DP Flow Transmitter Rerange Calculator
Rerange a DP flow transmitter on an existing primary element. Flow varies with the square root of DP, so changing one sets the other.
- ΔP2 = ΔP1 × (Q2 ÷ Q1)² = 250 mbar × (120 ÷ 100)² = 360 mbar
Square Root Rerange Formula
ΔP2 = ΔP1 × (Q2 ÷ Q1)²
Q1 and ΔP1 are the present full scale flow and DP, Q2 and ΔP2 the new ones. The bore, fluid density and discharge coefficient are assumed unchanged; if density changes too, use the orifice calculator.
Worked example
An orifice gives 100 m³/h at 250 mbar. To read 120 m³/h full scale, the new DP range is 250 × (120 ÷ 100)² = 360 mbar. Raising the range to 500 mbar instead gives 100 × √2 = 141.4 m³/h.
Frequently Asked Questions
Why does flow change with the square root of DP?
Bernoulli: DP across a restriction is proportional to velocity squared, so flow is proportional to √DP.
Do I need a new orifice to rerange?
Not if the new DP stays within the transmitter range and the meter limits. A new bore is needed for large changes or when density changes.
What else must be updated?
The transmitter range, the DCS or PLC flow scaling, the totaliser factor and the instrument datasheet.
Learn more on the blog
What factors affect the stability of flow meter readings?
A practical, field focused guide on instrumentationblog.in that explains the theory behind this calculator and how engineers apply it on real plants.
Read Full Article