SI units: slope vs. rate

modelica / ModelicaStandardLibrary

Free (standard conforming) library to model mechanical (1D/3D), electrical (analog, digital, machines), magnetic, thermal, fluid, control systems and hierarchical state machines. Also numerical functions and functions for strings, files and streams are included.

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SI units: slope vs. rate #4215

Closed tobolar closed 10 months ago

tobolar commented 11 months ago

There are defined units indicating change of the unit over time. They have a suffix "slope" or "rate".

The question is:

Shouldn't this be unified?
Shall such units even be a part of Units.SI at all? See https://github.com/modelica/ModelicaStandardLibrary/pull/4103/files#r1152856158

Slope:

Modelica.Units.SI.CurrentSlope
Modelica.Units.SI.TemperatureSlope
Modelica.Units.SI.VoltageSlope

Rate:

Modelica.Units.SI.MassFlowRate
Modelica.Units.SI.VolumeFlowRate
Modelica.Units.SI.EntropyFlowRate
etc.

HansOlsson commented 11 months ago

To me the first answer is 'No', and they are different even if the units are similar:

Slope indicates the derivative of a curve (w.r.t. time).
FlowRate indicates the rate at which the quantity flows past something (per unit of time). Thus a MassFlowRate doesn't necessarily indicate that the mass changes somewhere (especially not in a closed system), just that it flows past something (a valve etc).

With that line of thinking Current (not CurrentSlope) is a ChargeFlowRate. So derivatives have slopes and connectors may contain flow-rates.