Quick answer: To measure refrigerant superheat and subcooling with the testo 558s, 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 558s different
The testo 558s has a four-way valve block, a colour touchscreen and physical keys that can be used with gloves. The official page describes 30-minute trend curves and compatibility with specified A2L/A3 refrigerant applications. For a comparable four-valve workflow on another Testo manifold, see the testo 557s 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
Configure refrigerant and unit settings on the touchscreen, connect temperature probes and inspect the live/graph view after readings stabilize. Save the measurement with the operating mode rather than reading a single transient point.
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. For the pipe-temperature part of these calculations, see the testo 115i pipe-temperature guide.
Trend plots make cycling easier to notice but do not determine whether charge is correct without system-specific targets and operating context.
Worked example (hypothetical)
Hypothetical 558s trend: superheat appears as 14 K just after a restart and 8 K after stabilizing. Two points from different operating states should not be compared as though they represented a steady charging target; retain the full 30-minute graph with timing notes.
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.
Use the 558s trend to distinguish transients
The 558s offers a colour touchscreen and keypad input, so confirm units and refrigerant even if the interface is familiar from a phone. Testo describes a 30-minute graph of readings. Capture the curve during a controlled equipment cycle: a single point taken just after a compressor starts can be misleading. The graph can reveal the timing of a swing, but cannot by itself distinguish control modulation from incorrect charge. For the instrument’s setup procedure and common-error checks, see the Testo 558s setup guide.
Interpretation: A reading taken immediately after defrost and one taken ten minutes later should not be compared as if operating conditions were stable.
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.