A PT chart gives the saturation pressure of a refrigerant at each temperature. On a job you usually read it backwards. Find your gauge pressure in the right column, read across to the temperature, and you have the saturation temperature. Compare that with the line temperature and you have superheat or subcooling.
That's the whole method. Most mistakes come from reading the wrong column or the wrong refrigerant's chart.
If you'd rather skip the subtraction, the superheat and subcooling calculator takes the pressure and the line temperature and picks the column for you. The PT lookup on the home page also works backwards. Set Step 2 to Pressure and it shows the saturation temperature.
What one row tells you
Here are five rows from the R-448A PT chart, the way the site shows them:
| °F | Liquid (bubble, psig) | Vapor (dew, psig) |
|---|---|---|
| 36 | 82.1 | 65.9 |
| 38 | 85.5 | 69.0 |
| 40 | 89.0 | 72.1 |
| 42 | 92.6 | 75.3 |
| 44 | 96.3 | 78.6 |
Reference only. Check the equipment data plate and the manufacturer's charging data, and verify with your gauges.
Each row is a temperature where liquid and vapor sit together, boiling or condensing. Read the 40 °F row and it says R-448A vapor is saturated at 72.1 psig. Turn it around and a suction gauge reading 72 psig means the refrigerant in the coil is boiling at about 40 °F.
Single refrigerants such as R-22 and R-134a have one pressure per row. Blends like R-448A have two. Below 0 psig, the chart switches to inches of mercury vacuum. R-134a at -20 °F reads 3.7 inHg.
Reading backwards from a gauge
Start with R-410A on the suction side. The gauge says 118 psig. On the dew column, 40 °F is 118.4 and 38 °F is 113.9, so 118 psig lands at 39.8 °F. Your clamp on the suction line, near the evaporator outlet or the compressor, reads 52 °F. Superheat is 52 minus 39.8, or 12.2 °F.
Now the liquid side. The gauge reads 318 psig. On the bubble column, 100 °F is 318.5, so 318 psig works out to 99.9 °F. With the liquid line at 90 °F near the condenser outlet, subcooling is 99.9 minus 90. Call it 9.9 °F.
Superheat is always line temperature minus saturation. Subcooling flips it: saturation minus line temperature. Get the order wrong and you'll see a negative number.
On a blend, pick the column first
The rule is short. Use the vapor (dew) column for superheat and the liquid (bubble) column for subcooling.
On R-410A you'd barely notice a mistake. At 40 °F the two columns are 0.4 psi apart. R-448A is different. The chart lists a glide of about 10 °F at a 40 °F evaporator, and that gap shows up directly in your answer.
Take the same 72 psig suction reading and a 52 °F suction line. Read the dew column and saturation is 40.0 °F, so superheat is 12.0 °F. Read the bubble column by mistake and saturation drops to 29.8 °F, which makes superheat 22.2 °F. Same unit, same gauges, a 10 degree difference.
The liquid side goes wrong the other way. At 260 psig with a 95 °F liquid line, the bubble column gives 104.5 °F saturation and 9.5 °F of subcooling. The dew column would give 112.9 °F and 17.9 °F. Both mistakes make the system look further off than it is.
The wrong chart looks almost right
R-404A and R-448A both show up on commercial refrigeration work, and their charts aren't far apart. At 72 psig, the R-404A dew column says 31.8 °F. The R-448A dew column says 40.0 °F. With a 52 °F suction line, that's 20.2 °F of superheat on one chart and 12.0 °F on the other.
Neither number looks crazy, which is the problem. Read the refrigerant off the data plate before you pick a chart, even when you think you know what's in the system.
A chart pressure isn't a target
The 40 °F row doesn't mean the suction should sit at 72 psig. A PT chart only tells you the temperature the refrigerant is boiling or condensing at for the pressure you have. Whether 12 °F of superheat or 10 °F of subcooling is right comes from the equipment's charging chart or the manufacturer's instructions.
If you get a negative superheat or subcooling, check the readings before anything else. A probe in the wrong spot or the wrong refrigerant picked will do it.
For a sheet to keep in the van, the printable PT chart fits up to six refrigerants on one page.