### Why Orifice Plate is having a 3:1 Turndown Ratio in Practice?

It is a general practice in the differential type flow meter, using flow orifice plate as a flow element, to limit the using in a 3:1 to 4:1 **turndown ratio**. Where this general practice comes from?

## Orifice Plate Turndown ratio

The limitation of the DP type flow meter using orifice isnâ€™t comes from the beta ratio limitation of the flow orifice plate nor any other flow orifice plate dimension. The limitation is comes from the error reading caused by the un-linear relationship between the differential pressure across flow orifice plate and the calculated flow rate.

In simplify concept we can calculate the orifice flow rate by:

#### Qm = C x (Differential Pressure)^0.5

Where,

Qm : Flow rate

C : Meter Constant

Differential Pressure : differential pressure across flow orifice plate

From the above equation we know that the flow rate is proportional to the square root of the differential pressure. This un-linear relation produces a significant error in the lower flow when the turndown ratio is exceeding 3:1.

Basically the differential pressure transmitter it self can have a turndown ratio up to 100:1 but the square root extraction limits its capability in the upper range only.

See below calculation example that illustrates the increasing of error reading when the turndown ratio is below 3:1.

### Example of Orifice Flow Meter Error Reading Calculation

From the above example, we know that the % error reading (after perform the above square root calculation) is increasing along with the flow rate decreasing. The % error reading is exceeding 0.5% when the flow rate is lower than 2500 l/m.

Let set this flow rate as our minimum flow rate since the flow rate measurement is having high error below this point. If we look further, the 2500 l/m minimum flow and 7500 l/m maximum flow is correspond to the 3:1 turndown ratio. This is where the 3:1 turndown ratio is applied in our general practices.

If we insist to have minimum flow let say 1000 l/m then we will have a turndown ratio 7.5:1. This will lead us to get an error reading of 2.8 % which means a very bad measurement accuracy.

Thus some engineer usually simply use a 3:1 turndown ratio as their rule of thumb to decide whether they will use DP type flow orifice plate flow meter or not.

If the required turn down is higher than 3:1, then it will be better if we use another type of flow meter even though it is possible to have such higher turn down ratio by installing a parallel DP type flow orifice plate flow meter with a split range configuration.

I donâ€™t have any reason regarding this except the parallel configuration will give us a high cost of material procurement.

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## 1 comment

Nice article, however if the full scale of the transmitter is 200 inH2O and the transmitter accuracy is 0.1% of full scale, is the error in pressure measure not +/- 0.2 inH2O rather than 0.1?