Flow Measurement · Free Calculator
Tank Level Rate to Flow Calculator
Use a tank as a flow meter: from the change in level over a time and the tank size, find the net flow, and the unmeasured outflow (or inflow) when the other stream is known.
Level changeNet flowMass balanceFlow meter check
m²
Net flow (in − out)22.619 m³/h
Calculated outflow7.3805 m³/h
Volume change5.655 m³
Tank area12.57 m²
Level rate1,800 mm/h
How this result was calculated
- Net flow = A × Δh ÷ Δt = 12.57 × 0.45 ÷ 900 s = 22.619 m³/h
- Outflow = inflow − net flow = 30 − 22.62 = 7.3805 m³/h
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Level Rate Mass Balance
Qnet = A × (h₂ − h₁) ÷ Δt
Qout = Qin − Qnet
Qout = Qin − Qnet
A is the tank cross section at the liquid level (constant for vertical tanks and straight sided pits), h₁ and h₂ the levels at the start and end of the period and Δt the time between them. A rising level gives a positive net inflow.
Worked example
A 4 m tank rising from 2.10 to 2.55 m in 15 minutes gains 5.65 m³, a net 22.6 m³/h. With 30 m³/h known inflow, the outflow is about 7.4 m³/h.
Key insight: a level rate test is a quick way to check a suspect flow meter in the field: stop other flows, run through the meter into a tank and compare.
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Frequently Asked Questions
How accurate is flow from level change?
It depends on level resolution and tank area accuracy. Use a long enough period that the level changes by at least several centimetres.
Can I use it for horizontal tanks?
Only over small level changes near mid height, or use the strapping table volumes instead of area × level.
Does temperature matter?
For large temperature changes the liquid expands, which shows up as level change without flow.
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