High-glide refrigerants (zeotropic) cannot be charged and diagnosed like typical R-22 or R-410A (near-azeotropic). In addition to dew and bubble point, we must also understand when to use average or midpoint saturation.
When a refrigerant has glide, that means that the temperature of boiling and condensing changes. You will typically see glide in refrigerant blends. However, some blends are near-azeotropic, which means that there is very little change in the vapor and liquid properties between two refrigerants.
When using the temperature-pressure relationship on refrigerants with glide, we typically use the bubble point when the refrigerant is subcooled and the dew point when the refrigerant is superheated.
On the Refrigerant Slider, a refrigerant like R-22 doesn't allow you to select between dew and bubble point for the evaporator temperature. However, a refrigerant like R-422D gives you that option. At dew point and 75 PSI, the temperature near the end of the evaporator coil would be 45.5 degrees Fahrenheit. In the bubble range (at the beginning of the evaporator coil), it would be 39.3 degrees Fahrenheit; that is when R-422D would start boiling at 75 PSI. So, the saturation range would be between 39.3 and 45.5 degrees, and that's the glide.
To calculate the actual boiling temperature, we would find the midpoint; we would take the glide value (6.2) and divide it by 2 to get 3.1 degrees. We would then add that to the bubble point value (39.3) to get a ROUGH midpoint of 42.4 degrees.
If you have refrigerant in the static state, it will be at the bubble point. We test this on a tank of R-422D. The tank temperature is just over 70 degrees, and the pressure from our Testo instrument and the Refrigerant Slider confirm that the refrigerant would be at the bubble point. If it were at the dew point, then the temperature would have been significantly higher.
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