Key takeaway
A gas volume needs a state and a moisture basis. A different reference temperature can change the reported number without changing the gas passing the meter.
Recover the state hidden behind the unit
Two historian columns both say m³/h. One reports volume at the meter’s operating conditions; the other reports an equivalent volume at a reference state. A chart that overlays them can suggest a flow change that comes entirely from the reporting convention. The receiving question is whether the records can be put on a common, justified basis.
EPA Method 2 supplies a specific example: its dry stack-gas calculation declares standard absolute temperature and pressure, and includes a water-vapour adjustment. Those conventions belong to that method. They do not establish what a factory’s normal or standard label means. Recover the actual meter configuration and dictionary before borrowing any reference values.
The same dry flow produces three numbers
In this hypothetical calculation, a dry ideal gas flows at 100 m³/h at 400 K. Its absolute pressure equals the reference pressure throughout. Composition is unchanged, measurements are exact for the illustration and compressibility is assumed to be one. The reference-volume relation therefore reduces to Qref = Qactual × Tref / Tactual. These are invented conditions, not a correction for a real installation.
Using a declared 293 K reference gives 100 × 293 / 400 = 73.25 m³/h. Using a different, explicitly stipulated 273 K reference gives 68.25 m³/h. Both describe the same assumed gas flow. The 5 m³/h difference between those reference reports is not evidence of leakage, reduced production or a missing delivery.
| Representation | Declared temperature | Flow in this illustration | Decision |
|---|---|---|---|
| Actual dry volume | 400 K | 100 m³/h | Preserve operating-state basis |
| Reference A | 293 K | 73.25 m³/h | Valid only under stated ideal/dry assumptions |
| Reference B | 273 K | 68.25 m³/h | Different reference; same stipulated gas flow |
| Wet report, moisture unknown | Not established | Do not calculate a dry equivalent | Recover moisture basis first |
Pressure and moisture need their own evidence
The full state map must retain actual absolute pressure, reference absolute pressure, actual absolute temperature, reference temperature and whether the source is wet or dry. A gauge-pressure reading is not an absolute pressure. Record how any atmospheric contribution was obtained, rather than silently treating the gauge number as the required input.
NIST’s gas-flow standards use measured pressure and temperature with volume and an equation of state for the gas species. That is a useful boundary on the simple example: a real-gas correction, changed composition or uncertain measurement cannot be resolved by assuming that every instrument follows the same ideal model. Ask the metering owner which model and configuration applied during the recorded period.
Keep the reported and reconstructed series
Store the original report beside a derived common-basis series. The transformation record should identify meter/configuration version, source period, reference state, moisture source, calculation, rounding stage and excluded intervals. A corrected dictionary should create a traceable revision; it should not erase which convention an earlier report used.
For the illustration, accept the arithmetic relationship between the three dry representations. Hold the wet row because moisture is unknown. If a historical export retains only the final standardized number, describe that limitation and assign recovery of the configuration to the metering owner. More decimal places cannot recover missing conditions.
Choose a claim the history can support
A metadata description can state that gas-flow histories exist with a known reference basis for specified periods and unresolved configuration elsewhere. It should distinguish measured inputs from later calculations. This supports a focused evaluation question without presenting every populated hour as comparable physical evidence.
Use Inventory to list the periods and configuration gaps, then Due diligence to agree which common basis a recipient needs. Facility operations and commercial meter information remain subject to their own disclosure review. An interpretable flow record supplies neither a compliance finding nor permission to transmit the archive.