Compressed Air Units and Symbol Dictionary

Compressed Air Unitsand Symbol Dictionary

Keep the quantity, unit, reference state and measurement boundary together—before comparing a rating, importing a log or trusting a calculation.

1. Engineering answer: a unit does not explain the whole measurement

A compressed-air units and symbol dictionary defines what a quantity means, how it is written, which unit represents it and what context must travel with its value. It prevents unlike quantities from being compared merely because the digits, abbreviations or unit labels look similar. The essential record is not “500 CFM” or “7 bar” alone: it is the quantity, unit, reference basis, physical point, operating state and source that make the number interpretable.

A flow meter, a supplier rating and an actual pipe-flow calculation can use similar unit labels without sharing the same meaning.

Its entries explain pressure, airflow, temperature, moisture, power, energy, performance, geometry and evidence terms. Exact unit changes are separated from reference-state changes and from equipment-performance corrections. Where a source leaves a basis unresolved, the dictionary preserves that uncertainty instead of inventing the missing condition.

2. How this dictionary fits the engineering-tool sequence

The present page includes enough equations and examples to show why the terminology matters, but it is not a replacement for those specialist resources. The fixed page ID and next-page sequence are preserved.

Use the dictionary at the moment a value enters a workflow. Before comparing two offers, identify each flow basis. Before importing a logger file, identify pressure reference and timestamp convention. Before calculating specific energy, identify both the electrical and delivered-air boundaries. Before interpreting an oil result, preserve the contaminant fraction and reporting qualifier. A correct calculation cannot repair a quantity that was misidentified at import.

Symbols in this issue are project notation, not an assertion that every manufacturer uses identical lettering. Original symbols remain in the source record. The companion JSON dictionary supplies stable field identities so a developer can map different source labels without depending on typography. Its release is a new authored reference, not a claim that every historic RC page has already been migrated to it.

3. Separate quantity, symbol, unit and basis

A quantity is the physical or reporting property being described: pressure, volume flow, electrical input or elapsed time. Its symbol is a compact name within an equation. A unit is the scale used to express its value. A basis identifies such matters as gauge versus absolute pressure, actual versus reference flow, or electrical versus thermal energy. The physical boundary and operating state supply further meaning. These layers must remain distinguishable in both prose and software. [S1, S2; R3, R4]

Write the preferred quantity label as p_g = 7 bar, or “gauge pressure: 7 bar.” Familiar source labels such as bar(g), barg, bar(a), psig and psia may be retained for recognition, but the stored unit should remain bar or psi while pressure_reference carries the qualification. Likewise, store m³/h with a separate reference-state object instead of relying on the letter N in Nm³/h to carry all thermodynamic assumptions. This follows the principle that information about the quantity belongs with the quantity, not hidden inside a modified unit. [R3, R4]

Use a space between a numerical value and its unit: 25 °C, 75 kW and 8 bar. Unit symbols are case-sensitive; kW is not KW, mW is not MW, and Pa is not pA. Exponents apply to the whole prefixed unit: 1 mm² = 10⁻⁶ m² and 1 mm³ = 10⁻⁹ m³. A software import may recognise controlled aliases, but it must retain the original text and disclose the mapping. Do not solve ambiguity by indiscriminately lowercasing every symbol. [R1, R3]

4. Pressure dictionary: identify the zero and the two points

5. Pressure conversion reference: exact definitions versus displayed rounding

Pure conversion changes a scale, not a pressure reference. A gauge reading of 100 psi can be expressed as approximately 689.476 kPa gauge without knowing atmospheric pressure. Converting that reading to absolute pressure requires the additional reference. The factors below preserve this distinction. Decimal values marked approximate are display values; their trailing digits are not an assertion of instrument accuracy. [R1]

The exact-definition expression used for psi in the companion registry is (0.45359237 × 9.80665)/(0.0254²) Pa. It uses the international inch and avoirdupois pound with standard gravity. A source labelled only “kg pressure,” “inches vacuum” or “water column” remains incomplete: identify force versus mass, gauge versus absolute, fluid and any temperature-dependent or conventional definition before applying a factor.

6. Pressure and vacuum relationships without hidden assumptions

For a gauge measurement referenced to the stated atmosphere, p_abs = p_g + p_atm. For a positive vacuum depression, p_abs = p_atm − p_vac. Each term must use the same pressure unit. A stated standard atmosphere is a reference condition, not a universal replacement for site barometric pressure. A sealed-gauge sensor or a device using another reference needs its own documented relationship. [S1; R5]

For two compatible points sharing the same atmospheric reference, subtracting gauge readings gives the same pressure difference as subtracting their absolute values. Adding an atmosphere to that difference is an error. Conversely, a compression ratio requires the absolute pressures individually. These two operations look similar on a spreadsheet but answer different questions; quantity_kind must determine which operation is allowed.

Vacuum percentages need a definition. A proposed local reporting convention may express depression as 100 × (p_atm − p_abs)/p_atm, but that formula is not assumed for every supplier display. Record its reference before comparing sites. A value below zero absolute pressure is invalid for the present gas-state calculations; a negative gauge pressure is not invalid merely because it has a minus sign. Keep these validity rules separate from equipment safety limits.

7. Airflow and capacity dictionary: retain the reporting state

The CAGI glossary provides its own stated standard-cfm condition and explains inlet-related capacity terminology. That is useful source-specific context, not permission to assign the same reference to every scfm entry. This also resolves the apparent conflict between “actual” in a compressor rating and “actual” in a pipe-flow model. [R5]

8. Volume, flow and time conversion reference

The international foot is exactly 0.3048 m, giving 1 ft³ = 0.028316846592 m³. The same factor applies to ft³/min and m³/min without altering the reference state. A litre is 0.001 m³, and minute-to-hour changes use 60 minutes per hour. These are unit operations, not compressor-efficiency corrections. [R1]

Runtime multiplied by an appropriate interval-average flow gives volume. A logged totaliser already represents accumulated volume; multiplying it by time again is a dimensional error. For samples, exclude preparation and unmeasured pauses according to the collection record rather than assuming all elapsed time is accepted collection time. The dictionary should identify whether a field is instantaneous, an interval average or a cumulative counter before any integration is attempted.

9. Reference-state conversion is a different operation

For the same single-phase gas composition and mass flow, the state relationship is Q₂ = Q₁ × (p₁/p₂) × (T₂/T₁) × (Z₂/Z₁). Pressure is absolute, temperature is in kelvin and Z is the compressibility factor at the respective state. This relationship follows mass conservation and pV = ZnRT; it changes reporting state, not compressor operating capability. [S1; newly developed notation]

The calculation requires a declared scope. A dry-gas reference comparison is not interchangeable with moist-air total volume when moisture condenses or the composition changes. In a specifically defined ideal dry-air accounting model, dry partial pressure is total absolute pressure minus water-vapour partial pressure. That is a conditional model requiring humidity evidence, not a correction to apply to every flow reading. Missing humidity does not mean zero humidity.

Do not apply state conversion twice when a meter has already normalised its output. Keep raw_state, reported_state and target_state separately. Before an output can be used, the developer should establish which state the value currently occupies. Changing the reporting reference from 0 °C to 20 °C does not demonstrate that the compressor delivers more mass, can operate at higher discharge pressure or has gained capacity.

A complete reference-state record identifies absolute pressure, absolute temperature, moisture/composition basis, compressibility treatment, governing method and source revision. The well-known phrase “standard conditions” is insufficient by itself. The dictionary deliberately provides no auto-filled universal normal or standard state. That requirement prevents a familiar abbreviation from becoming an undocumented engineering assumption.

10. Mass, gas properties and dimensions dictionary

A dimensional check is necessary but not sufficient. Torque and energy can both be expressed using N·m, yet they are different quantities and must not be interchangeable fields. Likewise, electrical energy and thermal energy may both use kWh while measuring different boundaries. The dictionary therefore carries quantity_kind as well as unit; software that validates only dimensions can still accept a physically inappropriate substitution.

11. Temperature and moisture dictionary

A negative Celsius dew point is physically meaningful and must not be rejected by a generic “no negative temperatures” rule. Conversely, −300 °C is outside the thermodynamic domain of these gas calculations. A zero temperature difference can be a valid observation. Applying one sign rule to all temperature-related fields would destroy valid data and accept some invalid models. [R2; proposed software interpretation]

12. Power, energy and electrical dictionary

Horsepower requires a named convention. Mechanical horsepower is approximately 0.745699872 kW; metric horsepower is 0.73549875 kW; the electric-horsepower convention is 0.746 kW. They are not interchangeable exact labels. Converting a 100 hp motor rating into kW still gives a rating on its declared basis, not measured package electrical input. Retain the original horsepower description rather than assigning an unexplained factor. [R1]

For a constant 75 kW electrical load operating for eight hours, electrical energy is 600 kWh. For changing loads, sum P_i × Δt_i with time in hours. The shortcut “kW per hour” describes a rate of change of power, not the ordinary unit for purchased electrical energy. Displaying the correct symbol is part of the engineering meaning, not merely editorial style.

13. Performance ratios, percentages and uncertainty dictionary

When SP is expressed in kW per m³/min, the corresponding steady-state specific energy in kWh/m³ is SP/60. For kW per 100 cfm, first convert 100 ft³/min to 2.8316846592 m³/min, then reconcile the power-to-flow ratio. A smaller specific-power number cannot be ranked as better until its denominator convention and measurement boundary are known. Dimensional equivalence does not remove differences in discharge pressure or operating duty.

14. Geometry, pneumatics and mechanical dictionary

The same quantity/unit discipline applies to drilling annuli and pneumatic actuators. An annular area needs both actual diameters and the direction of the subtraction. A double-acting cylinder has different effective areas on cap and rod sides. A nominal receiver volume cannot be converted directly into minutes of reserve without the permitted pressure window, gas state, demand and recharge assumptions. Those detailed designs remain on their respective engineering pages.

15. Air-quality units and analytical-result dictionary

The fixed prefix change 1 mg/m³ = 1,000 µg/m³ is valid only with the same contaminant and gas-volume basis. Conversion from ppmv to mg/m³ is different: it involves composition and state, not just a thousandfold multiplier. Do not insert a standard molar volume without declaring its reference conditions. A dew-point temperature cannot be translated into an oil class, and a compressor technology label cannot substitute for a laboratory or point-of-use result.

16. Time, operating state and data-quality dictionary

A zero flow may be valid during an unloaded or stopped state. It is nevertheless an invalid denominator for a specific-power ratio. These statements are compatible: preserve the observation, disable the affected division and explain why the derived value is unavailable. Do not reject the whole row or silently turn the ratio into zero. Similarly, a missing five-minute interval remains missing rather than being filled with the preceding value merely to make a graph continuous.

17. Symbol collisions: familiar letters require a quantity name

Single letters are compact within a well-defined equation but weak database keys. Use descriptive identifiers for storage and leave display symbols available for engineering readability.

Search aliases can improve discovery without becoming conversion rules. Searching “barg” may find gauge pressure, and “CFM” may find the volume-flow entry. That lookup must not populate an unknown flow reference state or reinterpret a supplier guarantee. A successful search result is a definition suggestion, not proof that the source quantity has been identified.

18. Resolve ambiguous source labels without rewriting history

Preserve the original label, original value and original document revision. Add a proposed mapping, its evidence and a status such as confirmed, clarification required or not applicable.

Examples needing clarification include CFM without a basis; Nm³/h without reference conditions; kW without an input/output boundary; kg/cm² without an explicit force convention; ppm without a fraction basis; and a temperature rise stored with an absolute-temperature conversion. A supplier appendix, meter configuration or signed clarification may resolve the label. A plausible number, familiar brand or neighbouring column does not resolve it by itself.

When a source definition conflicts with the preferred dictionary convention, preserve both and map explicitly. Do not silently “correct” the source quotation. The quotation may be internally valid on its own basis while unsuitable for a direct comparison. A change affecting an issued result needs a correction record and a new derived issue; merely replacing the unit heading beneath unchanged values destroys traceability.

Where several values share one source, link them to the same evidence record. Repeated bookmark copies or multiple exports of one report are not independent corroboration. The companion dictionary separates general definitions from observation evidence so a reader cannot mistake a reference page for proof of a particular machine’s performance. No real supplier rating or customer measurement is introduced by the following examples.

19. Worked example 1: pressure, vacuum and ratio are different calculations

Fictional observations: a discharge gauge reads 7.00 bar, and the applicable measured atmospheric pressure is 0.95 bar absolute. The inlet is assumed, for this arithmetic example, to be at that atmospheric pressure. Discharge absolute pressure is 7.00 + 0.95 = 7.95 bar, and the pressure ratio is 7.95/0.95 = 8.368421. Dividing 7.00 by 0.95 would mix gauge and absolute quantities. It would give 7.368421, which is not the defined compression ratio.

At the same time, compatible upstream and downstream gauges read 7.00 and 6.72 bar. Their observed differential is 0.28 bar = 28 kPa. No atmospheric term is added to that difference. If the same atmosphere is added to both readings first, 7.95 − 7.67 still gives 0.28 bar. This demonstrates why reference conversion and subtraction must be distinct operations in the data model.

For a separate vacuum example under the same 0.95 bar atmosphere, a recorded depression of 0.70 bar corresponds to 0.25 bar absolute. Under the explicitly selected depression-per-atmosphere convention, the percentage is 100 × 0.70/0.95 = 73.684211%. These are synthetic demonstrations, not pressure settings, equipment limits or compressor performance predictions. A different sensor reference requires a different interpretation, not an automatic reuse of this example.

20. Worked example 2: flow units, reference state and line velocity

A fictional rating is 500 ft³/min on a declared dry reference basis of 1.00 bar absolute and 293.15 K. Pure unit conversion gives 500 × 0.028316846592 = 14.158423296 m³/min on the same reference basis. That number is not yet actual flow inside the discharge pipe.

For the same dry mass flow, an ideal-gas example with local line conditions of 8.00 bar absolute and 313.15 K gives Q_line = 14.158423296 × (1.00/8.00) × (313.15/293.15) = approximately 1.890547 m³/min. A real-gas model would additionally use Z_line/Z_ref. The example assumes single-phase dry gas and does not replace a supplier capacity correction or performance test.

For a circular pipe with an actual internal diameter of 50 mm, area is π × 0.050²/4 = 0.001963495 m². Actual volume flow is approximately 0.0315091 m³/s, so the illustrative local velocity is approximately 16.04746 m/s. Using the reference volume without state conversion would give approximately 120.18043 m/s for the same area. The discrepancy is a basis error, not evidence that the pipe or compressor has changed.

A second reference declaration at 273.15 K and the same absolute pressure would produce a different reference-volume number for the same mass flow. Comparing the two declarations therefore requires temperature reconciliation before any capacity comparison. The figure on a different reference basis must not be presented as extra delivered air. These calculations illustrate semantics and geometry; they do not select a permissible pipeline velocity.

21. Worked example 3: specific power and energy units

Fictional aligned measurements give electrical package input P_el = 120 kW and delivered reference airflow Q_ref = 20 m³/min. Specific power is 120/20 = 6.000 kW/(m³/min). During eight hours at these conditions, energy is 960 kWh and reference volume is 20 × 60 × 8 = 9,600 m³. Their ratio is 0.100 kWh/m³, the same result as 6/60.

The same ratio expressed in kW per 100 cfm is 6 × 2.8316846592 = 16.990107955 kW/(100 cfm). The larger displayed number does not describe worse performance; it uses a different volume-flow denominator. Conversely, comparing 16.99 kW/(100 cfm) directly with 6.00 kW/(m³/min) would create a false ranking.

Suppose the 20 m³/min is compressor delivery but only 18.5 m³/min crosses a downstream treatment boundary, while separate auxiliaries add 5 kW. The new ratio is 125/18.5 = 6.756756757 kW/(m³/min), or 0.112612613 kWh/m³. This is a changed system boundary, not a pure unit conversion. Both valid ratios may be retained, but each needs its own power and flow definitions. None of these synthetic figures is a customer saving or an equipment guarantee.

22. Worked example 4: temperature intervals and sample concentration

A fictional temperature reading of 104 °F converts to (104 − 32)/1.8 = 40 °C, or 313.15 K. A separate observed rise of 18 °F converts to 18/1.8 = 10 K. Applying the reading formula to the rise would produce −7.777778 °C, a category error. Adding 273.15 to that difference would be equally wrong. An importer must therefore know temperature_reading versus temperature_difference before choosing the transform.

For a separate sample-volume illustration, a verified flow of 2 L/min operates for 20 accepted collection minutes. V_sample = 2 × 20/1,000 = 0.040 m³ on the stated sample basis. Twelve accepted counts in a named, non-overlapping band give 12/0.040 = 300 particles/m³. A source reporting 25 elapsed minutes with a verified five-minute pause must not add the paused time to accepted collection volume.

An oil result of 0.010 mg/m³ would equal 10 µg/m³ on the same contaminant and volume basis. A report of <0.010 mg/m³ instead remains <10 µg/m³: the inequality is preserved. It must not become 0 µg/m³. This final example joins two otherwise different topics because both reveal the same design requirement: transformations must preserve quantity meaning and result qualifiers, not simply return a numeric value.

23. Editable definition and import-mapping record

Use this record when a quotation, logger export or earlier worksheet introduces an unfamiliar label. The blank fields are intended for manual completion. One row may describe one quantity definition shared by many observations, provided the effective period and source are controlled. Do not insert a universal reference state merely to complete an empty cell.

For an existing field named “pressure_bar,” a completed mapping may declare pressure_reference = gauge and point_id = HEADER_01. The dictionary version then defines how a derived absolute-pressure field is calculated. When the atmospheric input is missing, that derived field is held rather than substituting one atmosphere. The original gauge reading stays valid for uses that do not need absolute pressure.

For a field named “air_volume,” first determine whether it is a cumulative totaliser, interval volume or instantaneous flow mislabeled as volume. Those alternatives need different operations. A corrected name is useful only when accompanied by the verified meaning and an impact review of formulas that used the old field. This worksheet is a practical interface to source clarification, not a blanket approval of imported data.

24. Versioned machine-readable dictionary and quantity records

The companion package contains dictionary.json, unit_registry.json, reference_states.json, ambiguity_rules.json and worked_examples.json, together with an index, change history and structural-check report. The files are UTF-8 JSON. The dictionary entries are the same authored entries shown in the grouped tables above, not a second independently written glossary. No original supplier or customer file is embedded in the package.

Use stable semantic IDs such as pressure_absolute, flow_actual, energy_electrical and temperature_difference. A unit_registry entry supplies a scale conversion within a compatible quantity group. A reference-state object supplies state metadata. Neither a unit factor nor a search alias is permission to change the physical boundary. Defined-but-unpopulated fields are allowed to remain null with an explicit reason.

For a pressure observation, the recommended record includes observation_id, source_label, source_value, source_unit, quantity_id, unit_id, pressure_reference, point_id, observed_at, source_id, dictionary_version and quality_status. Flow observations additionally link flow_definition and reference_state_id. Where a flow is actual rather than reference, its state belongs to the actual measurement point.

Future revisions should distinguish editorial clarification from a material meaning or conversion change. A material change identifies affected calculators and issued datasets; it does not silently replace the historic dictionary beneath an unchanged result. This package does not claim to have converted all previous tools to the new field identities.

25. Expected-result tests and invalid-input behaviour

The cases below are acceptance references for implementing the dictionary and its conversions. Their arithmetic is checked during this document task. Structural checks of the delivered JSON are separate from testing a website importer or a deployed calculator. Exact factors and stored unrounded values govern calculations; display rounding must not alter equality, sign or a reporting qualifier.

Additional implementation checks should reject nonfinite numeric inputs, distinguish null from zero, retain unknown source units, preserve duplicate observations for review and test source-to-target-to-source round trips. A valid temperature interval of zero must remain valid. A negative gauge pressure must not fail an absolute-pressure sign check unless it has first been transformed into an invalid absolute value. Record the reason for every held result next to the affected field.

26. Engineering questions and direct answers

Is CFM the same as FAD?

No. CFM is a volume-flow unit. FAD is a defined way of expressing delivered compressor capacity. The quantity, reference state, operating pressure and reporting method must agree before values can be compared. A pure ft³/min-to-m³/min conversion does not establish those conditions.

What is the difference between bar gauge and bar absolute?

Gauge pressure is relative to the stated atmospheric reference; absolute pressure is relative to vacuum. Add the appropriate atmospheric pressure to convert the former to the latter. Do not add atmosphere to an already calculated differential.

Does Nm³/h always mean 0 °C?

Do not assume that from the label alone. Obtain the source’s exact reference temperature, absolute pressure, moisture basis and method. The dictionary stores these conditions explicitly rather than treating N as a universal complete definition.

Can I use FAD directly to calculate pipe velocity?

Not as actual line flow. First reconcile the gas state and composition to the pipe location, then express flow per second and divide by actual internal area. The resulting calculation still requires an appropriate flow model and actual geometry.

Why is kWh/m³ numerically different from kW per m³/min?

The time units differ. For compatible boundaries and the stated steady relationship, divide kW/(m³/min) by 60 to obtain kWh/m³. For a variable operating period, use total compatible energy divided by total compatible air volume.

Should 273.15 be added to a temperature rise?

No. The offset converts a Celsius reading to a kelvin reading. A Celsius-degree difference and a kelvin difference have the same size. Fahrenheit differences are divided by 1.8 without subtracting 32.

Can ppm be converted directly to mg/m³?

Not without knowing the fraction basis, species and relevant gas state. A mass fraction, mole fraction and mass-per-volume concentration are different quantities. A prefix-only unit converter cannot infer the missing physical inputs.

Does a 75 kW motor mean the compressor consumes 75 kW?

Not necessarily. Identify whether the figure is shaft rating, measured electrical input or another package quantity. Motor/drive losses and included auxiliaries affect the comparison. Changing units does not change that boundary.

Is a less-than laboratory result a zero?

No. Preserve the comparison sign and threshold. A result below a reporting threshold describes a qualified observation, not a quantified zero. Converted units must retain the qualifier and the method’s limits.

Does adopting the dictionary approve earlier calculations?

No. It provides controlled definitions and conversion references. Earlier records must be mapped and their affected outputs checked where meanings change. Calculator validation, equipment acceptance and operating approval remain separate activities.