How to Read a Pressure-Temperature Chart

Why this matters

A pressure-temperature chart is the most-consulted piece of paper in the trades and one of the most misread. Techs treat it as a list of correct pressures, which it is not. It is a conversion table, and the conversion is only the first half of a diagnosis. Read it as a lookup and you will condemn a good compressor, add charge to a system that is already overfull, or chase an airflow problem that never existed. Read it as a converter and the same two numbers that fooled the last tech will tell you which of two opposite faults you are standing in front of.

Before you take the reading

Any reading that requires you to open a cabinet exposing a rotating fan comes after you open the disconnect, apply your own lock and tag, and confirm the wheel has coasted to a stop. Mechanical isolation and stored energy on that equipment is 29 CFR 1910.147, and a coasting condenser fan is exactly the stored energy the standard is about.

Connecting to a pressurized circuit is its own hazard. Wear eye protection and gloves rated for the fluid, keep the hose ends pointed away from your face, and never crack a fitting on a system you have not verified is at a known state. On a steam or hot-water system, isolate the section and let it cool before you break a joint, because saturated liquid at pressure flashes to vapor the instant it sees atmosphere. If the reading forces you to open a refrigerant circuit at all, recover the charge into a recovery cylinder rather than releasing it: 40 CFR Part 82, Subpart F prohibits knowingly venting refrigerant during service, maintenance, repair, or disposal.

What the chart is actually telling you

A pressure-temperature chart plots the saturation curve of one specific fluid. Saturation is the condition where liquid and vapor coexist. At that condition, pressure and temperature are locked together: fix one and the other is determined. That is the entire content of the chart.

So the chart answers exactly one question. "For this fluid, at this pressure, what temperature is the boiling or condensing happening at?" It does not know your equipment, your ambient, your metering device, or whether the charge is right. It cannot tell you a pressure is wrong, because there is no such thing as a wrong pressure in isolation.

The same physics runs a steam table, the vapor pressure of liquefied petroleum gas in a tank, and the refrigerant chart in your pocket. A propane tank sitting at a given ambient sits at the vapor pressure for that ambient, and a tank that reads far off that value is either not full of what the label says or is not at the temperature you assume.

Fix these four things before you look anything up

Every misread I have seen traced back to one of these, decided wrong before the reading even started.

1. Which fluid. The chart column is fluid-specific and the columns are not close to each other. Confirm the fluid from the equipment nameplate or the service label, not from what the last tech told you and not from the color of the cylinder.
2. Gauge or absolute. A field gauge reads pressure above local atmosphere. Most service charts are printed in gauge units to match. A chart printed in absolute units sits about 15 units higher at sea level for the same condition, which is enough to move a saturation temperature by a meaningful amount at low pressures. Check the column header, every time.
3. Dew point or bubble point. A zeotropic blend does not boil at one temperature. It boils across a range, called glide, so it has two saturation temperatures at any given pressure: bubble point (where the last liquid is about to boil) and dew point (where the last vapor is about to condense). Superheat is measured from the dew point, subcooling from the bubble point. Using the wrong one shifts your answer by the full glide, which runs from under 1 degree F on a near-azeotropic blend to roughly 10 degrees F on a high-glide one.
4. Where the temperature probe actually is. The chart gives you a saturation temperature inside the circuit. Your comparison is a surface temperature on a line. A probe that is not clamped tight, not insulated from the surrounding air, or sitting on a fitting rather than the tube reads the room as much as the fluid, and the error lands directly in your calculated difference.

The rule, stated the way you have to use it

Per circuit, at one operating condition, hold the reading at steady state for at least 10 minutes: the chart converts each pressure to its saturation temperature, and the diagnosis is the difference between that saturation temperature and the measured line temperature at the same point. Two differences, both required, both compared against the band the equipment documentation gives for that metering type:

• Superheat is the low-side line temperature minus the low-side saturation temperature. It tells you how much vapor-only line you have past the point where boiling finished.
• Subcooling is the high-side saturation temperature minus the liquid line temperature. It tells you how much liquid-only line you have past the point where condensing finished.

Neither number alone decides anything. It is the pair that names the fault, and either one read alone will point you at the wrong repair about as often as the right one.

Case one: high superheat, low subcooling

Illustrative values on a fixed-orifice system with a documented band of 8 to 12 degrees F for both.

The low-side pressure converts on the chart to a saturation temperature of 40 degrees F. The clamped suction line probe reads 62 degrees F. Superheat is 62 minus 40, or 22 degrees F. Against a band whose top is 12, that is 10 degrees F high.

The high-side pressure converts to a saturation temperature of 105 degrees F. The liquid line reads 99 degrees F. Subcooling is 105 minus 99, or 6 degrees F. Against a band whose bottom is 8, that is 2 degrees F low.

High superheat says the evaporator ran out of liquid early, so the last stretch of coil was doing nothing but warming vapor. Low subcooling says the condenser had no reservoir of liquid to hold back. Both point the same way: there is not enough refrigerant in the circuit, either because it leaked out or because something upstream of the metering device is restricting how much can get through. That fork is the next test, not this one, and a leak search and a restriction check are different tests.

Case two: the same method, the opposite call

Same chart, same probes, same 8 to 12 degrees F band, different machine.

Low-side saturation converts to 40 degrees F, the suction line reads 48, so superheat is 8 degrees F, sitting right on the bottom edge of the band and technically inside it. High-side saturation converts to 120 degrees F, the liquid line reads 100, so subcooling is 20 degrees F, which is 8 degrees F above the top of the band.

Note what did not happen. The superheat number did not go out of band, so a tech reading only superheat would have reported the system in spec and gone looking somewhere else. The pair is what tells the story: liquid is stacking up in the condenser instead of leaving it. That is a system carrying more refrigerant than it has room for, or a condenser that cannot reject heat fast enough because airflow is blocked or a noncondensable is taking up volume. Every one of those raises high-side saturation, and the raised saturation temperature is the reading that told you, not the pressure itself.

The same conversion, the same two subtractions, and the correct action in case one is the exact opposite of the correct action in case two. That is why the chart cannot be a list of right pressures.

When the chart itself is the problem

• The chart is for a fluid the system no longer contains. A retrofit, a topped-off mixed charge, or a mislabeled cylinder puts a fluid in the circuit that no chart column describes. A mixed charge does not follow any published saturation curve, and every number you calculate from it is fiction. The tell is a pair of differences that will not agree with any single fault, and the fix is recovery and a clean weighed-in charge, not more reading.
• The gauge is out. A gauge that reads a couple of units off shifts your saturation temperature by a few degrees, and a few degrees is the whole width of some bands. Check a gauge set against a known reference at least seasonally and any time it has been dropped.
• The printed chart is stale. New fluids get added and older columns get reprinted with better data. A pocket card that predates a fluid simply will not have it, and a tech who picks the closest-looking column has invented a reading.
• The equipment documentation, not the chart, sets the band. The 8 to 12 degrees F used above is illustrative. Target superheat on a fixed-orifice system is a function of indoor wet bulb and outdoor dry bulb and is looked up on the equipment's own charging chart; a thermostatic or electronic expansion valve holds superheat near its own setpoint, so on those you diagnose primarily on subcooling. Applying a fixed-orifice band to a valve-metered system, or the reverse, produces a confident and wrong answer.

How to verify you read it right

Before you act on the pair, run these four checks in this order.

1. Re-read the column header. Fluid name, gauge versus absolute, and bubble versus dew. Most bad readings die here.
2. Sanity-check saturation against ambient. High-side saturation temperature should land above the air going into the condenser, and low-side saturation should land below the air going into the evaporator. If either is on the wrong side of its air, you have read the wrong column or the wrong gauge port.
3. Move the probe and re-read. Slide the clamp a foot along the same line, re-insulate, and let it settle. A shift of more than about 2 degrees F means the probe was reading air, not fluid, and the difference you calculated is not real.
4. State the pair out loud with both numbers and their band. "Superheat 22 against 8 to 12, subcooling 6 against 8 to 12." If you cannot say both against the same band in one sentence, you have not finished the reading, and a tech who reports only the number that was out of band has handed the next person half a diagnosis.

References

• 40 CFR Part 82, Subpart F, prohibition on knowingly venting refrigerant during service, maintenance, repair, or disposal
• 29 CFR 1910.147, control of hazardous energy, for isolating a fan or compressor before opening a cabinet
• ASHRAE Handbook, Fundamentals volume, thermophysical property data for refrigerants and steam
• Equipment manufacturer documentation for the charging chart and target superheat or subcooling band
• See related: Refrigerant Pressure-Temperature Quick Reference; Reading a Nameplate: What It Tells You