Interpret %LEL, account for sample-tube delay and air ingress, and recognise sensor faults, oxygen dependence and model-specific calibration factors.
Choose the measurement
Concentration describes how much gas is present in the sampled mixture. An instrument may display parts per million, percentage by volume or percentage of the lower explosive limit (%LEL). Check which quantity it measures and which gases it can detect.
An adjustable ticking leak seeker is useful for tracing a source, but its tick rate is not a calibrated %LEL measurement. To record that quantity, use a suitable concentration monitor with the correct target-gas setting, range and calibration. A pressure gauge or gas meter measures something else.
These are principles for professional portable instruments. They do not set positions for household alarms or establish a confined-space entry procedure.
Get the intended sample to the sensor
A diffusion instrument relies on gas reaching its exposed sensor. An aspirated instrument uses a pump to bring a sample through a probe or tube. A diffusion sensor alone cannot pull a sample from the far end of an extension line.
Use the approved sampling arrangement. Inspect the tube, connections, filter and instrument before use. Kinks, blockage, water and damage can prevent representative gas reaching the sensor.
A crack on the suction side of a pumped line can admit surrounding air. That air dilutes the sample: the gas fraction arriving at the detector is lower than at the intended sampling point. Repair the sample path and repeat the check as instructed. A stable number on the display is not proof that the sample was collected correctly.
Allow time for the complete system
Gas takes time to travel through a sample tube. The sensor and electronics also take time to respond. A longer tube can therefore add delay even when the sensor itself is fast.
A sensor’s T90 figure describes the time to reach 90% of its final response to a step change at its sample point. It does not automatically include the transit time through a separate extension tube. Use the instrument’s instructions for the actual tube and sampling arrangement before accepting a reading at a new location.
Check whether the result is valid
A zero accompanied by a sensor-fault warning is unreliable. A failed detector can display zero while gas is present. Follow its fault procedure and the site’s safety arrangements; restore effective monitoring before relying on results.
Sensor type matters too. A catalytic pellistor uses the heat produced when gas reacts with oxygen at a heated bead. Oxygen deficiency can interfere with that reaction and make the gas reading misleading. This is a limitation of the sensing method, not a way of judging whether air is breathable. Do not assume every detector uses a pellistor or shares the same operating limits.
Match calibration to the gas
A detector can respond differently to two gases at the same concentration. If the target gas differs from its calibration gas, use the manufacturer’s approved setting or response correction for the fitted sensor and model. Do not transfer a factor from another instrument merely because both detect combustible gas.
Relative density, calorific value and meter-volume correction describe other properties or measurements. None supplies the detector’s response factor.
Understand the %LEL scale
The lower explosive limit is the reference point for %LEL. A reading of 100%LEL means that reference concentration has been reached. It does not mean the sample is 100% fuel gas, and it does not refer to the upper explosive limit or maximum flame speed.
Use the calibration basis specified for the instrument. For example, where 100%LEL methane corresponds to 4.4% by volume:
- 25%LEL is one quarter of that reference.
- One quarter of 4.4% is 1.10% methane by volume.
- 50%LEL on the same basis is 2.20% by volume.
The arithmetic explains the scale. It does not choose an alarm threshold or permit work in an atmosphere. Follow the applicable monitoring and emergency procedures.