Reference Value - property tree

Hi EHS SAP Gurus.

I am trying to define the Reference value for Occupational Exposure Limit for an Agent Risk.

About this I have two questions:

1) I would put the Occupational Exposure Limit in the field "Limit Value" in the Standard Property Tree, but I am not finding the Standard Unit of measure that allow me to have for example 80 V, having 80 in the field of Limit Value and V (voltage) in the unit of measure field.

Is it correct? What do I need to change?

2) I created a phrase dB(A) and I assigned it to Unit of measurement, but when I call the reference value in the measurement project, the System doesn't recognise the unit of measurement and gives me an error about unit dimensions (they must have the same unit dimension) Where can I choose the unit of measure to fill correctly the "non standard unit of measure" in the value assignment instance in the standard property tree? Is there any specification or customizing activity for Property Tree?

Thanks in advance.

Regards,