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Level · Interactive Instrumentation Guide
Wet-Leg Tank Level
Enter your report LRV and URV to check the calibrated span, or select physical tank geometry to explore hydrostatic level.
Custom process schematic · not to scale · blue H connection / orange L connection, where applicable.
Tank geometry and application settings
Equation and live calculation
P_H = P_v + ρg(h − z); P_L = P_v + ρ_ref g(H − z); ΔP = P_H − P_L; I_ideal = 4 + 16(ΔP − LRV)/(URV − LRV)
Live substitution appears when the demonstration loads.
PV = process value; LRV/URV = lower/upper range values; I = current in mA. Pressure calculations use kPa internally. Selected units apply to inputs, limits, diagrams, and readouts; changing units converts the same physical values without changing the current. Tank calculations use meters internally; z is positive above the bottom tap. ρ = 1000 × specific gravity in kg/m³; g = 9.80665 m/s². H in tank equations means measured height, not the H port. Entered LRV/URV define the current scaling; calculated empty/full tank DP values are reference checks. Set and Reset Points define switch operation; deadband is their absolute difference. Temperature reference equations use °C internally. Mercury pressure units use conventional NIST conversion factors. Vacuum entries are negative gauge pressure; the absolute-pressure readout remains nonnegative. Models assume ideal instantaneous response. Displayed current is limited to the nominal 4–20 mA range: I_display = clamp(I_ideal, 4, 20), where clamp holds a value at its lower or upper limit. This teaching limit does not reproduce manufacturer-specific saturation or alarm currents.
Where it is used
Closed vessels with a stable liquid-filled reference leg, often used where vapor can condense.
What the demonstration shows
The wet leg contributes a constant reference head. Common vapor pressure cancels between H and L, so DP can be negative even when the tank is full. Entered-range mode interpolates applied DP between your LRV and URV; the tank graphic represents calibrated span. Geometry mode calculates physical tank DP independently. LRV and URV alone cannot define actual tank height or density.
Common mistake to avoid
Do not assume 0 kPa means an empty tank. Set LRV and URV from both liquid columns; a negative LRV represents zero elevation.
Model assumptions
Constant densities, hydrostatic equilibrium, completely filled liquid impulse lines, and no gas-column head. The upper tap is at the top of the measured span; zero level is the bottom tap. Change vapor pressure to see cancellation in a closed vessel.
Technical references
Emerson — Differential pressure level measurementNIST — SI unit conversion factors (conventional mercury units)Educational model. Consult the instrument manual for installation, configuration, limits, and approved test procedures. No connection to live plant equipment is made.