A differential pressure density meter is the oldest continuous density method still on plants, and also the most misunderstood. It does not vibrate, it does not radiate, and it has no part moving in the stream. It simply reads the weight of a known column of liquid and turns that weight into density. For the right duty — thick slurries, viscous liquors, two-phase streams — it outlasts instruments that are far more elegant on paper. For the wrong one, it quietly lies because the column it trusts is not as fixed as the maths assumes.
The equation that does all the work
The whole instrument rests on one line of hydrostatics:
ρ = ΔP / (g × H)
Pressure difference between two points, divided by gravity and the vertical distance between them, gives density. If the height H is fixed and known, and you measure the pressure drop ΔP with a differential pressure transmitter, the rest is arithmetic. No resonant frequency to track, no sound path to keep clear, no source to license.
That simplicity is the appeal and the trap. The meter is only as good as its belief that H — the distance between the two taps — is exactly what the drawing says, and that the liquid between them is the liquid you think it is. Disturb either and the number drifts without the transmitter ever failing.
From the field. On a tailings underflow line we were asked to replace a Coriolis cell that had stalled in a slurry too thick and gritty for it to keep flowing. The plant did not want a radioactive gauge. A hydrostatic arrangement — two flush diaphragm seals a fixed height apart on the same pipe, feeding one DP transmitter — kept reading through solids loadings that had stopped the Coriolis dead. The reading was not ±0.001 g/cm³, but it was stable enough to hold the circuit, which is what the operator actually needed.
What the instrument actually is
A practical differential pressure density meter is two flush-mounted diaphragm seals set a known vertical distance apart on the same process connection, linked by oil-filled capillaries to one DP transmitter. The diaphragms sense only pressure; they do not touch flow, viscosity, or entrained gas the way a vibrating element does. Because the sensing surface is flush, solids that would coat a tuning fork slide past, and there is no small bore for slurry to bridge.
The transmitter does the rest: it reads ΔP, applies its own temperature correction from a Pt100 at the sensor, and outputs density, concentration, or a derived unit on 4–20 mA or a digital bus. There is no moving part in the wetted path, which is why the method survives abrasive and scaling services that chew through more delicate cells.
The inline density measurement methods guide groups this technique with the other continuous principles and lays out where each fits. It is worth reading before you decide a technology on a brochure.
Where DP density quietly wins
The differential pressure density meter earns its place in three situations where resonant and acoustic methods struggle:
- Heavy or settling slurries. Thickener underflow, tailings, cement slurry, black liquor — anything with solids that load a fork or stall a Coriolis tube. A flush diaphragm does not care how thick the stream is.
- Viscous and two-phase liquors. The reading comes from pressure, not motion, so there is no minimum flow to maintain and no upper viscosity limit. Bubbles that scatter ultrasound barely register against a pressure signal.
- Non-nuclear duty by choice. For plants that will not carry a sealed radioactive source, a DP arrangement is a genuine non-nuclear path to slurry density, sitting beside the non-nuclear density measurement option for chemical industry.
Against a clean, single-phase liquid that needs the tightest accuracy, a tuning-fork density meter or a Coriolis density meter is usually the better fit. The DP method is not trying to win that fight.
The catch: it measures a column, not a point
A resonant meter measures the fluid at the element. A differential pressure density meter measures the average density of the entire column between the two taps. If that column height is not what the calculation assumes — because the pipe is not truly vertical, because level in a vessel swings, because the lower tap sits in a different regime than the upper one — the error lands straight in the density value.
This is why accuracy figures for the method span a wide range in the literature, from ±0.5% of reading on a well-controlled vertical span to several kg/m³ where level is uncertain. The transmitter is rarely the weak link. The installation is. A density measurement error diagnosis rarely blames the DP cell; it blames the geometry around it.
One more property worth knowing: because density maps to concentration for a given fluid, most DP density transmitters output percent concentration or °Brix directly through a stored correlation. The same hydrostatic reading that gives you density also gives you the process variable the control system actually wants — provided the fluid composition is stable enough that the density-to-concentration model stays valid. That is a strength of the method, and a reminder that the model, not the transmitter, sets the concentration accuracy.
DP versus fork versus Coriolis — pick on the duty
Most enquiries resolve on one axis, and the differential pressure density meter belongs at one end of it.
- Need ±0.001 g/cm³, custody transfer, or true mass flow? Coriolis. You are buying a flowmeter and a densitometer, and the cost is justified by what it replaces.
- Abrasive, viscous, or dirty stream where availability beats the last decimal? A vibrating element or a DP arrangement will likely deliver more years of service. Between those two, the fork measures a point and is simpler to insert; the DP arrangement has no wetted moving part and shrugs off the thickest slurry.
- Level in the vessel swings, or the line is not vertical? Neither DP nor a point probe should be trusted blindly — fix the geometry first, or read the inline versus laboratory density measurement note on independent verification.
The density meter selection guide walks through this decision with the fluid properties that actually decide it.
Installation that makes or breaks it
A differential pressure density meter lives or dies on three installation rules:
- Hold the height H fixed and known. Mount the two taps on a straight, truly vertical run, the specified distance apart, with nothing between them that changes the column. Re-measure H on site; do not trust the isometric.
- Keep both taps in the same fluid and the same regime. A tap that intermittently sees air or a different phase than its partner injects error no transmitter can remove.
- Capillary and seal matter. Use the seal fluid the service needs, purge where the process can crystallize, and route capillaries away from heat that would change the oil density.
- Mind the temperature and the range. The transmitter corrects for fluid temperature at the sensor, but the seal oil has its own temperature coefficient — keep capillaries out of direct sun or steam tracing. Set the zero and range against the actual process density span, not the catalogue default, so the 4–20 mA uses the full range where your process lives.
After commissioning, verify against a reference sample exactly as you would for any other instrument — the density measurement hub points to the verification routine, minus the element cleaning, because there is no element to clean.
Frequently asked questions
How accurate is a differential pressure density meter?
On a well-controlled vertical span it reaches roughly ±0.5% of reading. Where level or column height is uncertain, expect several kg/m³ instead. The transmitter is seldom the limiting part; the geometry is.
Does a DP density meter need flow?
No. It reads the hydrostatic weight of a fixed column, so there is no minimum flow and no upper viscosity limit. It keeps reading through two-phase and stagnant conditions that stall a Coriolis tube.
Can it handle slurry and abrasive fluids?
Yes, better than almost any alternative. Flush diaphragm seals have no wetted moving part and no small bore for solids to bridge, so heavy slurries and tailings read cleanly where a fork would foul and a Coriolis would stall.
DP density meter or Coriolis — which should I choose?
Choose Coriolis when you need the tightest accuracy, mass flow, or custody transfer. Choose the DP arrangement for thick, dirty, or two-phase streams where availability matters more than the last decimal, and where a non-nuclear path is preferred.
Is a differential pressure density meter non-nuclear?
Yes. It uses no radioactive source, so there is no licensing, no radiation safety officer, and no sealed-source disposal obligation. It is a legitimate non-nuclear route to slurry and process density.
About LONNMETER GROUP. We design and build inline density meters based on resonant elements — tuning fork and Coriolis — and characterize them to ISO 15212, with density values verified per ASTM D4052. This article covers the differential pressure principle as a comparison technology; our own line is the vibrating-element family, and we point to DP where a flush-diaphragm arrangement is the better fit for a given duty. Reference values and temperature ratings should always be confirmed against the device datasheet.
Explore density meter applications or talk to our application engineers about which principle suits your stream.