If a measurement sheet says only “gas 10,” the next shift will struggle to assess the hazard. Carbon monoxide at 10 ppm, oxygen at 10 %VOL and combustible gas at 10 %LEL describe different conditions despite sharing the same number.

Read portable gas detector units together with the gas name, sensor channel and reference gas. This article distinguishes where conversion is possible from where it must stop, and suggests how to improve site records.

1. Three display items to check before the number

Read each display line in the order “substance → value → unit.” When multiple channels share a screen, check whether the unit is next to the value and whether changing pages changes channels. The actual sensor configuration may differ from the arrangement a worker remembers, so take particular care on days when instruments are exchanged.

The US OSHA Technical Manual explains that multi-gas instruments display ppm, oxygen percentage or %LEL depending on the target. This is technical guidance on display systems, not a legal basis for setting alarms at Korean sites. OSHA technical information on direct-reading instruments

Saying “the detector measures four gases” does not establish that all required substances are included. Do not assess carbon dioxide or solvents absent from the sensor list using existing numbers. Correctly reading units starts with checking what the instrument does not measure.

2. ppm and %VOL express different scales of concentration

For gas concentrations expressed as volume fractions, ppm means parts per million and %VOL means percentage by volume. For the same gas on the same volume basis, 1 %VOL equals 10,000 ppm. Thus 0.1 %VOL equals 1,000 ppm. This arithmetic changes the expression of concentration; it does not determine an allowable concentration or a safe level.

Display

How to read it

Items that must remain in the record

CO 10 ppm

Carbon monoxide concentration by volume

The substance CO and ppm

O₂ 20.9 %VOL

Oxygen percentage by volume

O₂ and %VOL

Combustible 10 %LEL

Ratio to the lower explosive limit of the configured reference

Reference gas and %LEL

For example, the numerical size of a CO ppm reading cannot simply be compared with an oxygen channel's %VOL value. The hazards and reference substances differ. Combining gas concentrations into one “total hazard concentration” also falls outside the purpose of this table.

Distinguish mass concentration in mg/m³ separately. The relationship between ppm and mg/m³ involves molecular weight, temperature and pressure, so do not apply “one constant for every gas.” NIOSH's chemical information explains units and conversion conditions alongside substance-specific entries. NIOSH Pocket Guide introduction and unit explanation

3. The percentage in %LEL is not percentage by volume in air

The lower explosive limit (LEL) is the lowest concentration of a gas or vapor at which flame can propagate under ignition conditions. %LEL expresses a ratio to that reference. Reading 10 %LEL as gas occupying 10% of air confuses the basis of the unit itself.

NIOSH explains that interpretation can differ when the combustible gas in the field differs from the detector's calibration gas. Therefore, do not convert an LEL reading into the actual volume concentration of a specific gas when gas identity is unknown. NIOSH multi-gas monitor safety guidance

The calculation principle is “volume concentration of the gas = %LEL reading ÷ 100 × that gas's LEL volume concentration.” In an actual detector, however, reference gas, adopted LEL value, response characteristics and correction status must all match before this can be used to interpret concentration. Seeing the same gas abbreviation on different manufacturers' screens does not mean these conditions are identical.

4. Conditions that allow conversion and conditions requiring a hold

For a hypothetical training example, suppose an instrument document specifies a reference LEL of 4 %VOL for a gas, and the target gas and calibration/display conditions all match. Under these conditions, the arithmetic equivalent of 25 %LEL is 1 %VOL, or 10,000 ppm. Here, 4 %VOL is an assumption to explain calculation, not a recommended value for a particular substance or a site setting.

Confirmation status

Handling method

Same substance; ppm and %VOL on a volume basis

Convert units while retaining the original display

Substance, instrument reference LEL and sensor conditions all confirmed

Review whether conversion is allowed under manufacturer instructions

Reference gas known, actual gas unknown

Record the original %LEL and withhold a definitive actual concentration

Mixed gases or possible interference

Do not uniformly convert using one molecular weight or LEL

Overrange or fault symbol present

Apply stop-use and response procedures instead of calculating

Also distinguish the manual's “display range” from the “measurement range with verified performance.” Dräger's official sensor documentation lists display ranges and certified measurement ranges separately for individual sensors. A displayed number does not establish equal accuracy across every part of the range. Dräger sensor data

5. Hypothetical site: rebuilding a contractor's record sheet

A hypothetical maintenance team receives a contractor's sheet recording only “oxygen 20.9 / gas 0 / hazardous gas 0.” The person responsible intends to file it because the instrument was said to be normal, but cannot compare it with another team's measurements from a different detector. Missing channel information, rather than different values, is the cause.

The first action is not to guess the missing units. Consult the original operator and instrument records to recover actual channels, units and instrument IDs, leaving unverifiable items unknown. Erasing the original and retaining only converted results removes the basis for the correction.

A record form can separate original values from interpretation as below. This is an example of record improvement and does not replace an entire confined-space entry permit.

Record item

Example entry or verification question

Measurement date, time and location

Where and when was it measured?

Instrument ID and channel

Can the same instrument and actually installed sensors be traced?

Original display and unit

What appeared before rounding or conversion?

Reference gas and configuration document

Which document supports %LEL interpretation?

Converted value and assumptions

Were substance and conditions confirmed to match?

Reviewer and disposition

Who placed unclear results on hold?

6. Understanding units still leaves safety decisions to be made

Even at the same ppm value, the review differs with substance toxicity, exposure duration, instantaneous peaks and applicable criteria. Unit conversion changes only how the measurement is written; it does not replace alarm, evacuation or work authorization procedures. When an alarm occurs, do not stay to complete calculations: follow designated evacuation and notification procedures.

Retaining original instrument displays also improves traceability when standardizing units in site records. Give substance, original value and unit separate spreadsheet columns, and record the formula and document version in the conversion column. Review forms that automatically change units when another team pastes values.

Continue reading LEL displays and reference gas checks, time-weighted average and short-term exposure displays, and distinguishing measurement range and accuracy. Each article narrows the question addressed by a particular display.

Action checklist

  • [ ] Read substance, number and unit together on the screen.

  • [ ] Compare installed sensors against work substances.

  • [ ] Confirm ppm-to-%VOL conversion uses the same substance and volume basis.

  • [ ] Check for records that treat %LEL as %VOL.

  • [ ] Find the reference gas and adopted LEL value in the manual.

  • [ ] Withhold arbitrary conversions for unknown or mixed gases.

  • [ ] Separate original and converted values and record applicable conditions.

  • [ ] Confirm fault and alarm response procedures with workers.

Conclusion

Portable gas detector units identify the reference used to express a value. ppm and %VOL can be converted on the same volume basis, while interpreting %LEL additionally requires gas and instrument configuration checks.

Recordkeeping starts with preserving the original display accurately rather than standardizing units. Connect only results with confirmed substance, unit and reference to the designated safety management procedure.

Sources