SKC Limited AirChek 3000 Deluxe Pump Step By Step Guide - Manual (Page 6)

Page 4 224-G1 Issue D www.skcltd.com
Basic Formulae
Calculation of the volume of air sampled:
Volume of air sampled = Sample flow rate x sample time
Example:
Flow rate = 2 l/min
Sample time = 8 hours = 8 x 60 = 480 minutes
Volume of air = 2 (l/min) x 480 (minutes) = 960 litres
Volume of air = 960 / 1000 = 0.96 m³ (cubic metres)
Conversion of ppm (parts per million) to mg (milligrams):
mg/m³ = ppm x (molecular weight* / 24)
*If the molecular weight of the contaminant is not known it can be determined using a periodic
table.
Calculation of time weighted average (TWA):
To determine an 8 hour TWA from more than one sample within any 24 hour period:
TWA = (C1 x T1) + (C2 x T2) + .....(Cn x Tn)
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where C is the occupational exposure and T is the time for that exposure.
Example:
An operator is exposed to 0.12mg/m³ for 7 hours and 20 minutes.
The 8 hour TWA is therefore 7h20min @ 0.12mg/m³ (C1 x T1) plus 40min at
0mg/m³ (C2 x T2).
TWA = (0.12 x 7.33) + (0 x 0.67) = 0.11mg/m³
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Note: 7h20min = 7.33 hours, 40min = 0.67 hours