ppm and % vol
1 % vol is 10 000 ppm: ppm is a millionth of the volume, per cent a hundredth. 10 ppm of H2S is 0.001 % vol; oxygen at 20.9 % vol is 209 000 ppm. This step needs no data about the gas and does not depend on the temperature.
Reference tool — the units of a gas detector reading, nothing more
Gas concentration converter.
ppm · % vol · mg/m3 · % LEL
Reading and gas.
Type a line (H2S 10 ppm to mg/m3, methane 2.2 % vol LEL 4.4 %) or fill in the fields: the line and the fields stay in step.
Examples:
All 130 engineering tools
Enter a portable or fixed gas detector reading, in ppm, % vol, mg/m3 or % LEL, and the gas, and the converter gives it in every unit at once: ppm, % vol, mg/m3 at the temperature chosen, and % LEL when the LEL is known. It shows the working with your numbers, marks the reading on a log scale, and gives a line you can copy or add to a request.
The LEL is entered from the detector’s calibration certificate: published LEL figures differ between standards, methane for example 4.4 or 5 % vol, so a preset is given only for hydrogen and ammonia, where the figures checked agree. No exposure limits and no alarm set points here: alarm levels are set by the detector configuration and the ship’s procedures.
How it is worked out
Volume, mass and share of the LEL.
ppm and mg/m3
mg/m3 = ppm x M / Vm. M is the molar mass of the gas, Vm the molar volume of an ideal gas at 101.325 kPa: 22.414 x T / 273.15, which is 24.055 l/mol at 20 °C; the conversion tables at 25 °C use 24.45 l/mol. H2S at 10 ppm: 10 x 34.08 / 24.055 = 14.17 mg/m3 at 20 °C. The other way: NH3 at 25 mg/m3 = 25 x 24.055 / 17.03 = 35.31 ppm.
% vol and % LEL
% LEL = % vol / LEL x 100. Methane at 2.2 % vol with a LEL of 4.4 % vol is 50 % LEL; with a LEL of 5 % vol it would be 44 % LEL. The result depends on the LEL, so it is taken from the detector’s calibration certificate, not from a general table.
| Gas | M, g/mol | mg/m3 per ppm at 20 °C | LEL preset, % vol |
|---|---|---|---|
| Hydrogen sulphide H2S | 34.08 | 1.417 | none: from the certificate |
| Carbon monoxide CO | 28.01 | 1.164 | none: from the certificate |
| Carbon dioxide CO2 | 44.01 | 1.83 | – |
| Ammonia NH3 | 17.03 | 0.708 | 15 |
| Methane CH4 | 16.04 | 0.6668 | none: from the certificate |
| Oxygen O2 | 32.00 | 1.33 | – |
| Sulphur dioxide SO2 | 64.06 | 2.663 | – |
| Nitrogen dioxide NO2 | 46.01 | 1.913 | – |
| Propane C3H8 | 44.10 | 1.833 | none: from the certificate |
| Butane C4H10 | 58.12 | 2.416 | none: from the certificate |
| Methanol CH3OH | 32.04 | 1.332 | none: from the certificate |
| Hydrogen H2 | 2.016 | 0.08381 | 4.0 |
Molar masses are worked out from the standard atomic weights and rounded to 0.01 g/mol (hydrogen to 0.001). A LEL preset is given only where the published figures checked agree; “none: from the certificate” means the figures differ between standards and the LEL is to be entered from the calibration certificate; a dash means the converter sets no LEL for that gas.
Sources
The relation mg/m3 = ppm x M / Vm and the molar volume of 24.45 l/mol at 25 °C and 760 mmHg are given in occupational exposure conversion guidance, and 24.055 l/mol at 20 °C and 101.325 kPa in a national list of workplace exposure limits; both follow from the ideal gas molar volume of 22.414 l/mol at 0 °C. The molar masses come from the standard atomic weights and were checked against a chemistry reference database. The LEL figures were checked in the international standard on flammable gas data for explosive atmospheres, in gas data references and in gas detector makers’ tables: they differ for methane, propane, butane, hydrogen sulphide, methanol and carbon monoxide, and agree for hydrogen and ammonia. The relation % LEL = % vol / LEL x 100 is described in gas detection guides.
Before you convert
Questions about gas units
How are ppm and % vol converted?
1 % vol is 10 000 ppm, because ppm is a millionth of the volume and per cent a hundredth. 10 ppm is 0.001 % vol, 2.2 % vol is 22 000 ppm, and oxygen at 20.9 % vol is 209 000 ppm. This step needs no data about the gas.
How is ppm converted to mg/m3?
mg/m3 = ppm x M / Vm, with M the molar mass of the gas in g/mol and Vm the molar volume in l/mol. For an ideal gas at 101.325 kPa, Vm = 22.414 x T / 273.15, which is 24.055 l/mol at 20 °C; the conversion tables at 25 °C use 24.45 l/mol. H2S (M 34.08 g/mol) at 10 ppm is 10 x 34.08 / 24.055 = 14.17 mg/m3 at 20 °C, and 13.94 mg/m3 at 25 °C. The other way, ppm = mg/m3 x Vm / M: NH3 at 25 mg/m3 is 25 x 24.055 / 17.03 = 35.31 ppm at 20 °C.
Why does the temperature change the mg/m3 figure?
A cubic metre of warmer air holds fewer molecules, so the same ppm reading is fewer milligrams per cubic metre. Between 20 and 25 °C the difference is about 1.6 %: CO at 50 ppm is 58.22 mg/m3 at 20 °C and 57.28 mg/m3 at 25 °C. ppm and % vol do not depend on the temperature. The converter takes 20 °C unless another temperature is entered, and says so.
How is % LEL worked out?
% LEL = % vol / LEL x 100, with the LEL in % vol. Methane at 2.2 % vol with a LEL of 4.4 % vol is 50 % LEL; a reading of 25 % LEL with the same LEL is 1.1 % vol, or 11 000 ppm. The result depends wholly on the LEL used, so the converter shows which one it used.
Which LEL should be used?
The one the detector is calibrated to, from its calibration certificate. Published LEL figures differ between standards: methane is given as 4.4 % vol in one and 5 % vol in another, propane as 1.7 or 2.1 % vol. The converter therefore has no preset for those gases and asks for the figure. A preset is used only for hydrogen (4.0 % vol) and ammonia (15 % vol), where the published figures checked agree, and a figure entered from the certificate always replaces it.
Does the converter give exposure limits or alarm levels?
No. It converts a reading between units and nothing more: no exposure limits and no alarm set points. Alarm levels are set by the detector configuration and the ship’s procedures.
A sensor or calibration gas needed?
Send the line and the detector type.
The converter gives a gas detector reading in every unit. When sensors, calibration gas or detector parts are needed, send the line from the converter with the detector type and a photograph of its label, and Vessel Core checks the details before an offer goes back.
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