Why does the R-407C PT chart have two temperature columns?
R-407C is a zeotropic blend, so at any given pressure it boils and condenses across a range of temperatures rather than at one point. The lower temperature is the bubble point and the higher one is the dew point; the gap between them is the glide, about 9 to 11 degrees F for R-407C over typical operating conditions, widening toward the cold, low-pressure end of the chart. At 226 psig, for example, the bubble point is 100F and the dew point is 109.2F.
Do I use bubble or dew point for R-407C superheat and subcooling?
Use the dew point for superheat and the bubble point for subcooling. Superheat is measured on the suction line where the refrigerant is fully vapor, which is the dew side, so convert suction pressure to the dew temperature. Subcooling is measured on the liquid line where the refrigerant is fully liquid, which is the bubble side, so convert liquid pressure to the bubble temperature. Using the wrong column introduces an error equal to the glide, roughly 9 to 11 degrees F, which is enough to misdiagnose the charge.
What is the R-407C pressure at a 70F bubble point?
On the R-407C PT chart, a 70F bubble point corresponds to about 141 psig, where the dew point is 80.4F. R-407C is the common R-22 retrofit blend, so its pressures sit close to R-22 on a service gauge, but the glide means you must read the correct column for each measurement.
What units is this R-407C PT chart in, and how do I convert to kPa or Celsius?
The table is in psig (gauge pressure) and degrees Fahrenheit, the field-standard units in North America. To convert pressure to kPa (gauge), multiply psig by 6.895. To convert temperature to Celsius, use C = (F - 32) / 1.8.