Quick answer: To measure refrigerant superheat and subcooling with the testo 557s, 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 557s different
The testo 557s uses a four-way valve block rather than the two-way arrangement on the 550s. It supports refrigeration measurement programs and Bluetooth temperature/vacuum probes; this page concerns the 557s generation, not older testo 557 models. For related guidance, see Digital Refrigeration Gauges: Choose by Measurements, Probes and Model.
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
Check hose routing and each valve position with the appropriate service schematic. Pair both clamps, confirm the refrigerant and record stabilized high/low pressure and pipe temperatures, with particular attention to the extra valve paths.
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.
Extra manifold valves change servicing connections, not the superheat equation; incorrect open valves and a transposed temperature clamp remain common errors. For detailed setup checks and common error prevention, see the Testo 558s setup guide.
Worked example (hypothetical)
Hypothetical 557s reading: suction saturation is 5 °C and measured suction line is 15 °C, producing 10 K superheat. If the clamp is moved to another pipe and reads 9 °C, the calculated value falls to 4 K without a refrigerant charge change; first check clamp identity and valve routing. For more detail on clamp placement and pipe-temperature measurement, see the testo 115i pipe-temperature guide.
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.
A four-valve block changes service handling
The 557s has additional service-valve paths that can make evacuation or connection tasks more convenient, but they also create additional routing errors. Trace each hose and valve against the device’s service diagram and lock down the configuration appropriate for the current test. Pair the low-side temperature clamp with suction pressure and the high-side clamp with liquid pressure, not merely the physical left and right positions.
Interpretation: An unexpectedly flat pressure trace can result from a closed or isolated manifold path rather than a sealed or faultless refrigeration system.
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.