Quick answer: The testo 871 records apparent surface-temperature patterns. To compare images meaningfully, document emissivity, reflected radiation, palette scale and distance; a thermogram cannot directly detect water behind a wall.
What makes the testo 871 different
The testo 871 thermal imager supports building/maintenance surveys with thermal detail suited to differentiating surface patterns; available accessories and variants should be checked against the model number. For related guidance, see 890 Setup and Emissivity Guide: Lenses, Distance and Analysis.
Prepare and carry out the measurement
- Check model generation, lens and firmware manual, and allow camera/environment stabilization as specified.
- Set emissivity and reflected-temperature assumptions appropriate to the inspected surface; avoid polished targets.
- Capture overview and close-up images with repeatable distance, focus and temperature scale.
- Corroborate a suspected anomaly using suitable temperature, humidity or moisture tests and document conditions.
Model-specific use and interpretation
Document the scene with a consistent distance, angle and scale. Confirm any suspected cold spot with repeat views, avoiding shiny surfaces and direct reflected heat sources.
The colours on a thermal image represent a selected rendering of radiometric temperatures. Colour contrast alone is not a defect category. Apparent surface temperature patterns can be due to material, geometry, reflection, sunlight or airflow.
A more detailed thermal image still cannot identify the hidden material or prove the cause of dampness without independent investigation. For a comparable model-specific discussion, see the testo 872 thermal camera guide.
Worked example (hypothetical)
Hypothetical wall comparison: two painted surfaces show apparent temperatures of 14 °C and 18 °C under matched emissivity/scale settings. The 4 °C contrast identifies an area for investigation; it does not prove failed insulation or concealed dampness. For the relationship between surface temperature and dew point, see the wall condensation and dew point guide.
Interpretation limits and safe follow-up
- Shiny surfaces may reflect a hot or cold object that is not the target itself.
- A thermal bridge, air movement or damp material can show overlapping patterns; corroborate with appropriate independent measurements.
- Electrical and high-temperature assets must be examined from approved safe positions by competent personnel.
- If inspecting an older model, verify its own manual instead of adopting instructions from later 868s, 871s or 872s models.
Focus, repeatability and spatial context
Use the testo 871 to inspect suspected building defects at a repeatable distance and viewing angle. Keep the scale consistent when showing a “before” and “after,” and note heating-system, occupancy and weather conditions. Thermal contrast may reflect studs, airflow or solar loading; check the room temperature and air humidity before labelling a pattern as condensation risk. For independent moisture checks, see building moisture inspection with the testo 606-1.
Interpretation: A bright patch in an automatically rescaled colour palette can look dramatic even when the measured temperature difference is small.
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.