Equations Table for International Module W505
Air density
 0 = 1.2 
b0
bs

Ts
T0
 0 is the air density at non standard conditions, b0 is the Barometric pressure at non
standard conditions, bs is the Barometric pressure at standard conditions (760mm Hg), Ts is
the Absolute temperature at non standard conditions and T0 is the Absolute temperature at
standard conditions (2930K)
Under standard temperature conditions use:  =1.2 kg/m3 or kgm-3
Pressures and Velocity in ducts
Pv = Pt - Ps or Pt = Ps + Pv
Where Pv is velocity pressure Pt is total pressure and Ps is static pressure in Pascals (Pa)
Under Standard temperature conditions use:
V =1.29
where V is velocity in m/s or ms-1 or Pv = 0.6 V 2
Pv
Under other temperature conditions use the more general formulae :
V=
2 Pv

where V is velocity in m/s or ms-1 and  is the air density in kg/m3 or kgm-3
or Pv = ½  V 2 where V is velocity in m/s or ms-1 and  is the air density in kg/m3 or kgm-3
Volume and mass
Q  vA or v 
Q
Q
or A 
A
v
Where v is the average air velocity [m/s or ms-1 ] A is cross sectional area where the
velocity measurement was made in [m2] and Q is the volume flow rate in [m3/s or m3s-1]
Circular ducts
A
d 2
4
or d 
4A

Where A is the cross sectional area in m2 (metre2) of the duct and d is the duct diameter in
Metres
Dilution Ventilation
Q
r
C
Where Q airflow rate in m3/sec or m3s-1 r is emission rate mg/s or mgs-1 and C is
equilibrium concentration in mg/m3 or mgm-3
Protection Factors
For RPE often called Nominal Protection factor [NPF]
Protection Factor = Ambient Airborne concentration/ Concentration inside the respirator.
Required Minimum Protection Factor = Ambient Airborne Concentration/ Acceptable
Exposure Level or Standard.
Capture Velocities
Source Conditions
Released into still air with
no velocity
Released at low velocity
into moderately still air
Released at moderate
velocity into moving air
Released at high velocity
into a very turbulent air
stream
Typical situations
Evaporation of solvents from
degreasing tanks, paint
dipping/drying etc.
Welding
Soldering
Liquid transfer
Crushing
Spraying
Cutting
Abrasive blasting
Grinding
Capture Velocity m/s or ms-1
0.25 to 0.5
0.5 to 1.0
1.0 to 2.5
2.5 to >10
Duct /Transport velocities
Type of Contaminant
Gases (non condensing) and Vapours
Smoke and Metal fumes (e.g. welding, brazing etc.)
Light, medium density dusts and powders with a low
moisture content (e.g. cotton dust, sawdust, fine rubber
dust, plastic dusts etc.)
Average industrial dusts (e.g. grinding dust, silica, coarse
rubber dust, cement dust etc.)
Heavy and moist dusts (e.g. lead, metal turnings, moist
cement, quicklime etc.)
Transport Velocity m/s or ms-1
5
10
15
20
25