Quick answer: To measure refrigerant superheat and subcooling with the testo 550i, 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 550i different
The testo 550i is a compact screenless two-way digital manifold controlled from the testo Smart App. The app is required for its readings and measurement program; it is not the display-equipped 550s.
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
Prepare a compatible phone, install/update the Smart App and confirm Bluetooth pairing before connecting service hoses. Pair suitable pipe clamps and check that the app associates each probe with the intended line.
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.
Loss of phone connection does not establish a change in refrigerant pressure; separate communications faults from measurement faults before diagnosing the equipment.
Worked example (hypothetical)
Hypothetical 550i app session: pressure converts to 7 °C evaporating saturation while a Bluetooth clamp reports 16 °C at the suction line, giving 9 K superheat. A momentary missing clamp value means the result cannot be updated, not that superheat has become zero.
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.
Smart App and Bluetooth are part of the instrument
Unlike a display-equipped manifold, the 550i depends on its connected phone or tablet. Check OS/app compatibility, Bluetooth permissions, battery state and device selection before beginning a job. Set favorites only after confirming refrigerant choice and probe assignment; preserve screenshots or exports with the operating mode and units. A disconnected app is an information failure, not evidence that the refrigerant circuit suddenly changed. For related guidance, see Testo Smart Probes Compatibility: Pairing Steps and Probe Selection.
Interpretation: If the pipe temperature is missing from the superheat screen, confirm the appropriate clamp probe is paired; the 550i pressure block has not independently measured it.
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.