DP Flow Square Root Scaling Table
Convert between DP and flow for any DP flowmeter. The workbook builds a table from 0 to 100 % flow with the matching DP and transmitter output in linear and square root modes, and converts a live DP reading to flow.
Download
- Calculator sheet with a worked example
- How to Use sheet with steps
- 129 built in formulas, no macros
- Yellow input cells, grey results
- Calculation sign off block
- Excel, Google Sheets and LibreOffice
Worked Example
These are the results of the example already filled in the workbook. Replace the yellow cells with your own data.
Flow, DP and output
| Flow (%) | Flow | DP (mbar) | DP (%) | mA, linear DP output | mA, square root output | Cut off |
|---|---|---|---|---|---|---|
| 0 | 0.00 | 0.00 | 0.00 | 4.000 | 4.000 | Below cut off |
| 5 | 6.00 | 0.62 | 0.25 | 4.040 | 4.800 | Below cut off |
| 10 | 12.00 | 2.50 | 1.00 | 4.160 | 5.600 | |
| 15 | 18.00 | 5.62 | 2.25 | 4.360 | 6.400 | |
| 20 | 24.00 | 10.00 | 4.00 | 4.640 | 7.200 | |
| 25 | 30.00 | 15.62 | 6.25 | 5.000 | 8.000 | |
| 30 | 36.00 | 22.50 | 9.00 | 5.440 | 8.800 | |
| 35 | 42.00 | 30.62 | 12.25 | 5.960 | 9.600 | |
| 40 | 48.00 | 40.00 | 16.00 | 6.560 | 10.400 | |
| 45 | 54.00 | 50.62 | 20.25 | 7.240 | 11.200 | |
| 50 | 60.00 | 62.50 | 25.00 | 8.000 | 12.000 | |
| 55 | 66.00 | 75.62 | 30.25 | 8.840 | 12.800 |
… and 9 more rows in the download.
About this Calculator
DP flowmeters measure the square of flow, which confuses operators and catches out commissioning teams: half flow is only a quarter of the DP. A clear table of flow, DP and mA in both output modes avoids double square root errors and makes calibration quick.
Basis: Square law of DP flow elements: Q ÷ Qmax = √(ΔP ÷ ΔPmax).
How to use
- Enter the maximum flow, the DP at that flow and a low flow cut off.
- Use the table for calibration and for checking DCS readings.
- Type a measured DP into the live conversion to get the flow instantly.
- Remember: take the square root in the transmitter or in the DCS, not both.
Formulas used
- DP = DPmax × (Q ÷ Qmax)².
- Q = Qmax × √(DP ÷ DPmax).
- Linear DP output: mA = 4 + 16 × DP%; square root output: mA = 4 + 16 × flow%.
Related tools
Frequently Asked Questions
Why is 50 % flow only 25 % DP?
DP is proportional to flow squared, and 0.5² = 0.25.
Should square root be done in the transmitter or the DCS?
Either, but only once. Doing it in both gives a wildly wrong reading.
Why use a low flow cut off?
Near zero the square root amplifies noise; a cut off forces the reading to zero below a few % of flow.
Learn more on the blog
Differential pressure transmitter working principle
A practical guide on instrumentationblog.in that explains the engineering behind this workbook.
Read Full Article