A pressure-temperature chart for nine common refrigerants, with the three calculations a refrigeration mechanic does at the gauge set: superheat from the suction pressure and suction line temperature, subcooling from the liquid line pressure and temperature, and target superheat for a fixed-orifice system from indoor wet-bulb and outdoor dry-bulb. Work in °F and psig or in °C and kPa gauge. Every result shows its steps, so you can check the interpolation the way you will have to on the 313A exam, where the P/T chart is read by hand.
Pressure-temperature chart
| Temp (°F) | Saturation (psig) |
|---|---|
| -60 | 0.2 |
| -58 | 1.0 |
| -56 | 1.9 |
| -54 | 2.9 |
| -52 | 3.9 |
| -50 | 4.9 |
| -48 | 5.9 |
| -46 | 7.1 |
| -44 | 8.2 |
| -42 | 9.4 |
| -40 | 10.7 |
| -38 | 12.0 |
| -36 | 13.3 |
| -34 | 14.7 |
| -32 | 16.2 |
| -30 | 17.7 |
| -28 | 19.3 |
| -26 | 20.9 |
| -24 | 22.6 |
| -22 | 24.4 |
| -20 | 26.2 |
| -18 | 28.1 |
| -16 | 30.0 |
| -14 | 32.0 |
| -12 | 34.1 |
| -10 | 36.3 |
| -8 | 38.5 |
| -6 | 40.8 |
| -4 | 43.2 |
| -2 | 45.7 |
| 0 | 48.2 |
| 2 | 50.8 |
| 4 | 53.5 |
| 6 | 56.3 |
| 8 | 59.2 |
| 10 | 62.2 |
| 12 | 65.2 |
| 14 | 68.4 |
| 16 | 71.6 |
| 18 | 74.9 |
| 20 | 78.4 |
| 22 | 81.9 |
| 24 | 85.5 |
| 26 | 89.2 |
| 28 | 93.1 |
| 30 | 97.0 |
| 32 | 101.1 |
| 34 | 105.2 |
| 36 | 109.5 |
| 38 | 113.9 |
| 40 | 118.4 |
| 42 | 123.0 |
| 44 | 127.7 |
| 46 | 132.6 |
| 48 | 137.5 |
| 50 | 142.6 |
| 52 | 147.9 |
| 54 | 153.2 |
| 56 | 158.7 |
| 58 | 164.4 |
| 60 | 170.1 |
| 62 | 176.0 |
| 64 | 182.1 |
| 66 | 188.3 |
| 68 | 194.6 |
| 70 | 201.1 |
| 72 | 207.7 |
| 74 | 214.5 |
| 76 | 221.4 |
| 78 | 228.5 |
| 80 | 235.7 |
| 82 | 243.2 |
| 84 | 250.7 |
| 86 | 258.5 |
| 88 | 266.4 |
| 90 | 274.5 |
| 92 | 282.7 |
| 94 | 291.2 |
| 96 | 299.8 |
| 98 | 308.6 |
| 100 | 317.6 |
| 102 | 326.7 |
| 104 | 336.1 |
| 106 | 345.7 |
| 108 | 355.4 |
| 110 | 365.4 |
| 112 | 375.5 |
| 114 | 385.9 |
| 116 | 396.5 |
| 118 | 407.3 |
| 120 | 418.3 |
| 122 | 429.6 |
| 124 | 441.0 |
| 126 | 452.7 |
| 128 | 464.7 |
| 130 | 476.8 |
| 132 | 489.3 |
| 134 | 501.9 |
| 136 | 514.9 |
| 138 | 528.0 |
| 140 | 541.5 |
| 142 | 555.2 |
| 144 | 569.2 |
| 146 | 583.5 |
| 148 | 598.1 |
| 150 | 613.0 |
| 152 | 628.2 |
| 154 | 643.8 |
| 156 | 659.7 |
| 158 | 675.9 |
Gauge pressure at sea level (14.696 psia atmosphere). Bubble is the saturated liquid pressure, dew the saturated vapour pressure. For the blends, use dew for superheat and bubble for subcooling; a refrigerant shown with one column uses it for both. The find box matches the dew (vapour) column. Rows stop below the critical temperature.
Superheat
Within the typical 8 to 20 °F range for a fixed-orifice system in cooling (10 to 15 °F at a TXV outlet). Typical ranges vary by manufacturer; use the nameplate or charging chart.
Subcooling
Within the typical 8 to 15 °F range at the condenser outlet. Typical ranges vary by manufacturer; use the nameplate or charging chart.
Target superheat (fixed orifice, cooling)
Rule of thumb: target = ((3 × WB) − 80 − DB) / 2, in °F. It applies to fixed-orifice (piston, capillary) systems in cooling only. TXV systems hold superheat on their own and are checked by subcooling. Below about 5 °F the rule is not applicable.
01How to read a PT chart
A pressure-temperature chart lists, for one refrigerant, the pressure at which liquid and vapour exist together at each temperature. That is the saturation pressure, and the matching temperature is the saturation temperature. Inside an evaporator or condenser, where refrigerant is boiling or condensing, the two are locked together: read the pressure on the gauge, look it up on the chart, and you have the temperature at which the refrigerant is changing state. The chart says nothing about refrigerant that is all vapour or all liquid. That is the whole point of superheat and subcooling, which are covered below.
The chart on this page is laid out the usual way, temperature down the left and pressure across. The rows run from −60 °F in 2 °F steps up to just below the critical temperature of each refrigerant. Above the critical temperature there is no saturation pressure at all, because liquid and vapour stop being distinguishable, which is why the R-744 (carbon dioxide) table stops at 86 °F while the R-134a table runs to 200 °F. Pressures below atmospheric, a vacuum, are shown as negative psig or negative kPa gauge; on a compound gauge you would read them in inches of mercury vacuum.
Between rows, interpolate. The chart moves in 2 °F steps, and a gauge reading will usually land between two of them. Take the two rows the pressure falls between, work out how far along the gap it is, and apply that fraction to the 2 °F step. The calculators on this page show that arithmetic in full for every result. A printed chart in the exam room is read the same way, and the answer options are usually far enough apart that rounding to the nearer row is good enough, but know how to do the fraction when they are not.
For example, R-410A at 118 psig sits between the 38 °F row (113.9 psig) and the 40 °F row (118.4 psig). The reading is 4.1 psi above the lower row, out of a gap of 4.5 psi, so it is 0.91 of the way up: 38 + 0.91 × 2 = 39.8 °F. This is example 1 in the superheat section.
02Bubble point, dew point and why glide matters
A single-compound refrigerant such as R-22, R-134a, R-32, R-290 or R-744 boils and condenses at one temperature for a given pressure. So does an azeotrope, a blend whose components happen to boil together. Those refrigerants have one saturation column on the chart.
A zeotropic blend, which is what the 400-series numbers mean, does not. At a given pressure its components boil at different temperatures, so the mixture starts to boil at one temperature (the bubble point) and finishes at a higher one (the dew point). The difference is the temperature glide. On the chart the blend gets two columns: bubble, the pressure at which liquid at that temperature starts to boil, and dew, the pressure at which vapour at that temperature starts to condense. The bubble pressure is always the higher of the two at a given temperature.
How much the glide matters depends on its size. In this data R-407C has about 10.5 °F of glide, so at 40 °F its bubble pressure is 80.2 psig and its dew pressure 63.2 psig, a 17 psi gap. R-404A has under 1 °F of glide (86.9 against 85.4 psig at 40 °F), and R-410A about 0.2 °F (118.8 against 118.4 psig), which is why R-410A is treated as an azeotrope in practice and most charts print a single column for it. This chart shows both columns for R-407C and R-404A, where the glide is at or above 0.5 °F, and one column for the rest. For R-410A that single column is the dew pressure; the bubble pressure is at most about 1 psi higher, which is 0.2 °F, and the calculators use the single column for both superheat and subcooling on those refrigerants, as a single-column chart would.
The rule for blends is short: superheat uses the dew column, because superheat is measured on vapour that has finished boiling, and subcooling uses the bubble column, because subcooling is measured on liquid that has finished condensing. Get the column wrong on R-407C and the answer is off by the glide, about 11 °F, which is more than the superheat you are trying to measure. Example 3 below shows the size of that error with numbers.
Glide has a practical consequence beyond the chart. Because the components of a zeotrope boil at different rates, vapour leaving a cylinder of R-407C or R-404A is not the same mixture as the liquid in it. That is why blends are charged as liquid from the cylinder, a point the 313A standard lists under completing the system charge.
03Gauge pressure and absolute pressure
Every pressure on this chart is gauge pressure: what a gauge reads, which is the pressure above the atmosphere around it. Absolute pressure counts from a perfect vacuum, so at sea level absolute = gauge + 14.696 psi. The 313A formula sheet provided in the exam room gives it as psia = psig + 14.7, and that is the figure to use on the exam. In metric, standard atmospheric pressure is 101.325 kPa, so kPa absolute = kPa gauge + 101.3.
The distinction matters in two places. The first is the vacuum side of the chart. R-134a at −20 °F saturates at −1.8 psig, which is 12.9 psia; a low-temperature R-134a system running in a vacuum is the classic case where a technician reads the gauge in inches of mercury and has to convert. The second is any formula that uses pressure as a ratio, such as compression ratio, which is discharge absolute divided by suction absolute. Divide gauge pressures and the ratio is wrong.
The chart does not correct for altitude. A gauge zeroed at sea level reads about 2.5 psi high in Calgary relative to the true gauge pressure there, because the atmosphere is thinner, but the saturation pressure inside the system is an absolute quantity and does not change. For the accuracy this chart is used at, the correction is ignored at ordinary elevations; the exam does not expect you to make it unless the question gives an atmospheric pressure.
04How superheat is measured and why
Superheat is the number of degrees a vapour is above its saturation temperature at the pressure it is at. In a refrigeration system it is measured at the evaporator outlet (or, for compressor protection, at the compressor inlet) with two readings: the suction pressure from the low-side gauge, and the suction line temperature from a probe clamped and insulated on the pipe. Convert the pressure to a saturation temperature on the chart, then subtract that from the line temperature. Superheat = line temperature − saturation temperature.
It tells you two things. First, that the refrigerant leaving the evaporator is all vapour: any superheat above zero means boiling finished before the outlet, so no liquid is heading for the compressor. Second, how much of the evaporator is being used. A small superheat means the coil is full of boiling refrigerant almost to the outlet, which is efficient but close to flooding; a large superheat means boiling finished early and the last part of the coil is doing little. Typical figures are 8 to 20 °F for a fixed-orifice system in cooling and 10 to 15 °F at the outlet of a TXV, but these vary by manufacturer and by application, and the nameplate or the charging chart is the authority for a particular unit.
Example 1 (R-410A, single column). A residential air conditioner on R-410A shows 118 psig on the suction gauge and 52 °F on the suction line at the outdoor unit. On the R-410A dew column, 118 psig falls between 38 °F (113.9 psig) and 40 °F (118.4 psig): 38 + (118 − 113.9) / (118.4 − 113.9) × 2 = 39.8 °F. Superheat = 52 − 39.8 = 12.2 °F. That is inside the typical fixed-orifice range, and it is the example the tool loads with.
Example 3 (R-407C, the column matters). A rooftop unit on R-407C reads 60 psig suction and 50 °F on the suction line. On the dew column, 60 psig is between 36 °F (57.5 psig) and 38 °F (60.3 psig): 36 + (60 − 57.5) / (60.3 − 57.5) × 2 = 37.8 °F, so superheat = 50 − 37.8 = 12.2 °F. Read the bubble column by mistake and 60 psig lands between 26 °F (59.1 psig) and 28 °F (61.9 psig), giving 26.6 °F and a superheat of 23.4 °F. The wrong column turns a normal reading into one that looks like a starved coil, an 11 °F error, which is the glide of the blend at that pressure.
Two measurement points to keep straight. Evaporator superheat, at the coil outlet, is what a TXV controls and what a charging chart for a fixed-orifice system refers to. Total superheat, at the compressor inlet, includes whatever heat the suction line picked up on the way and is what protects the compressor; the difference between the two is the suction line superheat. A question that says where the temperature was taken is telling you which one it wants.
05How subcooling is measured and why
Subcooling is the number of degrees a liquid is below its saturation temperature at the pressure it is at. It is measured at the condenser outlet or on the liquid line with the same two readings as superheat, taken on the other side of the system: the liquid line pressure from the high-side gauge, and the liquid line temperature from a probe on the pipe. Convert the pressure to a saturation temperature, this time on the bubble (liquid) column for a blend, and subtract the line temperature from it. Subcooling = saturation temperature − line temperature. The order is reversed from superheat because liquid is colder than saturation, not warmer.
It tells you that the refrigerant leaving the condenser is all liquid and how much liquid the condenser is holding. Zero subcooling means vapour bubbles are reaching the metering device, which then cannot meter properly. Very high subcooling means liquid is backing up into the condenser, taking up surface that should be condensing. Typical figures are in the region of 8 to 15 °F at the condenser outlet, and TXV systems are charged to a subcooling target because the valve holds superheat on its own and a superheat reading says little about the charge. As with superheat, the nameplate or charging chart is the authority for the unit in front of you.
Example 2 (R-410A). The same air conditioner as example 1 shows 340 psig on the liquid line and 95 °F at the liquid line probe. On the R-410A column, 340 psig falls between 104 °F (336.1 psig) and 106 °F (345.7 psig): 104 + (340 − 336.1) / (345.7 − 336.1) × 2 = 104.8 °F. Subcooling = 104.8 − 95 = 9.8 °F. (On the bubble pressures, which the chart does not print for R-410A, the result is 104.6 °F and 9.6 °F, a difference of 0.2 °F, which is the glide.)
Example 4 (R-134a). A medium-temperature R-134a case reads 135 psig on the liquid line and 92 °F on the pipe. 135 psig is between 104 °F (132.7 psig) and 106 °F (137.2 psig): 104 + (135 − 132.7) / (137.2 − 132.7) × 2 = 105.0 °F. Subcooling = 105.0 − 92 = 13.0 °F.
Take the liquid pressure at the condenser outlet or the liquid line service port, not the discharge port. Discharge pressure includes the pressure drop across the condenser, small on most systems but not zero, and on a long liquid line or one with a filter-drier the pressure at the metering device is lower still. The chart converts whatever pressure you give it; the reading has to belong to the place the temperature was taken.
06The target superheat method and its limits
A fixed-orifice metering device (a piston or a capillary tube) has no way to adjust its flow, so the superheat it produces depends on the load: the indoor humidity and the outdoor temperature. A superheat figure on its own therefore cannot say whether the charge is right. The target superheat method compares the measured superheat with what a correctly charged system should show under the conditions at the time. Manufacturers publish that as a charging chart; the rule of thumb behind most of them is target superheat = ((3 × indoor wet-bulb) − 80 − outdoor dry-bulb) / 2, in °F.
Indoor wet-bulb is the return-air wet-bulb temperature, read with a sling or digital psychrometer at the return grille, not the dry-bulb. Outdoor dry-bulb is the air entering the condenser. With a 64 °F return wet-bulb and 85 °F outdoors: ((3 × 64) − 80 − 85) / 2 = (192 − 165) / 2 = 13.5 °F. If the measured superheat is well above that, the coil is starved; well below, it is flooded. Charts commonly allow a few degrees either way.
The limits are important, and the calculator applies them. The rule is for fixed-orifice systems in cooling mode only. A TXV system holds its own superheat and is checked by subcooling instead; applying a target superheat to it says nothing useful. The formula also runs out at the dry, hot end: when it gives less than about 5 °F (low indoor humidity, high outdoor temperature) the charging charts mark the condition as one not to charge in, and the tool reports it as not applicable. Inputs well outside the ordinary range (indoor wet-bulb below 50 or above 76 °F, outdoor dry-bulb below 55 or above 115 °F) are also reported as not applicable. Manufacturers' charts differ from the formula by a degree or two and take precedence for their equipment.
07Working in kPa and degrees Celsius
The Red Seal exam is written in Canada and the 313A formula sheet is imperial, but a question may state its readings in either system, and gauges, nameplates and charts in the field come both ways. The conversions are exact: 1 psi = 6.894757 kPa, and °C = (°F − 32) × 5/9. The unit toggle above converts the chart and every input and output, and the steps box shows the working in the units you chose.
Two things trip people up. First, a temperature difference converts differently from a temperature. 52 °F is 11.1 °C, but a superheat of 12.2 °F is 6.8 °C, because the 32 is not subtracted from a difference: divide by 1.8 and nothing else. The tool does this correctly; a calculator conversion of the final superheat figure will not. Second, kPa gauge and kPa absolute differ by 101.3, just as psig and psia differ by 14.7. Metric gauges read kPa gauge.
Example 5 (R-410A, metric). A heat pump in cooling reads 814 kPa gauge on the suction side and 12 °C on the suction line. 814 kPa is 814 / 6.894757 = 118.1 psig, which the dew column puts at 39.8 °F, or 4.4 °C. Superheat = 12 − 4.4 = 7.6 °C, which is 13.8 °F. Switch the tool to metric and enter those figures to see the same working.
08Common mistakes
Wrong column on a blend. Superheat from the bubble column or subcooling from the dew column. On R-407C the error is about 11 °F, enough to turn a correct reading into a wrong diagnosis (example 3). On R-404A it is under 1 °F and on R-410A about 0.2 °F, which is why many charts print one column for them, but the exam can still ask which column applies.
Gauge and absolute mixed. Reading psia off a chart printed in psig, or dividing gauge pressures for a compression ratio. The chart is gauge; add 14.7 when a formula needs absolute.
Temperature taken in the wrong place. A suction line probe next to the compressor gives total superheat, not evaporator superheat, and one taken downstream of a suction accumulator or a heat exchanger gives neither. A liquid line probe on an uninsulated pipe in the sun, or a probe not strapped tight and insulated, reads the air rather than the refrigerant. The chart cannot fix a temperature that does not belong to the pressure.
Subtracting the wrong way. Superheat is line temperature minus saturation temperature; subcooling is saturation temperature minus line temperature. A negative result means the order is reversed, the wrong column was used, or the reading is wrong.
Units mixed. A kPa gauge read against a psig chart, or a superheat in °F converted to °C by the full formula instead of dividing by 1.8. Check that pressure, temperature and chart are all in one system before you start.
Charging a fixed-orifice system to subcooling, or a TXV system to superheat. Each metering device is checked the other way round, and the target superheat rule applies only to the fixed-orifice case in cooling.
Reading a system that has not settled. Pressures and temperatures after start-up or a change in load move for ten minutes or more. The chart converts whatever it is given; the numbers have to be steady first.
09What the 313A exam expects
The Red Seal exam for Refrigeration and Air Conditioning Mechanic has 125 questions, and the Red Seal Program's published breakdown puts 28 of them in Block F, maintenance and service, 26 in Block D, installation, and 21 in Block E, commissioning. Our 313A exam guide works through what each block covers. Commissioning includes completing the system charge and setting up primary components, and maintenance and service includes troubleshooting from a set of readings. Reading a P/T chart, taking superheat and subcooling, and knowing which one to charge to for which metering device sit inside those tasks, so expect them to come up more than once, as procedure questions (do the calculation) and as critical thinking questions (here are the readings, what is the cause).
The exam provides a formula sheet and an acronym list. The sheet includes psia = psig + 14.7 and the kPa-to-psi conversion, and the acronym list includes P/T, SST, psia, psig and TXV, so the terms in this page are the ones the questions use. A P/T chart, when a question needs one, is given with the question. Practise the interpolation by hand: the tool shows the steps so you can check yours.
Typical questions in this area give a refrigerant, a gauge pressure and a line temperature and ask for the superheat or subcooling; give a set of readings and ask which is consistent with an overcharge, an undercharge, a restricted metering device or a dirty condenser; or ask which P/T column, which charging method or which charging state (liquid or vapour) applies to a named blend. The worked examples above are labelled as examples, not as exam questions, but they are the same arithmetic. Our free 313A sample questions include this material with explanations.
10Sources
- CoolProp: open-source thermophysical property library the pressure-temperature data on this page was computed with CoolProp 8, which implements the NIST-based Helmholtz equations of state for each refrigerant and the standard mixture models for the blends. Gauge pressure = absolute − 14.696 psia. The table is computed, not copied from any manufacturer's chart; a manufacturer's chart may differ from it by a psi or two because of the reference equations, the rounding and the blend composition tolerance it uses.
- Red Seal Program: Refrigeration and Air Conditioning Mechanic trade page the Red Seal Occupational Standard the current exam is based on, including the commissioning and maintenance tasks that cover charging, superheat and subcooling.
- Red Seal Program: Refrigeration and Air Conditioning Mechanic exam information the 125-question breakdown by block, the formula sheet (including psia = psig + 14.7) and the acronym list provided at the sitting.
- TicketPrep: what is on the 313A Refrigeration and Air Conditioning Mechanic Red Seal exam our guide to the six blocks, the question counts and where superheat, subcooling and charging fall in the standard.
11Questions people ask
- How do you calculate superheat from a PT chart?
- Read the suction pressure on the low-side gauge and the suction line temperature with a clamped, insulated probe. Find the pressure on the chart (the dew or vapour column for a blend) and read the saturation temperature, interpolating between rows if needed. Superheat is the line temperature minus that saturation temperature. For example, R-410A at 118 psig has a saturation temperature of 39.8 °F, so a 52 °F line gives 12.2 °F of superheat.
- How do you calculate subcooling from a PT chart?
- Read the liquid line pressure on the high-side gauge and the liquid line temperature at the condenser outlet. Find the pressure on the chart (the bubble or liquid column for a blend) and read the saturation temperature. Subcooling is that saturation temperature minus the line temperature. For example, R-410A at 340 psig saturates at 104.8 °F, so a 95 °F liquid line gives 9.8 °F of subcooling.
- Which column do you use for R-410A, bubble or dew?
- Either, in practice. R-410A is a zeotropic blend but its glide is about 0.2 °F, so the bubble and dew pressures differ by less than half a psi and most charts print one column. Strictly, superheat uses the dew (vapour) pressure and subcooling the bubble (liquid) pressure, and that rule matters for R-407C, where the glide is about 10 °F.
- What is a normal superheat and subcooling?
- Typical figures are 8 to 20 °F of superheat for a fixed-orifice system in cooling, 10 to 15 °F at the outlet of a TXV, and 8 to 15 °F of subcooling at the condenser outlet. These vary by manufacturer, by application and by conditions, so the nameplate or the charging chart for the unit is the authority. A fixed-orifice system is charged by comparing measured superheat with a target from indoor wet-bulb and outdoor dry-bulb; a TXV system is charged by subcooling.
- Is the PT chart in psig or psia?
- Gauge pressure, psig (or kPa gauge in metric), which is what a gauge reads. Absolute pressure is gauge plus atmospheric: at sea level psia = psig + 14.7, the figure on the 313A formula sheet, or kPa absolute = kPa gauge + 101.3. Use absolute pressures for a compression ratio or any other ratio of pressures. Readings below atmospheric appear as negative gauge pressures on this chart and as inches of mercury vacuum on a compound gauge.
12Practice for this exam
Superheat, subcooling and the P/T chart are procedure questions on the 313A exam, and procedure questions are more than half the paper. TicketPrep's 313A practice test follows the Red Seal Program's published block weights, so commissioning and maintenance get the share of questions they get on the day, and every answer is explained, including the chart column and the arithmetic. Try the free questions first.
Ten free questions per trade, no account: free practice questions. How our questions are written and checked: how our questions are made.