing leaves your browser.
How it works
- Choose the insulating fluid: mineral oil, natural ester (soybean, rapeseed, sunflower or unspecified), synthetic ester or silicone. For mineral oil, say whether the sample comes from the main tank, a main tank with a communicating on-load tap changer, or the tap changer compartment itself.
- Enter the gases in ppm from your laboratory report: H2, CH4, C2H6, C2H4 and C2H2 are needed for a diagnosis; CO, CO2, O2 and N2 are optional. Add earlier dated samples to get the rate of change, or paste a row copied from a spreadsheet.
- Read the summary: severity, fault type from each method, the reasons to treat the result with caution, and the recommended action. The detail shows every Duval triangle and pentagon with your sample plotted.
An empty field means "not measured" and is never read as zero. With one sample and no date, the calculator uses today's date; with several samples, each needs its date.
Method and standards
Mineral oil, main tank. Severity follows the status of IEEE C57.104-2019 §6.1.1 and its Tables 1 to 4 (gas levels by age and O2/N2 ratio, increments and rates). IEC 60599:2022 adds the typical values of Annex A (Tables A.2 and A.3), the gas ratios of Table 1 and the CO2/CO ratio of §5.5, shown as indicative. Fault type comes from Duval Triangle 1 with the 2022 arcing sub-zones in oil (-H) and in paper (-P), Duval Triangles 4 and 5 as activated in CIGRE TB 771 (2019) Appendix H.1, and Duval Pentagons 1 and 2. The oil-and-paper sub-zones are indicative: the tool says so next to the result and suggests confirming arcing in paper with other evidence, such as acoustic tests and the CO2/CO ratio.
Tap changer compartment. The main-tank tables do not apply. The calculator plots Duval Triangle 2 (IEEE C57.139 Table D.2) and gives no diagnosis when C2H2 and C2H4 are both below 10 ppm (CIGRE TB 443 (2010) §4.5). For the full IEEE C57.139-2015 check of a tap changer, with ratios, trend and gas per operation, use the OLTC DGA Check.
Natural and synthetic esters. Typical values from IEEE C57.155-2014 Table 1 and IEC 63585:2026 Annex B. Fault type from Duval Triangle 3, or 3b when the solid insulation is cellulose, Triangles 6 and 7 for soybean ester, and Pentagons 3, 4 and 4b. Rapeseed and unspecified esters use the soybean methods, marked as indicative. Mineral-oil methods are never applied to an ester.
Silicone. Only the silicone version of Duval Triangle 3; severity is not assessed.
Severity rules. IEC values above the typical range give severity 2, never 3, because the only action IEC 60599:2022 attaches to them is more frequent sampling. Esters reach severity 2 at most, following IEEE C57.155-2014 §6.2.
Agreement between methods. The calculator groups each method's zone into fault families (partial discharge, discharges, mixed, thermal, stray gassing) and reports whether the methods agree, are compatible or differ.
Does the Duval zone hold within laboratory uncertainty?
A Duval point near a zone boundary can change zone when a gas moves by a few ppm, and laboratory results are never exact. The calculator varies each gas across its analytical uncertainty and counts how much of that range keeps the same zone. It uses ±15 % above 10 ppm and ±30 % from 5 to 10 ppm, derived from the uncertainty figures of IEC 60599:2022 §6.2, and ±75 % below 5 ppm, following CIGRE TB 783 (2019) §5.3. For a triangle this is a grid of 9 levels per gas; for a pentagon, 5 levels on each of the five gases.
The zone is reported as stable only when it stays the same over the whole grid. Otherwise the result is borderline and the page lists the other zones with their share. The calculator does not turn this into a confidence percentage: it lists the reasons for caution (low or weak concentrations, borderline zone, methods that differ, single sample), each with its source.
Worked example
A high-temperature thermal fault, and a zone on a boundary
Mineral oil, main tank, transformer age 22 years, one sample: H2 150, CH4 260, C2H6 80, C2H4 520, C2H2 9, CO 250, CO2 2500, O2 2200 and N2 53000 ppm. This is the "T3" example built into the calculator.
- Duval Triangle 1: T3, thermal fault above 700 °C.
- Duval Pentagon 2: T3-H, thermal fault in oil only.
- The Triangle 1 zone is stable: it stays T3 across the whole uncertainty grid.
- The only reason for caution listed is that there is a single sample.
A second example sits on a boundary. With H2 50, CH4 80, C2H6 30, C2H4 20 and C2H2 0 ppm, Triangle 1 falls at 20 % C2H4, on the line between T1 and T2. The calculator marks the zone as borderline and names the alternative, so you know that one more sample, or a laboratory check, matters more than the label.
A third built-in example, with H2 160, CH4 20, C2H6 4, C2H4 14 and C2H2 60 ppm, gives Triangle 1 zone D1, discharges of low energy.
The T3 and D1 cases are in the tool's examples menu; the boundary case can be typed in.
What happens to "<1" and "ND"
A result such as "<0.5" is not zero: it only says the gas is somewhere between zero and 0.5 ppm. The calculator keeps the limit, runs each method at both ends, 0 and the limit, and gives a result only when both ends agree. When they disagree, that method abstains and a note names the method and the gas, following IEEE C57.104-2019 §5.2.1 on the large relative uncertainty near the detection limit. "ND" without a number stays unknown, and the page asks for the limit.
For example, C2H2 reported as "<0,5" with C2H4 at 2.0 ppm gives a C2H2/C2H4 ratio between 0 and 0.25, shown as a range, not as a single value.
Limits
- One transformer at a time. For a list of transformers from a laboratory CSV, use Fleet DGA Triage.
- Laboratory samples only. To compare on-line monitor readings with laboratory results, use the Online DGA Monitor vs Lab Check.
- IEC typical values apply to power transformers, not to distribution, furnace or instrument transformers.
- No health index, no remaining life, no PDF report (use your browser's print function).
- The result is an indication for planning, not a diagnosis. Decisions stay with a qualified engineer who knows the transformer's history, electrical tests and operating conditions.
Your data stays in your browser
The calculator runs entirely in your browser. Its Content Security Policy sets connect-src 'none', so the page cannot send the values you enter to any server, including ours. It uses no cookies and no browser storage: when you close the page, the values are gone. You accept the disclaimer at each visit, because the tool does not remember you.
The tool runs in your browser. Seetalabs® does not store or see the values you enter.
Frequently asked questions
Which Duval methods does the calculator use for natural ester?
Duval Triangle 3, or 3b when the solid insulation is cellulose, plus Pentagons 3, 4 and 4b. For soybean ester it also uses Triangles 6 and 7. Typical values come from IEEE C57.155-2014 Table 1 and IEC 63585:2026 Annex B.
Why is my result "borderline"?
Because the Duval zone changes somewhere within the laboratory uncertainty of the gases: ±15 % above 10 ppm, ±30 % from 5 to 10 ppm and ±75 % below 5 ppm. The page shows which other zones appear and how much of the range they take.
Can I enter a sample from the tap changer compartment?
Yes, for mineral oil. The calculator then uses Duval Triangle 2 of IEEE C57.139 instead of the main-tank tables, and gives no diagnosis when C2H2 and C2H4 are both below 10 ppm.
Does it calculate a health index?
No. It interprets dissolved gases, and optionally the condition of the fluid in service against IEC 60422 and the other fluid standards. A health index needs more data and belongs to Ronin AI.
From one transformer to the fleet
The calculator answers for one transformer and the samples you type in. Several transformers in one laboratory file? Rank them with Fleet DGA Triage.
If you follow more than a few transformers, or receive reports from more than one laboratory, Ronin AI imports the reports, keeps every sample by date and reads each gas trend against the IEEE C57.104-2019 change tables. Tell us how many units and which laboratories.
Tool version 1.0.4. Seetalabs®.
