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Square Root Flow and DP Table
Square root flow table for orifice, venturi and other DP flow elements, linking differential pressure, flow and transmitter output.
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DP percent to flow percent
| DP % | Flow % | Linear DP output mA | Square rooted output mA |
|---|---|---|---|
| 0 | 0.00 | 4.000 | 4.000 |
| 1 | 10.00 | 4.160 | 5.600 |
| 2 | 14.14 | 4.320 | 6.263 |
| 4 | 20.00 | 4.640 | 7.200 |
| 5 | 22.36 | 4.800 | 7.578 |
| 9 | 30.00 | 5.440 | 8.800 |
| 10 | 31.62 | 5.600 | 9.060 |
| 16 | 40.00 | 6.560 | 10.400 |
| 20 | 44.72 | 7.200 | 11.155 |
| 25 | 50.00 | 8.000 | 12.000 |
| 30 | 54.77 | 8.800 | 12.764 |
| 36 | 60.00 | 9.760 | 13.600 |
| 40 | 63.25 | 10.400 | 14.119 |
| 49 | 70.00 | 11.840 | 15.200 |
| 50 | 70.71 | 12.000 | 15.314 |
| 60 | 77.46 | 13.600 | 16.394 |
| 64 | 80.00 | 14.240 | 16.800 |
| 70 | 83.67 | 15.200 | 17.387 |
| 80 | 89.44 | 16.800 | 18.311 |
| 81 | 90.00 | 16.960 | 18.400 |
| 90 | 94.87 | 18.400 | 19.179 |
| 100 | 100.00 | 20.000 | 20.000 |
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Flow percent to DP percent
| Flow % | DP % | Linear DP output mA | Square rooted output mA |
|---|---|---|---|
| 0 | 0.00 | 4.000 | 4.000 |
| 5 | 0.25 | 4.040 | 4.800 |
| 10 | 1.00 | 4.160 | 5.600 |
| 15 | 2.25 | 4.360 | 6.400 |
| 20 | 4.00 | 4.640 | 7.200 |
| 25 | 6.25 | 5.000 | 8.000 |
| 30 | 9.00 | 5.440 | 8.800 |
| 35 | 12.25 | 5.960 | 9.600 |
| 40 | 16.00 | 6.560 | 10.400 |
| 45 | 20.25 | 7.240 | 11.200 |
| 50 | 25.00 | 8.000 | 12.000 |
| 55 | 30.25 | 8.840 | 12.800 |
| 60 | 36.00 | 9.760 | 13.600 |
| 65 | 42.25 | 10.760 | 14.400 |
| 70 | 49.00 | 11.840 | 15.200 |
| 75 | 56.25 | 13.000 | 16.000 |
| 80 | 64.00 | 14.240 | 16.800 |
| 85 | 72.25 | 15.560 | 17.600 |
| 90 | 81.00 | 16.960 | 18.400 |
| 95 | 90.25 | 18.440 | 19.200 |
| 100 | 100.00 | 20.000 | 20.000 |
About this Table
Square root flow table for orifice, venturi and other DP flow elements, linking differential pressure, flow and transmitter output.
Basis: Flow is proportional to the square root of differential pressure: Q% = 10 × √(DP%), DP% = Q%² ÷ 100.
How to use
- Know whether square root is done in the transmitter or the DCS: only once.
- For a linear DP transmitter, use the 'Linear DP output' column.
- For a transmitter with square root on, use the 'Square rooted output' column.
Notes
- Below about 10 % flow (1 % DP) the signal is very sensitive to zero errors; most systems apply low flow cut off.
- Doubling the flow needs four times the DP.
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Frequently Asked Questions
What flow is 25 % DP?
50 % flow.
What DP gives 50 % flow?
25 % of the DP range.
Why use low flow cut off?
Near zero, small DP noise gives large flow changes because of the square root.
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Mistakes in DP flow measurement
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