Quick answer: The testo 300 measures flue-gas quantities for combustion service. Confirm the exact sensor and instrument generation, zero as directed, record stable oxygen and carbon-monoxide readings and interpret them against the appliance specification—not a universal target.
What makes the testo 300 different
The testo 300 Smart-Touch combustion analyzer combines O₂/CO flue measurements with on-device job documentation. The referenced 0633 3002 70 package specifies a CO sensor with a range to 4,000 ppm; do not assume the same sensor configuration for every kit. For a comparable model, see our guide to the testo 330 combustion analyzer setup.
Prepare and carry out the measurement
- Verify that a qualified technician has confirmed the appliance operating safely and identified an approved sampling point.
- Inspect the flue-gas probe, sampling hose, condensate protection and sensor status; complete fresh-air zeroing exactly as specified in the corresponding manual.
- Select the actual fuel and operating mode, and record burner load, flue/combustion-air temperatures and other installed sensor channels.
- Sample at the specified point with proper probe placement, wait for stable O₂ and CO and save the values together with the operating condition.
Model-specific use and interpretation
On the touch interface, confirm the correct appliance and fuel, establish the fresh-air zero as the manual directs, and save the report with the burner firing state. Review flue-gas and ambient measurements as separate applications. For related guidance, see Carbon Monoxide Tester for Ambient Air Checks: Safe Steps.
O₂ indicates residual oxygen, CO indicates incomplete-combustion product and flue temperature supports the calculated flue-loss estimate. A calculated combustion-efficiency percentage is not seasonal appliance efficiency. Sensor configuration and fuel choice change what can be displayed. For a broader explanation, see our guide to flue gas analyzer readings and efficiency.
A drop in calculated flue loss is meaningful only when fuel, firing rate, probe position and temperature measurement method are comparable.
Worked example (hypothetical)
Hypothetical record: at one stable burner condition O₂ is 5.2%, CO 18 ppm and flue temperature 150 °C. At another condition the values are 4.7%, 26 ppm and 165 °C. The changes are -0.5 percentage points O₂, +8 ppm CO and +15 °C; they cannot be judged safe or optimal without appliance criteria and comparable load conditions.
If the readings change unexpectedly
- Recheck probe location, blocked sampling hose, condensate trap, sensor zero and the fuel setting before concluding that combustion changed.
- If O₂ and CO drift together, review appliance operating state and combustion-air supply under an approved service protocol.
- If there is unsafe CO, spillage, incomplete combustion or any hazard indication, stop the measurement and follow the applicable emergency and professional service procedures.
Using the reporting workflow effectively
On the testo 300 Smart-Touch version, the job-report workflow matters as much as the initial reading. Confirm the site and appliance identifier before saving, and retain fuel selection, operating state and sampling location. If a client later compares two visits, a missing load condition can invalidate the apparent efficiency change. Check whether draft/pressure or ambient CO were measured with installed sensors and the appropriate accessories; do not add unmeasured values to a template report. For guidance on report contents, see our guide to boiler combustion test reports.
Interpretation: A practical check is whether the saved report ties the O₂ and CO results to a burner stage. Without the stage, a comparison between low fire and high fire is not meaningful.
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. For related guidance, see Gas Leak Detection vs Tightness Testing: Methods and Limits.