How measureQuick Calculates Superheat & Subcooling

Once in a while, we get a question on superheat and subcooling calculations and why they don't appear to agree. You will notice this difference on blended refrigerants only.

So we have a blended refrigerant here, R-458A, and you'll see what looks like a saturation temperature of 45 degrees and a suction line of about 65 degrees. Let's just keep the math simple: it looks like a superheat of about 20. Then on the high side, see 90.5 & 102. It looks like a subcooling of approximately 12 degrees, yet the superheat and the subcooling are 14.7 and 7.3.

If we look at the Fieldpiece app, you'll see we have the same suction line temperature but the vapor saturation temperature is 50 and liquid saturation is 98. On our app, we are at 45 & 102. They're definitely different. But if you're looking at superheat and subcooling at 14.7 and 7.3, those are almost identical. So why does measureQuick do some things differently than most other apps?

measureQuick does things differently because we're doing them the right way: showing you the average coil temperature instead of the dew point or the bubble point.

If you tap on the target for a gauge (as seen in the image above), you'll see that this refrigerant is a high glide refrigerant. The center of the glide is 45° and it's got a dew point temperature of 49.6°. (Remember: Dew for heat and Bub for cool.) We use dew point for calculating the superheat and bubble point for calculating the subcooling. If we go back to the mQ app (1st image) and say okay, 49 (dew point temp) minus 64 (suction line temp), that's where we get our 14.7 degrees of superheat.

A lot of people have not seen this before, but this actually goes back to some work that Emerson has done on refrigerants.

This is an Emerson presentation about average coil temperature. To get the average evaporator coil temperature of a blended refrigerant, we take the pressure in the bubble point column and the pressure in the dew point column, find the corresponding temperatures, and use the following equation:

Ave Coil Temp = 0.40(bubble point temp) + 0.60(dew point temp)

If we go to the condenser, you'll see that to find the average condensing temp, we take the bubble point plus the dew point divided by two, and that gives us an average.

In measureQuick, we're calculating average coil temperature because we're calculating where those targets should be for the high and the low side temperature. We use dew point and bubble point in the background to calculate superheat and subcooling. You'll notice that our app always agrees with other apps on superheat and subcooling because they use dew point and bubble point too. The difference is we display average coil temp under your gauge, which is much more representative of the temperature you should be looking for (especially in refrigeration) than the dew point or bubble point alone.

When you select a blended refrigerant, the app displays an in-app explanation confirming that superheat is calculated from the dew point temperature and subcooling from the bubble point. This is the same methodology described above.

Watch Jim Explain

If your saturation temperature doesn't match your PT chart or other apps, see Why Doesn't My Saturation Temp Agree? For other refrigerant-related questions, see Refrigeration FAQs.