DP Flow Transmitter Rerange Calculator
When a flow loop needs a new maximum flow, this sheet calculates the new DP range for the same orifice, checks it against the transmitter URL and builds a flow, DP and mA table.
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- Calculator sheet with a worked example
- How to Use sheet with steps
- 58 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.
New range table
| Flow (% of new max) | Flow | DP (mbar) | DP (% of span) | mA (linear transmitter) | mA (square root transmitter) | Notes |
|---|---|---|---|---|---|---|
| 0 | 0.00 | 0.00 | 0.0 | 4.000 | 4.000 | |
| 10 | 5.00 | 3.91 | 1.0 | 4.160 | 5.600 | |
| 20 | 10.00 | 15.62 | 4.0 | 4.640 | 7.200 | |
| 30 | 15.00 | 35.16 | 9.0 | 5.440 | 8.800 | |
| 40 | 20.00 | 62.50 | 16.0 | 6.560 | 10.400 | |
| 50 | 25.00 | 97.66 | 25.0 | 8.000 | 12.000 | |
| 60 | 30.00 | 140.62 | 36.0 | 9.760 | 13.600 | |
| 70 | 35.00 | 191.41 | 49.0 | 11.840 | 15.200 | |
| 80 | 40.00 | 250.00 | 64.0 | 14.240 | 16.800 | |
| 90 | 45.00 | 316.41 | 81.0 | 16.960 | 18.400 | |
| 100 | 50.00 | 390.62 | 100.0 | 20.000 | 20.000 |
About this Calculator
Plant capacity changes often require reranging DP flow loops. This sheet does the square law calculation, checks the transmitter can still measure the new range and produces a test table.
Basis: For a fixed orifice and fluid, DP ∝ Q²: DP_new = DP_old × (Q_new ÷ Q_old)². Flow % = √(DP %). Coefficient changes with Reynolds number are neglected; resize the orifice for large changes.
How to use
- Enter present maximum flow and DP.
- Enter the new maximum flow and transmitter URL.
- Read the new DP span and URL check.
- Use the table to set and test the transmitter.
- Update all related documents and settings.
Formulas used
- DP_new = DP_old × (Q_new ÷ Q_old)².
- DP % = (flow %)².
- mA linear = 4 + 16 × DP %; mA square root = 4 + 16 × flow %.
Related tools
Frequently Asked Questions
Can I rerange without a new orifice?
If the new DP stays within the transmitter URL and a sensible DP range, yes.
Why is turndown important?
A small span on a large URL sensor reduces accuracy.
What else must change?
DCS scaling, totaliser, alarms and documentation.
Learn more on the blog
Mistakes in DP flow measurement
A practical guide on instrumentationblog.in that explains the engineering behind this workbook.
Read Full Article