Quick answer: The testo 805i measures apparent surface temperature from infrared radiation. It is not a contact probe, internal food thermometer or air-temperature probe.
What makes the testo 805i different
The testo 805i is a Bluetooth infrared surface thermometer. It measures apparent radiative surface temperature; it cannot measure the internal temperature of food or the temperature of air directly.
Prepare and carry out the measurement
- Check the instrument’s target-size requirements and whether the target fills its optical spot.
- Choose an appropriate emissivity setting and avoid reflected heat sources on glossy targets.
- Take repeat readings from a consistent distance and angle on a stable surface.
- Verify surprising values with a contact measurement or a suitable high-emissivity reference patch if safe.
Model-specific use and interpretation
Check distance-to-spot ratio and target size, use a suitable emissivity setting and avoid reflective or transparent targets. Allow a stable reading while holding the instrument within its field-of-view requirement.
An emissivity setting and reflected apparent temperature can dominate uncertainty on polished metal or other low-emissivity targets. Better digital resolution does not eliminate those errors. For a focused discussion of reflective reference surfaces, see the IR reflective tape guidance.
On shiny metal, reflected radiation may dominate the display; verify with a contact sensor or suitable high-emissivity reference surface.
Worked example (hypothetical)
Hypothetical surface target: a warm pipe shows 46 °C on a stable contact probe but 34 °C by IR on a reflective surface. The 12 °C disagreement prompts a method check, not an assumption that the pipe cooled.
Check the target material before changing emissivity
The 805i reads infrared radiation from the visible spot and reports an apparent surface temperature. Painted matte surfaces are generally easier than mirror-like metal, where reflected radiation can dominate. Work with stable geometry and verify the spot is smaller than the homogeneous target. The testo Smart App can help document the reading, but a photographed temperature number still depends on the optical assumptions. For related guidance, see Infrared Pyrometer Guide: Improve Accuracy with Emissivity and Distance.
Interpretation: A temperature change that disappears when you move to a stable high-emissivity patch is evidence of an IR method effect, not necessarily a change in the underlying material. For related guidance, see Infrared Thermometer Calibration Certificates: What to Check.
When an infrared comparison is invalid
Repeat the reading when a glossy pipe reflects a hot light fixture, when a target is smaller than the instrument optical spot or when the target surface changes temperature during the measurement. Do not compare an IR result with an immersed contact probe as if the two methods sampled the same depth. For moving parts and hot equipment, perform contact verification only under an approved safe-access procedure. For a detailed explanation of why these methods disagree, see infrared and probe temperature differences.
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.