DP (hydrostatic) level
A liquid column exerts a pressure proportional to its height and density. A DP transmitter connected between a lower tap (HP side) and the vapour space or an upper tap (LP side) measures that head:
ΔP = ρ · g · h (in mmH₂O: ΔP = SG × h[mm])
The output depends on density: if the specific gravity changes (temperature, composition), the indicated level changes even if the real level doesn't. Keep that in mind for hot services and for liquids with variable composition.
Dry leg, wet leg and remote seals
- Open tank or dry leg: the LP side sees only atmosphere or non-condensing vapour. If the transmitter is mounted below the lower tap, the liquid in the HP impulse line adds a constant head: the zero is suppressed (LRV > 0).
- Wet leg: on a closed vessel with condensing vapour (steam drums, many hydrocarbon vessels), the LP leg fills with liquid. Now the LP side always carries a full column, so the DP is negative at low level: the zero is elevated (LRV < 0).
- Remote diaphragm seals: capillaries filled with silicone or another fill fluid replace the impulse lines. They are used for corrosive, viscous, hot or plugging services. The fill-fluid head between the seals must be included in the calculation, and temperature effects on the capillaries are a known error source.
Worked example: wet leg
The taps are 3000 mm apart (H), the process SG is 0.85, the wet-leg fill SG is 1.0, and the transmitter is mounted 500 mm (d) below the lower tap. The HP side is connected to the lower tap.
LRV (0 %) = SGp·(0 + d) − SGf·(H + d) = 0.85·500 − 1.0·3500 = −3075 mmH₂O
URV (100 %) = SGp·(H + d) − SGf·(H + d) = 0.85·3500 − 1.0·3500 = −525 mmH₂O
Span = SGp · H = 2550 mmH₂O
Notice that the span depends only on the process SG and the tap distance. The mounting height and the fill fluid only shift the zero. Check your own numbers with the DP Level Calibration calculator.
Radar and guided wave radar (GWR)
Non-contact radar measures the time of flight of a microwave pulse (or the frequency shift of an FMCW sweep) reflected from the liquid surface. Modern 80 GHz instruments have a narrow beam that avoids most internal obstructions. They are unaffected by density, pressure and temperature changes, which is a big advantage over DP.
Guided wave radar sends the pulse down a rod or cable probe. It handles foam, turbulence and low dielectric liquids better, and can measure interface between two liquids. It is a common replacement for displacers.
Other technologies
- Displacer: Archimedes' principle; buoyancy on a displacer is measured via a torque tube. It is proven and good for interface, but it is mechanical and density-dependent.
- Ultrasonic: cheap and non-contact, good for water and wastewater sumps. It is affected by vapour, foam, condensation and temperature.
- Magnetic level gauge (float): excellent local indication, often fitted with a transmitter or switches.
- Nuclear (gamma): for extreme services (very hot, toxic or highly corrosive) where nothing can touch the process. It requires radiation licensing.
- Level switches: vibrating fork, float and conductive switches for high/low alarms and pump protection.
Interface and density
When both taps are submerged, a DP transmitter measures the average density between the taps. With two immiscible liquids (oil over water), that gives the interface position:
0 % = all lighter liquid between taps : ΔP = H · (SGupper − SGfill)
100 % = all heavier liquid between taps : ΔP = H · (SGlower − SGfill)
The span is H · (SGlower − SGupper), which is often small. Tap spacing and density difference must give enough DP for a stable reading. See the Interface & Density calculator.
Selection guide
| Technology | Best for | Avoid when |
|---|---|---|
| DP | General liquids, pressurised vessels, interface | Density varies a lot; plugging lines (use seals) |
| Non-contact radar | Storage tanks, reactors, most liquids | Very low εr without still pipe; heavy foam |
| Guided wave radar | Interface, foam, small chambers, displacer replacement | Heavy coating/build-up on the probe |
| Displacer | Interface, existing cages | Varying density; maintenance-limited sites |
| Ultrasonic | Water, wastewater, open sumps | Vapour, foam, pressurised vessels |
| Nuclear | Extreme, inaccessible services | Any other option is practical |