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

## Why Does Orifice Plate Have a 3:1 Turndown Ratio in Practice?

The limitation of the DP-type flow meter using an orifice doesn’t come from the beta ratio limitation of the flow orifice plate nor any other flow orifice plate dimension.

The limitation comes from the error reading caused by the non-linear relationship between the differential pressure across the flow orifice plate and the calculated flow rate.

## Orifice Plate Turndown Ratio

In simplified 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 non-linear relation produces a significant error in the lower flow when the turndown ratio exceeds 3:1.

Basically, the differential pressure transmitter itself 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 below that illustrates the increase in 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 performing the above square root calculation) is increasing along with the flow rate decreasing. The % error reading exceeds 0.5% when the flow rate is lower than 2500 l/m.

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

If we insist on having minimum flow let’s say 1000 l/m then we will have a turndown ratio of 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 a 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 a higher turn-down ratio by installing a parallel DP-type flow orifice plate flow meter with a split range configuration.

I don’t have any reason regarding this except the parallel configuration will give us a high cost of material procurement.

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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?