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Type AB Weir Slot / Air Gap Calculator

Compliant to BS EN 13077:2008

Devices to prevent pollution by backflow of potable water - Air gap with non circular overflow (unrestricted) family A, type B.

Airgap Calculator

Calculations based on a maximum allowable flow rate of 3m/s unless a lower flow rate is known.
Input data required: Number of cistern inlets and their respective diameters "D", width "l" and % open area of slot, weir thickness "Cw" and unobstructed vertical deptth of wier sill upstream face "Uw"

Select "Calculate" and total maximum flow rate "Q", air gap "A", head over weir "h" and weir slot height "Ow" are automatically calculated.

Inlet Sizes "D"

Enter Inlet sizes here Calculate

Add or remove extra Inlets

Flow Rate "Q"

Only enter data when using manual entry of a known value of "Q" (Litres/Min) Calculate

Weir Dimensions

Enter the dimensions of the weir Calculate

Calculation Results

Results valid for single or multiple inlet arrangements if the following conditions are met. Calculate

"h" = head over Weir at Total Maximum Flow.
  1. Width (l) is greater than or equal to 10h at the spillover level. (If not, increase l)
  2. Weir crest thickness (Cw) is less than or equal to 5h
  3. Upstream face of weir (Uw) is vertical to a depth greater than or equal to 5h
  4. The depth of the weir (Ow) is greater than or equal to 2D+h and never less than 20mm. For multiple inlets (D) is the largest feed pipe bore diameter.
All dimensions in mm Acceptable Weir Sizes Validation Criteria see above
Weir Width
Ow (4)
Weir Height
h 1 2 3
Unrestricted Weir. (No insect screen)
Weir Width l increased to account for insect screen obstruction
Slot Height Ow increased to account for insect screen obstruction