Instrumentation
Orifice Plate Calculator
Square-edged concentric orifice, simplified ISO 5167 form. Verify against full ISO 5167 for custody-transfer accuracy.
Standard method — stated scope limits apply
Geometry (Fixed — From Datasheet/Nameplate)
Live Process Readings
Enter values and calculate.
About This Calculation
An orifice plate is a thin plate with a precisely machined bore, clamped between two pipe flanges. Forcing flow through the smaller bore accelerates it and drops its pressure; measuring that pressure drop with a DP transmitter gives a reading proportional to the square of the flow rate. It remains the most common flow element in industry because it has no moving parts, is cheap to replace, and its behaviour is described by a published international standard (ISO 5167) rather than a manufacturer’s proprietary curve. The trade-off is a real, permanent pressure loss and lower turndown than a venturi or flow nozzle.
Frequently Asked Questions
What beta ratio (bore/pipe diameter) should I use?
Most general process applications use a beta ratio between 0.4 and 0.7. Lower beta gives a larger, easier-to-measure differential pressure but a bigger permanent pressure loss; higher beta reduces both. Beta above about 0.75 starts to lose accuracy under ISO 5167.
How accurate is an orifice plate compared to a venturi?
A well-installed, well-maintained orifice plate is typically accurate to around ±0.5–1% of reading under ISO 5167. A venturi meter is usually similar or slightly better and has a much lower permanent pressure loss, but costs several times more and needs more installation length.
Why does this calculator ask for pipe diameter AND bore separately, not just the ratio?
The discharge coefficient and expansibility factor in ISO 5167 depend on the actual Reynolds number and beta ratio together, not the ratio alone — the same beta at a different pipe size behaves slightly differently.
Does the orifice plate need straight upstream pipe?
Yes — ISO 5167 specifies minimum straight-run lengths (often 10–44 pipe diameters upstream depending on the upstream fitting) to let the velocity profile settle before the plate. Skipping this is one of the most common real-world sources of orifice-flow error.