What a PT chart shows
A pressure temperature (PT) chart lists the saturation pressure of a refrigerant at each temperature. ASHRAE defines saturation pressure as the pressure at which vapor and liquid exist in equilibrium at a given temperature. Inside an evaporator or condenser the refrigerant is boiling or condensing, so the DOE Building America diagnostics guide reads the evaporator saturation temperature from the low side gauge and the condenser saturation temperature from the high side gauge. R-410A boiling at 40°F sits at 118.8 psig; condensing at 100°F it sits at 318.5 psig.
The chart only describes saturated refrigerant. ASHRAE defines superheat as how far a vapor is above its dew point and subcooling as how far a liquid is below its bubble point, both at the same pressure. Comparing a line temperature with the chart value is how you measure both; the superheat and subcooling calculators do that step for you.
How to pick the right chart
- Read the rating plate. Payne's installation instructions say to check the outdoor unit rating plate to determine the proper refrigerant before charging. If a system was converted, look for a retrofit label: Chemours' R-407C retrofit guidelines tell the installer to label the system with the refrigerant and lubricant now in it.
- Do not identify a refrigerant by its pressure. At 40°F, R-410A reads 118.8 psig and R-32 reads 121.0 psig. That gap is inside the plus or minus 3.5 psi accuracy California's Title 24 charge test requires of a digital suction gauge. R-454B is lower, at 107.0 psig on its dew line, but the DOE guide's minimum evaporator temperatures run from 35 to 45°F depending on indoor wet bulb, which on R-410A alone spans 107.3 psig to 130.1 psig. A suction reading cannot tell these refrigerants apart.
- Use the chart for the refrigerant in the system. A gauge or chart set to the wrong refrigerant gives the wrong saturation temperature, and so wrong superheat and subcooling. A reading of 118.4 psig is a 40°F dew point on R-410A but 45.3°F on R-454B.
- Check the units. Charts here read psig and °F by default. The tool converts to kPa, bar and °C.
Bubble point and dew point
ASHRAE describes a zeotropic refrigerant as a blend whose temperature changes as it evaporates or condenses at constant pressure. A single compound such as R-22, R-32 or R-134a has one saturation temperature at each pressure, so one column is enough. A zeotropic blend gets two. ASHRAE defines the bubble point as the liquid saturation temperature, where liquid refrigerant first begins to boil, and the dew point as the saturated vapor temperature. The difference between them is the temperature glide.
For R-407C at 40°F, the bubble pressure is 80.2 psig and the dew pressure is 63.2 psig. Read the dew column for suction (superheat) and the bubble column for liquid (subcooling). Chemours' PT guide marks its columns the same way: saturated vapor to calculate superheat, saturated liquid to calculate subcooling. The bubble point, dew point and glide guide explains why.
Gauge pressure vs absolute pressure
Absolute pressure (psia) is measured from a perfect vacuum. ASHRAE defines gauge pressure (psig) as absolute pressure minus the surrounding pressure, so a gauge open to air reads zero. ASHRAE gives standard sea-level atmosphere as 14.696 psi, so psia = psig + 14.7 at sea level. Every chart here is in psig referenced to sea level. Below atmospheric pressure, values are shown in inches of mercury (in Hg) of vacuum: R-134a at -20°F is 3.7 in Hg vacuum.
Altitude
A gauge measures against the local atmosphere, and the atmosphere thins with height. The U.S. Standard Atmosphere, 1976 gives 845.55 mb at 1,500 m and 840.38 mb at 1,550 m, which puts 5,000 ft (1,524 m) at about 12.2 psia, about 2.5 psi below sea level. For the same refrigerant temperature, a gauge at 5,000 ft therefore reads about 2.5 psi higher than a sea-level chart. Subtract that difference from the reading before you look it up. On the CoolProp data used here, 2.5 psi is about 1°F on an R-410A suction reading near 40°F and about 2.5°F on R-134a at the same temperature, because R-134a pressure changes less per degree.
How these charts are computed
Pressures come from CoolProp 8.0.0, an open-source property library, at every whole degree Fahrenheit, with bubble and dew points computed separately for blends. The methodology page lists the checks against manufacturer charts. The overview table below gives every refrigerant at 40°F and 100°F, and the lookup converts either way between temperature and pressure.