Read the saturation temperatures off the gauge scale for the actual refrigerant (or convert the pressure with a PT chart). Superheat = measured suction line temp − suction saturation temp; subcooling = liquid saturation temp − measured liquid line temp. Target values come from the manufacturer — low superheat risks liquid floodback, high superheat starves the evaporator. Not a substitute for commissioning data.
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This calculator works out superheat and subcooling from the readings you take at commissioning. Enter the suction and liquid saturation temperatures from your gauges and the measured line temperatures, and it returns the superheat and subcooling in kelvin.
It is for air conditioning and refrigeration engineers checking the charge on a system. Superheat and subcooling are the two numbers that tell you whether a system is correctly charged and the metering device is working — this tool does the subtraction so you can focus on what the values mean.
The two figures come straight from the readings: superheat is how far the suction gas has warmed above its saturation temperature, and subcooling is how far the liquid has cooled below its saturation temperature. Read the saturation temperatures off the gauge scale for the actual refrigerant.
Superheat = suction line temp − suction saturation temp; Subcooling = liquid saturation temp − liquid line temp
Superheat is the difference between the measured temperature of the suction line and the saturation temperature the low-side pressure corresponds to — it shows the refrigerant has fully evaporated and warmed. Subcooling is the difference between the high-side saturation temperature and the measured liquid line temperature — it shows the refrigerant has fully condensed and cooled.
Take the low-side (suction) pressure from your gauge and read the corresponding saturation temperature off the scale for the actual refrigerant, or convert it with a PT chart.
Measure the actual temperature of the suction line at the point specified by the manufacturer, usually near the compressor or evaporator outlet, with a clamp probe.
Take the high-side (liquid) saturation temperature from your gauge, and measure the actual liquid line temperature near the condenser outlet.
The calculator shows the superheat and subcooling. Compare them with the manufacturer's target values to judge the charge and metering.
Superheat and subcooling tell you different things. Superheat protects the compressor — too little risks liquid returning and slugging the compressor, too much means the evaporator is starved and short of refrigerant. Subcooling confirms the condenser is filled with liquid and is a good charge indicator on systems with a fixed metering device or where the manufacturer specifies a target.
Always work to the manufacturer's target figures, not a rule of thumb, especially on TXV/EEV systems where the valve holds superheat roughly constant and subcooling becomes the charging metric. A negative superheat or subcooling reading means liquid where there should be vapour, or vapour where there should be liquid — investigate before running the system.
Accurate saturation temperatures are everything. Read them for the actual refrigerant blend, and for zeotropic blends use the correct dew and bubble points. This tool does the arithmetic; it does not replace the manufacturer's commissioning data or your own judgement about what the numbers are telling you.
Read the saturation temperatures off the gauge scale for the actual refrigerant (or convert the pressure with a PT chart). Superheat = measured suction line temp − suction saturation temp; subcooling = liquid saturation temp − measured liquid line temp. Target values come from the manufacturer — low superheat risks liquid floodback, high superheat starves the evaporator. Not a substitute for commissioning data.
Superheat is the measured suction line temperature minus the suction saturation temperature. Read the saturation temperature from the low-side gauge pressure for your refrigerant, measure the actual line temperature, and subtract. For example, a suction line at 12°C with a 5°C saturation temperature is 7 K of superheat.
Subcooling is the liquid saturation temperature minus the measured liquid line temperature. Take the high-side saturation temperature from your gauge, measure the liquid line near the condenser outlet, and subtract. A liquid saturation of 45°C with a line at 39°C gives 6 K of subcooling.
It depends on the system and the manufacturer's specification, so always work to their figures. As rough context, many systems target superheat in the region of 5–10 K and subcooling around 3–8 K, but TXV systems, fixed-orifice systems and specific manufacturers vary, so the data plate or commissioning sheet is the authority.
A negative superheat means the suction line is at or below its saturation temperature, so liquid refrigerant is present where there should be vapour. That risks liquid returning to the compressor and causing damage. If you calculate a negative value, stop and investigate the charge and metering before continuing to run the system.
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