How to read a PT chart

Reading saturation temperature from a gauge

A PT chart lists, for each temperature, the pressure at which that refrigerant boils or condenses. To use it at the unit:

1. Read the gauge in psig on the port you care about: suction (vapor line) for the evaporator, liquid line for the condenser.
2. Find that pressure in the chart column for the refrigerant on the nameplate.
3. Read the temperature in the same row. That is the saturation temperature, the temperature at which the refrigerant is changing phase at that pressure.

Example from the R-410A data on this site: 118.4 psig on the suction is a 40°F dew point. The PT calculator does the reverse lookup and interpolates between rows, so you do not have to.

The saturation temperature is not the line temperature. Superheat is the measured suction line temperature minus the saturation temperature. Subcooling is the saturation temperature minus the measured liquid line temperature. The superheat and subcooling calculators take both readings.

Bubble vs dew: which column on a blend

Single-component refrigerants (R-22, R-32, R-134a) and azeotropes boil at one temperature for a given pressure, so their charts have one column. Zeotropic blends such as R-454B, R-407C and R-448A boil over a range. The bubble point is the temperature where liquid starts to boil. The dew point is where the last liquid evaporates. The difference at one pressure is the temperature glide.

The rule: dew for superheat, bubble for subcooling.

• At the evaporator outlet, the refrigerant has just finished boiling, so the suction pressure maps to the dew point.
• At the condenser outlet, the vapor has just finished condensing, so the liquid pressure maps to the bubble point. Rheem’s R-454B installation instructions tell you to take the measured liquid pressure and find the “bubble point saturation temperature” for the subcooling calculation.

Chemours’ pocket PT guide follows the same logic in a single column: for blends, values at 50°F and below are saturated vapor (dew), and values at 52°F and above are saturated liquid (bubble).

What a wrong column costs, using the R-454B data on this site:

On R-410A the glide is only about 0.2°F, so the columns nearly match. On R-454B it is about 2.7°F across the normal range. The temperature glide calculator shows the glide at any pressure, and every profile (for example R-454B) prints both columns per row.

Gauge pressure, absolute pressure and vacuum

Gauges read psig: pressure above the surrounding air. Absolute pressure (psia) is measured from a perfect vacuum. psia = psig + local atmospheric pressure, which is 14.696 psia at sea level.

When a refrigerant’s saturation pressure is below atmospheric, the gauge goes below zero and charts switch to inches of mercury (in Hg) vacuum. On this site’s R-134a chart, -20°F is 3.7 in Hg vacuum and -40°F is 14.8 in Hg vacuum. Low-pressure chiller refrigerants live there in normal operation: R-123 at a 40°F evaporator is 18.2 in Hg vacuum at sea level.

In Hg vacuum is a gauge unit, so it shifts with elevation too. Microns are absolute, which is why evacuation is measured in microns (see evacuation and microns). Metric charts differ silently: some print kPa gauge, others kPa absolute. This site prints kPa absolute. Check the header before you compare charts.

Elevation: why a sea-level chart reads wrong in the mountains

PT charts in psig assume sea-level air (14.696 psia); the Chemours guide states its psig values are at sea level. Refrigerant inside the system does not care about the air outside; its saturation depends on absolute pressure. Your gauge, though, measures against local air.

From the standard atmosphere, local air pressure at 5,000 ft is about 12.23 psia, roughly 2.5 psi less than at sea level. So the same refrigerant state reads about 2.5 psi higher on the gauge at 5,000 ft:

• R-410A at a 40°F dew point: 118.35 psig at sea level is 133.05 psia. At 5,000 ft the gauge shows 133.05 − 12.23 = 120.8 psig. A sea-level chart puts 120.8 psig at about 41°F, so you read the evaporator about 1°F warm.
• R-134a at 40°F: 35.05 psig at sea level reads about 37.5 psig at 5,000 ft. A sea-level chart puts that at about 42.5°F.
• R-123 at 40°F: 18.2 in Hg vacuum at sea level reads about 13.1 in Hg at 5,000 ft. A sea-level chart puts 13.1 in Hg near 55°F.

The lower the operating pressure, the bigger the error. The PT calculator and pressure converter take an elevation and correct for it.

Why the chart only applies to saturated refrigerant

The PT relationship holds only where liquid and vapor exist together: inside the evaporator and condenser while boiling or condensing, or in a cylinder holding some liquid. Superheated vapor in the suction line and subcooled liquid in the liquid line are not at saturation. Their temperatures differ from the chart value, and that difference is exactly what superheat and subcooling measure.

The pressure reading is still taken from those lines because pressure is nearly the same through the coil, minus line losses. The chart converts that pressure into the temperature at which the refrigerant was changing phase inside the coil.

A system that is off and has equalized also follows the chart, but only where liquid remains. A system with little charge left may hold only vapor, and then the pressure is below the chart value for the ambient temperature.

Identifying refrigerant in a cylinder at rest

A cylinder that holds liquid, has sat long enough to match a known ambient temperature, and is not being drawn from will sit near the refrigerant’s saturation pressure at that temperature. Compare the gauge with the bubble column, since the liquid sets the pressure. From this site’s data at 75°F:

The limits are real:

• Close refrigerants overlap. R-410A and R-32 are about 4.5 psi apart at 75°F, while R-410A rises about 3.6 psi per °F between 75°F and 80°F. A cylinder temperature off by little more than 1°F covers the gap.
• Temperature must be known. A cylinder from a hot truck or in direct sun is not at ambient.
• Air and other non-condensables raise the pressure above the chart, so a contaminated or mixed cylinder can match nothing or the wrong refrigerant.
• Nearly empty cylinders may hold only vapor and read low.

A PT check can rule a refrigerant out. It cannot prove what is in the cylinder. Use the label, and for recovered or unknown refrigerant handle it as mixed until tested.

Common chart errors to watch for

Compared with the data on this site, published web charts show these problems:

• Single column for a zeotropic blend. One site gives R-454B at 40°F as 102 psig, below both the dew point (107.5) and the bubble point (113.2).
• Wrong values. One chart lists R-410A at 100°F as 331.2 psig; this site’s data gives 317.6 (dew) to 318.5 (bubble) psig. Another drifts up to about 31 psi off at higher R-410A temperatures.
• Copy-paste rows. One R-22 chart repeats the 65°F value at 70°F; this site’s data has R-22 at 70°F as 121.4 psig.
• Unlabeled kPa, gauge on one site and absolute on another.
• No altitude note, even though every psig chart assumes sea level.
• “Azeotropic” used where zeotropic is meant. Azeotropes have no glide; zeotropes need bubble and dew columns.

Every refrigerant profile on this site lists its validation against a published manufacturer or NIST chart. For R-454B, the computed values match Chemours’ XL41 data within 2.5 psi on the bubble side at 100°F and within 1 psi on the dew side. See data sources for the method.