🔬 Virtual Experiment Setup
🎛️ Experiment Controls
Manual ModeToggle cutaway internal sensing diaphragm view
| Sr. No. | Tank Level (%) | Liquid Height h (m) | HP Side Pressure P_HP (kPa) | LP Side Pressure P_LP (kPa) | Differential Pressure ΔP (kPa) | Output Current I_out (mA) | Tank Process State |
|---|
Graph 1: Tank Level (%) vs Differential Pressure ΔP (kPa)
Graph 2: Tank Level (%) vs Output Current I_out (mA)
1. Hydrostatic Head Principle
The liquid column inside a vertical vessel exerts a downward hydrostatic head pressure at the bottom of the tank proportional to the liquid column height and liquid specific gravity.
Bottom pressure increases directly with fluid level height.
For standard water service:
- At 0% Level (0 m): High-Pressure side registers 0 kPa (0 mmH₂O).
- At 100% Level (1.02 m): High-Pressure side registers 10.00 kPa (1000 mmH₂O).
2. Differential Pressure Measurement
In a closed process vessel, the transmitter measures the difference between High Pressure (HP) bottom tap and Low Pressure (LP) vapor tap:
Net Pressure Difference = HP Head Pressure − LP Vapor Pressure
Because static vapor space pressure acts equally on both HP and LP chambers, it cancels out completely, ensuring exact liquid level measurement regardless of vessel pressure!
3. 4–20 mA Current Loop Standard
Industrial instrumentation standards use a 4–20 mA current loop for noise-immune analog signal transmission across long plant distances:
Linear output current directly tracks measured level percentage.
- 4.00 mA = Live Zero (0% Level / 0 kPa) - enables wire-break fault detection (0 mA = cable break).
- 12.00 mA = 50% Level (5.00 kPa).
- 20.00 mA = Full Scale Span (100% Level / 10.00 kPa).
4. Sensing Diaphragm Deflection
When High Pressure exceeds Low Pressure, the isolation diaphragm transfers pressure through silicone fill fluid to a central capacitive cell. The central sensing diaphragm flexes proportionally to differential pressure, altering internal cell capacitance which the electronics module converts into a precise 4–20 mA current output.
🏭 Closed Tank & Boiler Drum Level Control
DP transmitters are the gold standard in power plants and refineries for boiler steam drums, distillation columns, and pressurized chemical reactors where top vapor pressure is high.
⚖️ Zero Elevation & Zero Suppression
When impulse lines fill with condensed liquid (Wet Leg configuration), the LP tap has a constant head. Engineers apply Zero Elevation calibration to offset the wet leg head so 0% level still outputs 4.00 mA.
🛠️ 3-Valve Manifold Operation
The 3-valve manifold allows zero checking and maintenance without shutting down the process line:
- Normal Operation: HP Valve OPEN, LP Valve OPEN, Equalizing Valve CLOSED.
- Zero Check Mode: Close HP/LP Valves, OPEN Equalizing Valve. Both chambers equalize (Delta P = 0), transmitter must read exactly 4.00 mA.
- 3-Valve Manifold Sequence: Prevents single-sided overpressure damage during commissioning.
2. Advantages & Technical Specifications
- Suitable for open tanks and pressurized vessels
- High accuracy (+/- 0.075% span)
- Insensitive to liquid foam, turbulence, or dielectric constant changes
- Standardized 4-20 mA & HART digital communication