What the calculator is for
A DP transmitter measures level as the hydrostatic head of liquid above its high-pressure (HP) tap: ΔP = SG × ρw × g × h. The range you configure depends on where the taps and transmitter sit and on what fills the low-pressure (LP) side. Getting it wrong shifts every level reading, so calculate it before configuring the transmitter.
- Open tank: LP side vented to atmosphere.
- Closed tank, dry leg: LP impulse line full of vapour or gas, so the vessel pressure cancels.
- Closed tank, wet leg: LP line full of liquid (condensate or seal fill). Its head acts against the process head, giving a negative LRV: an elevated zero.
Worked example
A closed vessel holds a liquid of SG 0.85. The 0 % level is 300 mm above the HP tap, the span is 1,500 mm and the LP tap is 2,200 mm above the HP tap with a wet leg of water (SG 1).
At 0 %: ΔP = 0.85 × 0.3 m − 1.0 × 2.2 m = −1.945 m H₂O = −190.74 mbar. At 100 %: 0.85 × 1.8 m − 2.2 m = −0.67 m H₂O = −65.705 mbar. The span is 125.03 mbar; the transmitter is ranged −190.74 to −65.705 mbar. Load this case from the examples above to see each step.
Head of one metre of water
| Unit | 1 m H₂O = |
|---|---|
| mbar | 98.0665 |
| kPa | 9.80665 |
| psi | 1.42233 |
| inH₂O | 39.3701 |
| mmH₂O | 1,000 |
Common mistakes
- Design SG instead of operating SG. Density changes with temperature and composition; a 5 % SG error is a 5 % level error.
- Forgetting the transmitter mounting height. A transmitter below the HP tap sees the extra head of the filled impulse line: a suppressed zero.
- A wet leg that drains or a dry leg that fills. Both move the zero. Condensing vapours need a wet leg or seals, and the wet leg must stay full.
- The LP tap below the maximum level. The process then floods the LP leg and the measurement no longer works.
Basis
Hydrostatic pressure p = ρgh with the conventional water density 1000 kg/m³ and standard gravity 9.80665 m/s², the definitions behind mmH₂O and inH₂O (4 °C). Some transmitters use inH₂O at 60 °F or 68 °F; check the unit configured in the device.