Pattern
Generic reference ready
Enter one or both gauge readings to compare them with the reference envelope.
- Use the correct test state and refrigerant.
- Measure ambient at the equipment or vehicle.
AC pressure chart · live field reference
Check the pressure pair, not one number.
Choose car A/C or home HVAC, set the refrigerant and ambient temperature, then compare low- and high-side readings with a transparent reference method.
Input panel
System conditions
Live result
Pressure relationship
REFERENCE
Expected low
45–55 psig
Expected high
225–250 psig
Open measured saturation
Superheat
—
dew saturation
Subcooling
—
bubble saturation
Limits & safety3 notes
P–T ≠ running pressure clearly separated
No extrapolation outside reviewed car range
Device-first inputs stay local
Bubble + dew handled for blends
Gauge pattern matrix
Two readings tell a better story.
These patterns narrow the next checks; they do not identify one failed part or prove the charge. Select a row to load a representative sample into the live checker.
Pressure–temperature explorer
Move both directions on the P–T curve.
Convert saturation temperature to gauge pressure, or pressure back to saturation temperature. Blends expose bubble and dew instead of hiding glide.
Dew pressure at 70°F
201.1 psig
Bubble 201.8 psig
Dew 201.1 psig
Difference 0.7 psi
R-410A saturation curve
Bubble Dew
Open full 5°F pressure chart R-410A · °F / psig
| Temperature | Bubble | Dew | Difference |
|---|
Use the right number for the right job
Saturation pressure is not running pressure.
A P–T chart describes phase equilibrium. A running system adds compressor ratio, load, airflow, heat exchange, metering, and control behavior.
1Compressorraises pressure
2Condenserrejects heat
3Metering devicedrops pressure
4Evaporatorabsorbs heat
Line temperature − dew saturation
Use suction pressure to find the dew saturation temperature, then subtract it from the measured suction-line temperature.
SH = Tsuction line − Tdew @ suction pressure
Bubble saturation − line temperature
Use liquid-side pressure to find the bubble saturation temperature, then subtract the measured liquid-line temperature.
SC = Tbubble @ liquid pressure − Tliquid line
Targets depend on equipment, metering device, load, and manufacturer procedure. This site calculates the values; it does not invent a universal target.
Method, limits, and provenance
Every range shows where it came from.
The site deliberately separates thermodynamic P–T data, generic operating references, and equipment-specific targets.
Jump to sourcesNormalize units
Inputs convert to °F and PSIG internally so every UI, table, test, and API response uses one canonical calculation path.
Build the reference
Car mode interpolates only inside the reviewed 65–105°F chart. Home mode converts editable evaporating and condensing saturation bands through the selected refrigerant curves.
Apply local gauge context
When elevation is entered, P–T-derived gauge values use a standard-atmosphere estimate. The separate automotive operating chart is deliberately left unchanged.
Compare the pair
Each side is marked below, within, or above. The combined pattern suggests checks, while explicitly preserving uncertainty.
Calculate thermal values
Dew is used for superheat and bubble for subcooling. Optional OEM targets are user supplied and shown only as signed deltas.
Open method
Sources, data version, and API.
Pressure values, assumptions, and machine-readable endpoints are visible so the result can be checked instead of merely trusted.
HVAC PT Charts / CoolProp 7.2.0
Reviewed 5°F checkpoints from published saturation tables. R-407C and R-454B preserve bubble and dew values. Licensed CC BY 4.0.
Open source siteMastercool R-134a / R-1234yf gauge reference
Generic 65–105°F running-pressure bands. The manufacturer instruction explicitly directs users to vehicle data for exact information.
Open manufacturer pageNASA Glenn Earth atmosphere equation
Local atmospheric pressure is estimated only within −1,000 to 15,000 ft, then used to translate P–T absolute pressure into a local gauge reading. It is not live weather or an instrument-calibration service.
Open atmosphere methodData reviewed August 6, 2026
Engine 1.1.3 · PT dataset 2026.08.06-coolprop-7.2-5f. No external request is needed to calculate a result.
READ-ONLY JSON API
Use the same checked engine from a script.
No key is required for deterministic GET requests. Responses include units, assumptions, source metadata, and structured validation errors.
/api/v1/check?mode=home&refrigerant=r410a&ambient=95&low=120&high=400
Common pressure-chart questions
What the chart can—and cannot—answer.
What should car A/C pressure be?
It depends on refrigerant, outside temperature, test procedure, compressor/control design, humidity, airflow, and vehicle specifications. The car mode supplies a generic 65–105°F reference for R-134a and R-1234yf, then labels readings as below, within, or above that band.
Can static pressure tell me whether the system is fully charged?
No. After equalization, static pressure mainly tracks refrigerant saturation temperature. A system with very different refrigerant quantities can show a similar static pressure as long as both liquid and vapor remain. Use leak testing, charge-by-weight/OEM procedures, and operating performance checks.
Why are R-407C bubble and dew pressures so different?
R-407C is a zeotropic blend with meaningful temperature glide. Use the dew curve for evaporator/superheat calculations and the bubble curve for condenser/subcooling calculations. Using the wrong curve can materially distort the result.
Is an “inside the range” result proof that the charge is correct?
No. It means the pressure pair is plausible for the selected generic reference. Cooling output, airflow, line temperatures, compressor control, and manufacturer targets still matter.
Why does the home HVAC range change when I edit the advanced assumptions?
The home range is computed from an evaporating saturation band and a condensing-temperature split above ambient. Those are transparent assumptions, not hidden universal targets. Equipment-specific charging data remains authoritative.
Why does elevation change P–T gauge pressure?
A gauge reads pressure relative to local atmospheric pressure. At higher elevation, atmospheric pressure is lower, so the same refrigerant saturation absolute pressure appears as a higher local gauge reading. The correction applies to P–T-derived values, not the separate generic automotive running chart.