How to Read a PT Chart
A pressure reading is only half the story. The PT chart is what turns that number into a saturation temperature you can actually diagnose with. Here is how to read one fast, for any refrigerant on the truck.
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In This Guide
What a PT Chart Actually Tells You
A pressure-temperature (PT) chart lists the exact relationship between a refrigerant's pressure and its saturation temperature— the temperature at which that refrigerant is boiling or condensing at a given pressure. As long as liquid and vapor coexist in the coil, pressure and temperature are locked together. Know one, and the chart gives you the other.
That saturation temperature is the single most useful number in a refrigerant-side diagnosis. It is the reference point for two of the most important measurements you take on any system:
- Superheat — how many degrees the suction vapor sits above the evaporator saturation temperature. It confirms the coil is fully boiling off liquid before it reaches the compressor.
- Subcooling — how many degrees the liquid line sits below the condenser saturation temperature. On a TXV system it is your primary charging target.
The Core Relationship
Gauge pressure (psig) → PT chart → Saturation temperature (°F)
Every refrigerant has its own curve. There is no universal PT chart — R-410A at 118 psig is 40°F, but R-22 at 118 psig is roughly 65°F. Always match the chart to the nameplate.
psig vs. psia: Read the Right Number
Standard field PT charts are printed in psig (pounds per square inch gauge), and every analog and digital manifold you own reads in psig. Gauge pressure is referenced to atmospheric pressure — a gauge reading zero means the system is at atmospheric, not in a vacuum.
You only need psia (absolute) for a handful of calculations — most commonly compression ratio, where you add 14.7 to both the suction and discharge gauge readings before dividing. For a straight PT lookup, stay in psig. Mixing the two up will throw your saturation temperature off by a large margin.
Quick reminder
psia = psig + 14.7
Use psig for PT chart lookups. Convert to psia only for compression ratio and similar absolute-pressure math.
Bubble Point vs. Dew Point (Blends)
Open a modern PT chart and many refrigerants show two temperature columns for each pressure: a bubble point (liquid) and a dew point (vapor). This is because most current refrigerants are zeotropic blends — mixtures whose components boil at slightly different temperatures. As the blend changes phase, its saturation temperature drifts. That drift is called temperature glide.
Getting the column right is where most techs go wrong. The rule is simple once you anchor it to what you are measuring:
Superheat → Dew Point
Superheat is measured where refrigerant is fully vapor (suction line). Use the dew point column at your low-side pressure for the evaporator saturation temperature.
Subcooling → Bubble Point
Subcooling is measured where refrigerant is fully liquid (liquid line). Use the bubble point column at your high-side pressure for the condenser saturation temperature.
Glide varies a lot by refrigerant
R-410A and R-454B are near-azeotropic — their glide is only a fraction of a degree, so bubble and dew are practically the same number. But a blend like R-407C glides roughly 10°F. On a high-glide refrigerant, using the wrong column can swing your superheat or subcooling by 5°F or more and send you chasing a charge problem that isn't there.
Reading the Chart, Step by Step
Whether you use a laminated card, a slide chart, or an app, the sequence is the same:
Step 1 — Confirm the refrigerant.
Read it off the nameplate. Never assume — a system that looks like an R-410A unit may have been converted, and post-2025 equipment increasingly ships with R-454B or R-32.
Step 2 — Read the gauge in psig.
Let the reading stabilize. Suction (low side) for evaporator saturation, liquid/discharge (high side) for condenser saturation.
Step 3 — Pick the correct column.
Dew point for low-side/superheat, bubble point for high-side/subcooling.
Step 4 — Find the pressure row and read across.
If your pressure lands between two rows, interpolate. Example: if 40°F is 118 psig and 45°F is 130 psig, then 124 psig is about halfway — roughly 42.5°F.
Step 5 — Subtract to finish the measurement.
Superheat = suction line temp − evap saturation. Subcooling = condenser saturation − liquid line temp.
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Sample PT Chart: R-410A, R-22, R-454B
Below are the pressures (psig) that correspond to the saturation temperatures techs look up most. R-410A and R-454B numbers are near-identical — part of why R-454B is the designated R-410A replacement — while R-22 runs at much lower pressures. Values are rounded to whole psig for field use.
| Sat. Temp (°F) | R-410A (psig) | R-454B (psig) | R-22 (psig) |
|---|---|---|---|
| 32 | 101 | 99 | 58 |
| 40 | 118 | 116 | 69 |
| 45 | 130 | 128 | 76 |
| 50 | 142 | 140 | 84 |
| 95 | 296 | 293 | 182 |
| 100 | 317 | 314 | 196 |
| 110 | 365 | 361 | 226 |
| 120 | 418 | 414 | 260 |
A quick sanity check for R-410A: suction around 118–135 psig on a mild day puts the evaporator near 40–46°F, and a head pressure near 400 psig puts the condenser around 118°F. If your readings are wildly off these, suspect airflow, charge, or a restriction before you trust the chart.
Worked Example: Superheat & Subcooling
Scenario
A residential R-410A split system with a TXV. You measure a suction pressure of 118 psig with a suction line temperature of 52°F, and a liquid line pressure of 365 psig with a liquid line temperature of 98°F. Is the charge good?
Step 1 — Evaporator saturation from the low side.
118 psig on the R-410A dew point column → 40°F saturation.
Step 2 — Calculate superheat.
SH = 52°F − 40°F = 12°F
Step 3 — Condenser saturation from the high side.
365 psig on the R-410A bubble point column → 110°F saturation.
Step 4 — Calculate subcooling.
SC = 110°F − 98°F = 12°F
Result: Both in target range
A TXV system typically wants 8–14°F superheat and 10–18°F subcooling (always confirm against the manufacturer's charging chart). At 12°F and 12°F, this system is properly charged and metering correctly. None of it is possible without first converting those two pressures to saturation temperatures.
What the numbers would tell you if they were off
Low subcooling (say 4°F) with high superheat points to an undercharge or a liquid-line restriction. High subcooling (over 18°F) with low superheat points to an overcharge. The saturation temperatures are what let you read that story.
Field Tips & Common Mistakes
- Wrong chart, wrong answer. The most common error is grabbing the R-410A chart out of habit on an R-22 or R-454B system. Match the nameplate every time.
- Bubble/dew mix-ups on blends. On low-glide refrigerants it barely matters; on R-407C and similar high-glide blends it is a real error. Dew for superheat, bubble for subcooling.
- Don't skip interpolation. Reading to the nearest printed row can cost you a few degrees. On a tight superheat target, that can be the difference between "add charge" and "leave it alone."
- Let pressures stabilize. Take your reading after the system has run long enough to settle — typically 10–15 minutes — or your saturation temp is chasing a moving target.
- Handle A2L refrigerants correctly. R-454B and R-32 are mildly flammable (A2L). Follow ASHRAE 15 practices, use A2L-rated tools and leak detection, and keep ignition sources away when connecting gauges.
Pro tip: memorize a couple of anchors
Good techs carry a few R-410A reference points in their head — 118 psig is 40°F, and about 365 psig is 110°F. Anchoring on those two lets you gut-check a system in seconds before you ever pull out a chart or an app.
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