DP Level Transmitter Range Calculator (Wet Leg and Dry Leg)
By Santaji GadepatilinUpdated October 28, 2026
A DP transmitter measures level as the pressure of the liquid column, so its calibration range depends on liquid density, tap positions and any fill in the impulse lines. Getting LRV and URV right is the key step — see wet leg and dry leg level measurement and zero elevation and zero suppression. This DP level range calculator works out both.
How to use the DP level range calculator
Enter the specific gravity of the process liquid, the measuring span (distance between minimum and maximum level), and the height of the minimum level above the HP tap. Choose open/dry leg or wet leg; for a wet leg, enter the fill fluid’s SG and the leg height. The calculator gives LRV, URV and span in mm of water column (1 mmH₂O ≈ 0.0981 mbar).
DP level range formulas
LRV = SGp × h₀ − SGs × Hs
URV = SGp × (h₀ + H) − SGs × Hs
The wet-leg term is zero for open tanks and dry legs.
Example: wet-leg tank
Process SG 1.0, span 3,000 mm, minimum level 500 mm above the HP tap, wet leg filled with water (SG 1.0) 4,000 mm high:
- LRV = 500 − 4,000 = −3,500 mmH₂O
- URV = 3,500 − 4,000 = −500 mmH₂O
- Span = 3,000 mmH₂O
Choosing the reference leg
| Tank | LP side | Range type |
|---|---|---|
| Open to atmosphere | Vented | Suppressed or zero-based |
| Closed, non-condensing gas | Dry leg | Suppressed or zero-based |
| Closed, condensing vapour (steam drums) | Wet leg | Elevated (negative LRV) |
| Closed, either | Remote diaphragm seals (capillary) | Elevated, fill fluid SG |
Tips
- Wet legs must stay full — a partly drained leg causes large level errors.
- With diaphragm seals, use the capillary fill fluid SG and the vertical distance between seals.
- Density changes shift the reading; for varying density see DP level measurement with varying density.
Frequently asked questions
How do I calculate LRV and URV for a DP level transmitter?
LRV = SG × (height from HP tap to min level) − (wet-leg SG × wet-leg height); URV = LRV + SG × span. Results are in mm (or inches) of water column.
What is zero suppression?
When the minimum level is above the HP tap, the transmitter already sees pressure at 0% — the LRV is positive.
What is zero elevation?
With a wet leg, the reference side has a constant head that makes the DP negative at low level — the LRV is negative and the range is "elevated".
Does the span depend on the wet leg?
No. Span is always the process liquid SG × measuring height; the wet leg only shifts the range.