How does changing units in the Flow Properties (of elementary flows) affect the processes using the flow?

Question

Using Ecoinvent, in some cases, I want my flow units to be in mass and not in volume, as they are set by default, but I'm not sure what are the effects on the processes that use those flows.

For example, I have water vapor emissions in tonnes, but they are defined in m3 in Ecoinvent. Going to the flow properties of say "Elementary flows/Emissions to air/unspecified/Water", I can add Mass as a flow property and then choose e.g. "t - Mass" as units for the flow in my process. However, the flow properties page only gives the Reference unti in kg, not in t, while the Ref unit for Volume is m3. And if I'm still selecting Volume as my reference (check the box in "Is reference" column), my Mass property shows a formula "1 m3 = 1 kg".

Does this mean that if I now use this flow in my process, but choose "t - Mass", as the flow units, my flow value will be incorrectly scaled by a factor of 1000? Or is it safe to simply add the Mass flow property and then use the mass units normally the process?

Answer 1

Hi, good question. If you add a flow property to a flow (mass to a flow that so far understands only volume) you enable additional units to be used (for mass, all mass-related units, kg but also t etc.). It is your responsibility to specify the conversion factor correctly, the software does not know this of course. In your case, 1kg of water is not 1m3 of water, so the factor you set seems incorrect. You need to enter a different value (0.001 m3 = 1 kg, for water). Once you have set it correctly, you can use both "unit groups" (mass, and volume) as you wish, for that flow, and the conversion works correctly across the different units. Maybe this is clear now?

Comments

Thank you for the answer. However, I'm unable to change the conversion factor. In the flow properties window (the upper picture), none of the cells are editable. How should I do this?