Pressure & Level Measurement

Learn gauge, absolute and differential pressure; manometers, Bourdon tubes, diaphragms, strain-gauge, capacitive and piezo-resistive sensors; smart DP transmitters, impulse lines, manifolds and remote seals; and level by DP (hydrostatic), zero elevation/suppression, bubblers, displacers, floats, radar, ultrasonic, guided-wave radar, capacitance, nuclear and level switches.

Pressure is measured everywhere — for its own sake (boiler drums, pipelines) and as the basis for flow (DP across an orifice) and level (hydrostatic head). The differential-pressure (DP) transmitter is probably the most common instrument in process plants.

Level measurement keeps tanks from overflowing or running dry. There are many technologies because liquids differ: foaming, sticky, corrosive, changing density, high pressure. Picking the wrong one is a classic cause of spills.

An analogy

Measuring level with pressure is like judging how deep a swimming pool is by how hard the water pushes on your ears at the bottom — the deeper (and denser) the water, the harder the push.

Pressure references

Gauge pressure is relative to atmosphere (tyre pressure), absolute is relative to vacuum (barometers, vapour-pressure work), differential is between two points (filters, orifices, level). Units: Pa, kPa, bar, psi, mmH₂O, mmHg — always state gauge (g) or absolute (a).

Pressure sensors

Mechanical elements turn pressure into motion, then electronics into a signal:

Installing DP transmitters

Impulse lines connect process taps to the transmitter: slope them so gases vent (liquid service, transmitter below) or liquids drain (gas service, transmitter above). A 3- or 5-valve manifold allows zeroing and isolation. Remote diaphragm seals with capillaries avoid plugging and freezing but add temperature effects and slower response. Plugged or leaking impulse lines are the most common DP failure.

Level technologies

Select by fluid and vessel:

Wet legs, interfaces and density compensation

In closed pressurised vessels a DP transmitter's low side connects to the vapour space. If vapour condenses, a wet (filled) reference leg is used; its head makes the DP negative when the tank is full → zero elevation (negative LRV). Two-DP or DP + GWR combinations measure interface level and density. Boiler drum level requires density compensation because steam/water density changes with pressure — a famous cause of boiler accidents.

Formulas

Worked example: Absolute pressure

Given: Gauge reads 250 kPa(g), barometer 100 kPa

Find: Absolute pressure.

Answer: 350 kPa(a)

Worked example: DP level range

Given: Oil SG 0.85, 0–4 m, transmitter 0.5 m below tap

Find: Calibrated range.

Answer: 4.17 to 37.51 kPa

Worked example: Radar level

Given: Tank 10 m, echo time 40 ns

Find: Level.

Answer: 4 m

Ranging a DP level transmitter

g = 9.80665
rho = 850.0                 # oil, SG 0.85
h_max = 4.0                 # m of level to measure
d_below = 0.5               # transmitter mounted 0.5 m below the lower tap

span = rho * g * h_max / 1000            # kPa
lrv = rho * g * d_below / 1000           # zero suppression
urv = lrv + span
print(f"span {span:.2f} kPa, LRV {lrv:.2f} kPa, URV {urv:.2f} kPa")
for h in (0, 1, 2, 4):
    dp = lrv + rho * g * h / 1000
    mA = 4 + 16 * (dp - lrv) / span
    print(f"level {h} m -> dP {dp:6.2f} kPa -> {mA:5.2f} mA")
# span 33.34 kPa, LRV 4.17 kPa, URV 37.51 kPa
# level 0 m -> dP   4.17 kPa ->  4.00 mA
# level 1 m -> dP  12.50 kPa ->  8.00 mA
# level 2 m -> dP  20.84 kPa -> 12.00 mA
# level 4 m -> dP  37.51 kPa -> 20.00 mA

Because the transmitter sits below the tap, liquid in the impulse line adds 4.17 kPa even when the tank is 'empty'. Ranging the transmitter 4.17–37.51 kPa (zero suppression) makes 4 mA = 0 m. If density changes, the level reading changes — DP level really measures mass/head, not height.

Common mistakes

Applications

Key takeaways

Going further