The testo 558s measures refrigeration-system pressure and, with the required temperature sensors and correct refrigerant selection, supports superheat and subcooling checks. Set up the exact refrigerant, sensor connections and measurement program before interpreting the numbers. The manifold’s touchscreen and trend graphs do not eliminate installation errors or confirm a refrigerant charge by themselves.
Do not treat a pressure-to-temperature conversion as a temperature reading
The manifold derives refrigerant saturation temperature from measured pressure and a selected refrigerant. Actual pipe temperature needs a separate clamp probe. Confirm the refrigerant, account for blends where dew and bubble points differ, and pair the correct clamp with the correct service line before trusting superheat or subcooling.
Verify the manifold and its accessories
Check the instrument type and current firmware, hose ratings and intended refrigerant class. According to Testo, the 558s has a four-way valve block, touchscreen plus physical keys, app integration and a 30-minute trend display. A manufacturer-compatible accessory may be required for some extended logging or vacuum measurement functions; do not assume every kit contains the same probes. Before relying on app-connected accessories, check Testo Smart Probes compatibility.
What must be measured for superheat or subcooling?
| Check | Required inputs | Interpretation |
|---|---|---|
| Evaporator-side superheat | Suction pressure → saturation temperature; suction-line temperature | Difference between measured line temperature and saturation temperature |
| Condenser-side subcooling | Liquid pressure → saturation temperature; liquid-line temperature | Difference between saturation temperature and measured line temperature |
| Evacuation | Appropriate vacuum instrument and connection method | Requires procedures and readings beyond an ordinary pressure display |
Measurement preparation for qualified technicians
- Confirm refrigerant identity from reliable equipment records. Incorrect refrigerant selection gives incorrect saturation temperatures.
- Inspect manifold, seals, hoses, sensor connections and probe placement; follow the manual for safe connection and isolation.
- Allow the refrigeration system to stabilize at a representative operating state.
- Acquire pressure and correctly coupled pipe-temperature readings from the intended circuit locations.
- Record operating conditions and the resulting superheat/subcooling values; compare with the equipment manufacturer’s commissioning procedure.
Hypothetical calculation
If a correctly selected refrigerant pressure corresponds to an evaporating saturation temperature of 4 °C and the suction-line probe reads 11 °C, superheat is 11 − 4 = 7 K (12.6 °F temperature difference). This illustrates the arithmetic, not a target charge or universal acceptance range. Superheat must be interpreted with system design, load and the manufacturer’s guidance. For the separate clamp-probe step, see the guide to pipe-temperature measurement and superheat.
Frequent 558s mistakes
- Using the wrong refrigerant or an obsolete refrigerant profile.
- Confusing pressure-unit display with saturation temperature.
- Attaching a pipe clamp to an unsuitable location, with poor contact or no insulation.
- Assuming a vacuum reading can be inferred from ordinary gauge-pressure resolution.
- Assuming A2L/A3 equipment compatibility removes ventilation, ignition-control or technician certification requirements.