About This Tool This calculator combines three related air sampling calculations into one tool. Enter your start and end flow rates along with the sampling time to get the mean flow rate, total air volume sampled, and the number of graticule areas to count on the microscope.
How to Use Enter the start flow rate and end flow rate from your sampling pump (in litres per minute), and the total sampling time in minutes. The calculator will automatically compute the mean flow rate (rounded to 1 decimal place), the total volume of air sampled (rounded to the nearest whole number), and the number of graticule areas to count (always rounded up).
Mean Flow Rate = (Start Flow + End Flow) / 2, rounded to 1 decimal place.
Volume = Sampling Time × Mean Flow Rate, rounded to the nearest whole number.
Number of Graticules = 96,000 / Volume, always rounded up. The constant 96,000 comes from the filter effective area divided by the graticule area for standard 25mm membrane filters with a Walton-Beckett graticule.
Reference Based on HSG248 guidance for fibre counting methodology. The reference table in the calculator shows typical combinations of mean flow rate, sampling time, volume and graticule counts for standard clearance air monitoring.
Revision history 3 March 2026: Initial release
Disclaimer This tool is provided for educational and general information purposes only. It is not a substitute for professional engineering advice, design or verification.
Diggy and its contributors are not licensed engineering consultants and no results generated by this tool should be used directly for construction, design or safety-critical decisions.
All values and outputs are based on published empirical correlations and should be independently checked and confirmed by a qualified geotechnical engineer before use.
By using this tool, you accept full responsibility for how you interpret and apply the information provided.
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