A pure refrigerant boils at a single temperature at 3 bar. A blend, at the same pressure, starts boiling at one temperature (bubble point, liquid point) and becomes fully vapour a few degrees higher (dew point, vapour point). The difference between them is the glide. At 0.7 K for R404A it is negligible; at 4–6 K for R449A and R448A, 7 K for R407C and 6–8 K for R454C it must be taken into account.
Glide shows itself in three places. In the evaporator: the inlet is colder than the outlet, part of the fins frost more heavily, and the room temperature is set against the "mean evaporating temperature". In measurement: superheat is calculated from the dew temperature of the suction pressure, subcooling from the bubble temperature of the condensing pressure; using the wrong column means a 4–5 K error. In charging: the refrigerant is charged as liquid.
Expansion valve and pressure switch settings must be redone for a refrigerant with glide; this is the most frequently neglected step in conversions from R404A to R449A. Our refrigerant pressure–temperature chart gives bubble and dew values side by side for R449A, R448A, R407C and the other zeotropic refrigerants.