Refrigerant Glide, Dew Point and Bubble Point in the Field
Why this matters
On a single-component refrigerant, one pressure means one saturation temperature, and nobody has to think about which column of the chart they are reading. On a blend with real glide, the same gauge pressure has two saturation temperatures several degrees apart, and picking the wrong one moves a superheat or subcooling reading by that whole gap. The tech who reads an R-407C suction pressure against the bubble column sees superheat 11 F higher than it really is and adds charge to a system that did not need it. The tech who reads the liquid pressure against the dew column sees subcooling 8 or 9 F higher than real and pulls charge out. Both are making the same mistake from opposite ends. This card is about reading the right column, and about the handful of other things glide changes on a service call.
Glide in one paragraph
A blend is two or more refrigerants mixed in fixed proportions. When the components boil at different temperatures and do not behave as one substance, the blend is called zeotropic, and it does not boil at a single temperature at a given pressure. At the bubble point, the first bubble of vapor forms in saturated liquid. At the dew point, the last drop of liquid has just evaporated. Between them the blend is a two-phase mix whose temperature climbs as it boils, because the more volatile components leave first and the remaining liquid gets richer in the less volatile ones. The difference between the dew-point and bubble-point temperatures at one pressure is the glide. A blend's R-number designation and nominal composition are assigned under ASHRAE Standard 34, which is why two cylinders carrying the same R-number share one chart.
The chart, with both columns
Below is an excerpt of an R-407C pressure-temperature chart laid out by saturation temperature with separate dew and bubble pressure columns, which is how most makers and refrigerant suppliers print it for blends. Values are rounded to the nearest psig and computed at standard atmospheric pressure.
R-407C SATURATION PRESSURE, PSIG
Temp F Dew (vapor) Bubble (liquid)
35 56 72
40 63 80
45 71 89
50 79 98
95 181 209
100 196 225
105 212 242
110 229 260
115 247 279
Use DEW for suction and superheat.
Use BUBBLE for liquid and subcooling.
Read it two ways. Across a row, 45 F is reached at 71 psig on the dew side and 89 psig on the bubble side. Down a column at a fixed gauge reading, 71 psig is a 45 F dew point, while on the bubble side the same 71 psig falls below the 35 F row, at about 34 F. That 11 F spread at suction pressure is R-407C's glide in the evaporator range. At condensing pressure the spread is a little smaller: 260 psig is a 110 F bubble point and about a 118 F dew point. For the full tables across common refrigerants, see Refrigerant Pressure-Temperature Quick Reference.
Why superheat uses dew and subcooling uses bubble
The rule follows from what each number is supposed to measure.
Superheat is how far the vapor leaving the evaporator is above the temperature at which the last liquid boiled off. That temperature is the dew point. Below it there is still liquid in the vapor; above it there is none. So superheat is suction line temperature minus dew-point temperature at suction pressure. Read against the bubble point, you would be measuring from a temperature where the refrigerant had barely started to boil, and the figure would include the whole glide.
Subcooling is how far the liquid leaving the condenser is below the temperature at which the last vapor condensed. That temperature is the bubble point. So subcooling is bubble-point temperature at liquid pressure minus liquid line temperature. Read against the dew point, you would be counting the condenser's two-phase glide as subcooling it never produced.
The method itself, where to take each reading and how to settle the system first, is owned by a sibling; see related: HVAC Superheat and Subcooling Method Reference. Charge targets come from the maker's chart or nameplate: target superheat for a fixed orifice, target subcooling for a TXV or EEV.
Which refrigerants have glide worth worrying about
| Refrigerant | Type | Glide in the field |
|---|---|---|
| R-22 | Single component | None |
| R-32 | Single component | None |
| R-410A | Near-azeotropic blend | A few tenths of a degree F; dew and bubble are effectively one number |
| R-454B | Zeotropic blend (A2L) | Small, roughly 1 to 2 F; makers still publish both columns and charging charts read them |
| R-404A | Near-azeotropic blend | About 1 F in the evaporator range |
| R-407C | Zeotropic blend | Large, about 8 to 11 F across residential operating range |
| R-448A, R-449A | Zeotropic blends (commercial refrigeration) | Large, in the same range as R-407C |
Two practical rules come out of the table. First, on R-410A you can ignore the distinction and never be wrong by enough to matter, which is why techs who learned on it never built the habit. Second, on R-454B the gap is small but it is not zero, and on a TXV system charged to a 10 F subcooling target, using the wrong column moves the reading by a meaningful fraction of the target. Set the digital manifold to the right refrigerant and let it pick the column, or read the right column by hand. For the refrigerant families and their handling rules, see Refrigerant Types + EPA 608 Reference and, for R-454B and R-32, A2L Refrigerant Handling (R-454B and R-32).
Worked example: one R-407C system, read both ways
A light commercial split system running R-407C on a TXV. Maker's target subcooling from the chart: 10 F. Readings after 15 minutes of steady run with airflow confirmed:
- Suction pressure 71 psig, suction line temperature 55 F at the outdoor unit.
- Liquid pressure 260 psig, liquid line temperature 100 F at the outdoor unit.
Read correctly.
- Suction: 71 psig on the dew column is 45 F. Superheat is 55 minus 45, which is 10 F.
- Liquid: 260 psig on the bubble column is 110 F. Subcooling is 110 minus 100, which is 10 F.
- Against the 10 F target, subcooling is on target. Superheat of 10 F is a reasonable TXV figure. The charge is right.
Read with the columns swapped.
- Suction against bubble: 71 psig is about 34 F. Superheat reads 55 minus 34, which is 21 F. A tech who stopped here and treated it as high superheat would suspect low charge.
- Liquid against dew: 260 psig is about 118 F. Subcooling reads 118 minus 100, which is 18 F, 8 F over the 10 F target (18 minus 10). A tech who trusted this number would recover refrigerant.
The swapped readings contradict each other: high superheat suggests short charge and high subcooling suggests overcharge. That contradiction is itself the tell. When superheat and subcooling both read high on a TXV system with good airflow and no restriction evidence, check which columns you used before you chase a restriction.
What would change it. Had this been R-410A at the equivalent pressures, swapping columns would have moved each reading by a few tenths of a degree and nothing else would change. Had the system been R-454B, the swap would move each reading by roughly the 1 to 2 F glide, enough to push a subcooling reading from on target to borderline but not to flip the diagnosis outright.
Why blends are charged as liquid
A zeotropic blend in a cylinder separates into a liquid and a vapor of different compositions, because the more volatile components concentrate in the vapor. Draw vapor off the top and you charge the system with a mix rich in the volatile components, and the cylinder's remaining liquid drifts the other way as you go. So blends leave the cylinder as liquid: through the dip tube on a two-valve cylinder, or with the cylinder inverted on a single-valve one, per the cylinder's marking.
Liquid must not reach the compressor as a slug. Meter it into the suction side slowly, flashing it through the manifold valve or a charging restrictor so it enters as vapor and wet mist, with the system running; or charge liquid into the liquid line with the system off, per the maker's instructions. Either way, weigh it in and record what went in.
The same physics applies at the leak. A vapor leak takes the volatile components faster than the liquid ones, so a system that has lost charge through a vapor-side leak may have shifted composition. Blend suppliers have generally found that the shift from a typical service leak is small enough that topping off with the same refrigerant is acceptable. On a system that has lost most of its charge, or one whose history nobody knows, recovering and weighing in a fresh charge removes the question. The equipment maker's guidance governs where it says otherwise. Recovery, leak-repair and recordkeeping duties are owned by a sibling; see related: EPA Section 608 Rules on a Residential Service Call.
Other places glide shows up on a call
Coil inlet temperature. On a high-glide blend the refrigerant entering the evaporator is colder than the dew point read at the suction port, because it is still near the bubble end of its boil. A coil inlet that frosts at a suction pressure whose dew point is above freezing is not proof of a problem on R-407C; check airflow and the full readings before blaming the metering device.
Mean temperature for coil sizing. Engineers sizing a coil often use the midpoint between dew and bubble as the evaporating temperature. That is a design convention, not a charging one. Charging stays on dew for superheat and bubble for subcooling.
Retrofits. A system moved from R-22 to a high-glide replacement has a different chart, a different oil requirement in some cases and a TXV that may need adjusting. The charge targets come from the replacement refrigerant's guidance and the equipment maker, not from the R-22 chart that is still taped inside the panel.
How to verify you used the right column
- The refrigerant on the nameplate matches the refrigerant set on the gauge or the chart in hand.
- Superheat was computed from a dew-point temperature and subcooling from a bubble-point temperature, written on the ticket with both saturation temperatures shown.
- On a blend with large glide, superheat and subcooling tell a consistent story; if they disagree, the columns were checked before any refrigerant moved.
- Any refrigerant added left the cylinder as liquid and was weighed.
References
- Refrigerant manufacturer pressure-temperature data for the specific blend (dew and bubble columns).
- ANSI/ASHRAE Standard 34, Designation and Safety Classification of Refrigerants (current edition; blend designations and composition, a consensus standard referenced by codes and EPA rules).
- See related: Refrigerant Pressure-Temperature Quick Reference; HVAC Superheat and Subcooling Method Reference; Refrigerant Types + EPA 608 Reference; A2L Refrigerant Handling (R-454B and R-32); EPA Section 608 Rules on a Residential Service Call.