For an ideal constant-volume gas at constant temperature, the free-air equivalent associated with a gauge-pressure rise is proportional to volume and pressure rise. This calculator applies that simplified relationship to entered values.
Trunk Air Compressor Interactive Calculator
Enter a constant volume, start and final gauge pressures, average free-air delivery over that interval and constant current. Results are ideal arithmetic—not a compressor curve, battery-runtime estimate, pressure recommendation or tire-service procedure.
Equation Used
- Gas behavior is treated as ideal with constant tire volume and temperature.
- Ambient absolute pressure is fixed at 14.7 psia.
- Entered flow is average free-air delivery across the entered pressure interval.
- Entered current is constant and energy uses a fixed 13.8 V arithmetic supply.
- Leakage, hose restriction, compressor curve, duty cycle, thermal cutoff, battery voltage sag, tire deformation, measurement uncertainty and safe pressure limits are omitted.
Ideal Constant-Volume Pressure-Rise Arithmetic
NASA states the ideal-gas equation pV=nRT. With volume and temperature held constant, gas amount is proportional to absolute pressure; using a gauge-pressure difference removes the common atmospheric term.
What This Calculator Represents
The tool converts entered positive gauge-pressure rise and constant volume to a free-air equivalent using a fixed 14.7 psia ambient reference. It divides by entered average free-air flow and applies entered constant current.
The entered flow must already represent average delivered free air over the pressure interval. The tool does not derive it from compressor hardware.
Ideal Fill Arithmetic
free-air equivalent = V × pressure rise / 14.7 psia
ideal fill time = free-air equivalent / entered average CFM
current-time product = I × time
13.8 V arithmetic energy = 13.8 × I × time / 60
Worked Arithmetic Example
For 3.8 ft³, 20 to 35 psig, 1.2925 CFM average free-air flow and 30 A constant current, the ideal free-air equivalent is about 3.88 ft³ and fill time is 3.00 minutes. The current-time product is 90 A·min and fixed-13.8 V arithmetic energy is 20.7 Wh.
Model Boundary
- Calculated: ideal free-air, time and electrical arithmetic from entered constants.
- Not calculated: compressor flow curve, leakage, heat, duty cycle, thermal cutoff, hose drop, tire deformation, battery sag, runtime, charging-system behavior, accuracy, equipment ratings, allowable pressure or service safety.
Frequently Asked Questions
Is average CFM predicted?
No. It is entered by the user.
Does this predict battery runtime?
No.
Why is 14.7 psi used?
It is the fixed ambient absolute-pressure reference in this simplified arithmetic.
What if final pressure is below start pressure?
The positive pressure rise and all dependent results are zero.
Does the result include heating or leakage?
No.
Does this recommend a safe tire pressure?
No; use applicable vehicle, tire and service information.
References
- NASA Glenn, Equation of State (Ideal Gas): pV=nRT basis for the constant-volume arithmetic.
- U.S. Department of Energy, Improving Compressed Air System Performance: receiver fill/restoration relation using volume, pressure difference, atmospheric pressure and free-air capacity.
- OSHA 29 CFR 1910.177: servicing safety requirements for covered rim wheels; the calculator is not a service procedure.
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