Choosing
the element
Start with the element: the first table sets the differential pressure elements side by side. For an orifice, two more choices follow — how the plate is held in the line, and what the plate itself looks like. Everything after that is arithmetic — and we do that part.
Which element
The differential pressure elements side by side, with the values their standards give.
| Element | Standard | Uncertainty of C, uncalibrated | Permanent pressure loss | Upstream straight run | Typically chosen for |
|---|---|---|---|---|---|
| Orifice plate | ISO 5167-2 | 0.50 % at β 0.50, 0.75 % at β 0.75 (ISO 5167-2) | About 45 – 95 % of the differential, falling as β rises | About 20D after a single 90° bend at β 0.50, more than twice that at β 0.75 (ISO 5167-2) | The most widely used differential pressure element |
| Venturi tube | ISO 5167-4 | 1 – 1.8 % machined, 1.5 % fabricated convergent | 5 – 20 % of the differential | Typically 5D after a single bend (ISO 5167-4) | Where permanent pressure loss is expensive |
| Flow nozzle | ISO 5167-3 | 0.8 – 1.2 % ISA-1932, 2.0 % long radius | About 25 – 84 % of the differential (ISA-1932, β 0.80 – 0.30) | Set by β and the upstream fitting (ISO 5167-3) | High velocity, high temperature and erosive service |
| Venturi nozzle | ISO 5167-3 | 1.2 – 1.7 %, set by β | 5 – 20 % of the differential | Set by β and the upstream fitting (ISO 5167-3) | Hard service, with most of the loss recovered |
| Cone meter | ISO 5167-5 | 5 % (ISO 5167-5) | About 48 – 72 % of the differential (β 0.75 – 0.45) | 3D – 6D after a single bend (ISO 5167-5) | Lines without the straight pipe a plate needs |
| Wedge meter | ISO 5167-6 | 4 % (ISO 5167-6) | About 53 – 79 % of the differential (h/D 0.5 – 0.2) | 7D – 22D by the upstream fitting (ISO 5167-6) | Slurries, heavy oil, abrasive or viscous service |
| Averaging pitot tube | Not covered by ISO 5167 | Stated on the calculation sheet | Low; stated on the calculation sheet | Set by the probe and the line; on the calculation sheet | Large lines and rectangular ducts, through one connection |
Uncertainty of C applies within the limits of the standard named. Rangeability is the same for all of them: flow follows the square root of the differential, so one transmitter over 10 : 1 in differential pressure covers about 3 : 1 in flow; a second transmitter extends it. The calculation sheet states the uncertainty for your duty.
How the plate is held
Scope of supply increases down the list. The last two are the only constructions built around reaching the plate.
| Family | What you receive | Typically considered when | Plate access |
|---|---|---|---|
| Single Orifice Plate | The machined plate only, for existing orifice flanges or fittings | The flanges or fitting are already in the line | Line isolated |
| Orifice Flange Union | Plate, matched flange pair, tappings, studs, gaskets, jack screws | A new measurement point on an existing line | Line isolated |
| Compact Orifice | Short-body element with integrated tappings, minimal impulse piping | Space is tight, or impulse lines are unwelcome | Line isolated |
| Small Bore Meter Run | Element plus controlled upstream and downstream bore sections | Small line where field alignment is hard to control | Line isolated |
| Single Chamber Fitting | Serviceable fitting body with one plate-access chamber | Plate changes are frequent but outages are acceptable | Simplified, line isolated |
| Dual Chamber Fitting | Lower measurement chamber, upper service chamber, isolation mechanism | The line cannot be taken down to change a plate | Designed for process continuity |
What the plate looks like
Bore geometry and inlet profile are separate choices — a plate can have a concentric bore and a square-edge inlet at the same time.
| Design | Typically considered for | Design dimension |
|---|---|---|
| Concentric square-edge | Clean single-phase liquid, gas and steam — the conventional choice | Bore + inlet |
| Quadrant edge | Viscous fluids and lower Reynolds number service | Inlet profile |
| Conical entrance | Viscous fluids at the low end of the Reynolds range | Inlet profile |
| Eccentric bore | Entrained gas, liquid or suspended material in the stream | Bore position |
| Segmental opening | Contaminated fluids needing a larger open passage | Opening geometry |
| Multi-hole (conditioning) plate | Limited upstream and downstream straight pipe length | Flow conditioning |
These are general starting points, not selections. The final choice is confirmed by calculation against the applicable standard and your service data — send it over and we will do that part.
Narrowed it down?
Send the pipe size, flow rate, pressure, temperature and medium. We come back with a sized element, a calculation sheet and a price.