is not a smart thing to do with it, destroying its “quantity flavour”. var Number gas_unit = ...
and here you’re asking for the result as non-quantity too. In general, just let the system sort that part out.
This should be fairly bombproof var gas_unit = Total_gas.state as QuantityType<Volume> + 0.01 | m³
but you might have a problem with your starting value
which seems to have no units to begin with.
I suppose that’s a leftover from previous attempts.
I suspect this might get you out of that hole - var gas_unit = 0.01 | m³ + Total_gas.state as QuantityType<Volume>
as it usually gives the result in the units of the first operand