# Monitored Volume

An airbag is defined as a monitored volume. A monitored volume is defined as having one or more 3 or 4 node shell property sets.

The defined surface must be closed (the normal to shell elements must be all oriented outward, as shown in Figure 1. The shell normal must be oriented outside the volume. It is possible to reverse the shell normals for a given property set (by entering a negative property number). Dummy properties (/PROP/TYPE0) and materials (/MAT/TYPE0) can be used.
There are five types of MONITORED VOLUME:
1. AREA Type: volume and surface output (post processing option, no pressure)
2. PRES Type: user function defining pressure versus relative volume
3. GAS Type: adiabatic pressure volume relation. $P{\left(V-{V}_{inc}\right)}^{\gamma }=Cst$ with $\gamma =\frac{{c}_{p}}{{c}_{v}}$
4. AIRBAG Type: Single airbag $PV=nRT=mrT$

$P=\rho rT=\rho \left({c}_{p}-{c}_{v}\right)T=\left(\gamma -1\right)\rho {c}_{v}T=\left(\gamma -1\right)\frac{E}{V}$

5. COMMU1 Type: Chambered, communicating, folded airbag (airbag with communications)

Same basic equations:

Typical use of monitored volume is for tire, fuel tank, airbag.
• For tire use PRES or GAS type monitored volume.
• For fuel tank use PRES or GAS type monitored volume.
• For simple unfolded airbag use AIRBAG type monitored volume.
• For chambered airbag use two ore more COMMU1 type monitored volumes.
• For folded airbag use a set of COMMU1 type monitored volumes.