Mass unit used internally during simulation when UnitConversion is on
This property defines the mass unit that SimBiology uses internally
during model simulation when
on. You can set this to any string representing a mass unit such as
gram, or gram with any valid prefix. It can also be a custom unit
if it is consistent with mass as its dimension. The default is
which means SimBiology automatically selects a mass unit for simulation.
SimBiology examines the units on all of the states and selects a mass
unit such that
the states in mass or mass per volume is at least as stringent as
the simulation absolute tolerance multiplied by the smallest mass
unit. This stringency is relaxed appropriately for states that become
It is recommended that you use the default unit (
or choose units for states such that the simulated values are neither
too large (greater than 106) or too small
(less than 10-6).
However, for some edge cases, you may need to change
Suppose you have a model with a state that takes on values around
gram for the entire
simulation, and you need to use
gram as its unit.
Then it may be appropriate to set
In this case, the internal simulation values would be around 1, instead
of around 10-12 as in the default case.
AbsoluteTolerance of the simulation is
determined using this internal value. Thus by choosing
the mass unit, you effectively reduce the size of
MassUnits property is closely related
AbsoluteTolerance when considering
the effects on simulation results.
Even when using the default unit, it may be still necessary
For details, see Selecting Absolute Tolerance and Relative Tolerance for Simulation.
If you need to recover the simulation behavior from releases prior to R2015b:
However, if the model has quantity units in
set the unit to
If you have a custom function and
on (whether or not you are using the default unit
follow the recommendation below.
Non-dimensionalize the parameters that are passed to the function if they are not already dimensionless.
Suppose you have a custom function defined as
y = f(t) where
the time in hour and
y is the concentration of
a species in mole/liter. When you use this function in your model
to define a repeated assignment rule for instance, define it as:
= f(time/t0)*s0, where
time is the simulation
t0 is a parameter defined as 1.0 hour,
a parameter defined as 1.0 mole/liter, and
the concentration of a species in mole/liter. Note that
not have to be in the same units as
but they must be dimensionally consistent. For example, the
can be set to minute and picomole/liter, respectively.
|Applies to||Object: |
|Data type||Character vector|
Character vector specifying any mass unit. The default
Read/write for properties of