VOL vs ppm: Our recommendation for CO₂ detection under MSC.581 (110)

IMO Resolution MSC.581(110) sets 5,000 ppm (0.5% VOL) as the upper limit for CO₂ exposure during personnel work in enclosed spaces.

However, the resolution does not specify which unit of detection must be used, and many suppliers continue to offer %VOL-based sensors. Bruusgaard has made a deliberate choice to specify ppm. Here is our reasoning.

Sensor resolution matters at the 5,000 ppm threshold

Both 5,000 ppm and 0.5% VOL describe the same concentration, with ppm providing a more fine-grained reading. The choice of unit is usually based on the intended application of the sensor.

A dedicated ppm-range CO₂ sensor is designed specifically for detection around occupational exposure limits and enclosed space entry requirements. Its calibration, resolution and response time are optimized for the concentrations where enclosed space entry decisions are made.

By contrast, %VOL CO₂ sensors are traditionally used for process monitoring, inerting verification and applications that involve higher carbon dioxide concentrations. This is appropriate for these applications, where detections are typically made in the volume-percent range.

A sharper reading for enclosed space entry

For enclosed space entry, personell safety decisions are based on a 5,000 ppm exposure limit. A dedicated ppm-range sensor provides a detection range, calibration span and indication directly aligned with the concentrations that are relevant to personnel exposure and entry decisions.

When measurements are taken close to the exposure limit, clear interpretation becomes particularly important. If operators are unsure how close conditions are to the 5,000 ppm threshold, they may perform additional ventilation, repeat detection routines, or delay entry until compliance can be confirmed.

By operating, alarming and calibrating directly within the concentration range of interest, a dedicated ppm-range sensor provides a straightforward basis for these decisions.

Our recommendation

%VOL-based sensors serve their intended applications well. However, for enclosed space entry applications where compliance is assessed against a 5,000 ppm exposure limit, a ppm-based sensor will give you a more precise result.

If you have questions about sensor selection for your vessel or fleet, we are happy to help you work through the options.