Quick answer: To measure refrigerant superheat and subcooling with the testo 550s, combine pressure-derived saturation temperature with an actual pipe-temperature reading. Check whether the named instrument supplies both quantities or needs a separate clamp probe.
What makes the testo 550s different
The testo 550s is a two-way Bluetooth digital manifold with a large graphic display. The Smart App supports reports and refrigerant updates; temperature measurements can be supplied by compatible wired or Bluetooth clamp probes. For a comparable wireless refrigeration workflow, see testo 549i superheat and subcooling guide.
Prepare and carry out the measurement
- Only a certified technician should attach gauges to a charged refrigerant circuit. Confirm refrigerant type, equipment state, hose ratings and the system procedure.
- Measure the relevant low-side and high-side pressures with the appropriate manifold or pressure probe, and select the exact refrigerant in the instrument/app.
- Attach suitable temperature clamps to correctly identified suction and liquid lines. Allow readings to stabilize under a documented operating condition.
- For superheat subtract suction saturation temperature from measured suction pipe temperature. For subcooling subtract measured liquid-line temperature from condensing saturation temperature. Document dew/bubble conventions for blends.
Model-specific use and interpretation
Select the refrigerant, connect the manifold under approved service procedures, pair the correct suction and liquid-line clamps and allow pressures and pipe temperatures to stabilize. Check whether the temperature probes are included in your kit. For details on clamp-based pipe-temperature readings, see the testo 115i pipe-temperature guide.
Superheat = suction-pipe temperature − evaporating saturation temperature. Subcooling = condensing saturation temperature − liquid-line temperature. These are differences in K or °F, not percentages. The correct target depends on equipment design, metering device and test conditions; never use one universal charging target.
A displayed superheat/subcooling result depends on pressure conversion plus the pipe sensor. A wrong refrigerant selection or clamp position can create a plausible but false calculation.
Worked example (hypothetical)
Hypothetical stabilized system: the selected refrigerant yields suction saturation 3 °C and condensation saturation 41 °C. Clamps read 11 °C suction and 34 °C liquid line, giving 8 K superheat and 7 K subcooling. Do not use those figures as universal charge targets.
Interpretation and common errors
- Unexpected readings can be caused by unstable load, low airflow, incorrect probe placement, poor clamp contact, wrong refrigerant, or a sensor that has not paired.
- A high or low value does not identify refrigerant charge on its own. Expansion-device type, manufacturer charts and control strategy matter.
- Do not charge, recover or open the system based solely on this article. Refrigerant work carries pressure, electrical and potentially flammability hazards and is for qualified personnel.
Two-valve setup and clamp assignment
On the 550s, use the large graphic display to confirm which pressure side and clamp each channel represents. A two-way valve arrangement is not interchangeable with the four-way workflow of the 557s. Temperature probes may be wired or Bluetooth depending on the kit; neither can be assumed to be included when purchasing the bare manifold. Save a measurement after the compressor has reached the mode specified by the manufacturer. For the contrasting four-valve workflow, see the testo 557s four-valve setup guide.
Interpretation: If superheat appears negative, verify the refrigerant selection and clamp placement before drawing conclusions about charge or refrigerant flow.
Limits and safety
Follow the exact instrument manual and the equipment manufacturer’s acceptance criteria. The example is not field evidence, a legal limit or a commissioning target. Work on energized electrical equipment, combustion systems, charged refrigerant circuits or hot surfaces requires qualified personnel and the site’s approved protective procedures.