Quick answer: The testo 115i clamps onto a pipe to measure surface temperature; it is not a refrigerant pressure sensor. The reading can support superheat/subcooling calculations when paired with a compatible pressure measurement.
What makes the testo 115i different
The testo 115i is a Bluetooth clamp thermometer measuring pipe surface temperature, not refrigerant pressure or flow. The Smart App or compatible manifold displays the reading. For related guidance, see Weigh and Track Refrigerant Charging With the testo 560i.
Prepare and carry out the measurement
- Identify the correct pipe and a clean, straight metal section away from fittings or direct external heat.
- Secure the clamp so the sensor makes firm contact and wait for the reading to settle; avoid moving it between readings.
- Read the value in a compatible app/manifold and record pipe name, load state and ambient conditions.
- Compare with a pressure-derived saturation temperature only when the selected refrigerant and system operating condition are verified.
Model-specific use and interpretation
Clean the pipe contact area, clamp on straight accessible tubing with good contact, allow thermal equilibrium and note nearby ambient effects. Document which pipe and point the reading represents.
Clamps sense surface temperature, which may differ from the refrigerant fluid temperature during transients or with poor contact. The probe does not calculate saturation temperature without pressure and refrigerant data.
To obtain refrigerant superheat, compare a suction-line temperature from the 115i with saturation temperature calculated from a compatible pressure measurement. For a broader pressure-and-temperature workflow, see the guide to testo 549i superheat guide.
Worked example (hypothetical)
Hypothetical pipe readings: suction pipe 13 °C and independently derived saturation 5 °C give 8 K superheat. Moving the clamp onto an uninsulated section in a warm airstream can change the temperature without changing system charge.
Measurement limitations
- Do not place the clamp on an unsafe energized component.
- A poorly coupled sensor can read closer to air temperature than pipe temperature.
- Large transient changes may require additional stabilization time.
- Contact temperature is only one part of a complete HVAC diagnosis.
Improve contact before questioning a pipe temperature
The 115i reads pipe surface temperature, so a poor clamp fit, paint, heavy corrosion or air movement can affect the sensor’s thermal contact. Put it on a clean section within the clamp’s specified range and allow the reading to stabilize. If a refrigeration manifold shows implausible superheat, the clamp is often an easier first check than opening the sealed circuit. To better distinguish pipe temperature from surrounding-air effects, see guidance on measuring ambient air temperature accurately.
Interpretation: If two clamps on the same pipe disagree, inspect their positions, equilibration and hardware accuracy before averaging the numbers. For help relating probe type, contact method and accuracy, see the guide to temperature probe types and accuracy.
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.