Industrial DP transducers and transmitters for process engineers, HVAC and building controls, filtration monitoring, and flow and level measurement. Accurate, stable output across demanding applications, from inches of water column to thousands of PSI.
The fundamentals
A differential pressure transducer measures the difference between two pressures, applied to its high and low ports, and converts that difference into an electrical signal. It is not a gauge instrument, which reads pressure against the local atmosphere, and not an absolute instrument, which reads against a vacuum reference. The measurement is the difference itself, and that is exactly what most process questions actually ask: how much drop across this filter, how much head in this vessel, how much higher is this room than the corridor outside it.
The output side is just as important as the sensing side. Industrial units deliver a 4-20 mA current loop, an amplified voltage such as 0-5 V or 0-10 V, or a digital signal, with protocols such as HART layered on the current loop and fieldbus or industrial Ethernet options on newer instruments. Current loops dominate process plants because they run long cable distances without losing accuracy and can power the instrument over the same two wires; voltage outputs are common in HVAC panels and test equipment where runs are short.
A note on terminology: strictly speaking a transducer outputs a raw or amplified signal while a transmitter conditions the signal and drives a current loop, often with diagnostics and a configurable range. In practice the industry uses both words loosely, and the same physical product may be sold under either name. The datasheet's output specification, not the product title, tells you which one you are buying.
Why the reference matters to a buyer
Subtracting two separate gauge readings to get a small difference stacks the error of both instruments and can bury the measurement entirely. A true DP transducer makes the subtraction mechanically, at the diaphragm, so the full accuracy of one instrument is applied to the difference itself. That is why a dedicated DP instrument, not a pair of gauges, is the standard answer wherever the difference is the measurement.
Gauge pressure
vs atmMeasured relative to the local atmosphere. The everyday reading on a pump discharge or an air receiver. Atmospheric weather changes ride along with the reading.
Absolute pressure
vs vacuumMeasured relative to a sealed vacuum reference. Used where atmosphere is irrelevant or misleading: vacuum processes, barometric compensation, altitude.
Differential pressure
port A vs port BMeasured between two process points, high port minus low port. The instrument this site is about. Flow elements, filters, level in closed vessels and room pressurization all reduce to this measurement.
Typical output signals
Inside the instrument
At the core of nearly every DP instrument is a sensing diaphragm with the high-side pressure on one face and the low-side pressure on the other. The diaphragm deflects in proportion to the difference, and the sensing element converts that deflection into an electrical change. Because both pressures act on the same element, common-mode pressure largely cancels: the instrument can resolve a small difference even when both ports ride at a high line pressure.
Three sensing technologies dominate. Strain gauge designs bond or deposit resistive elements on the diaphragm and read the change in resistance as it flexes; they are robust and cover wide ranges. Capacitive designs use the diaphragm as one plate of a capacitor and measure the change in capacitance; they excel at low differentials and long-term stability, which is why the classic process DP cell is capacitive. Piezoresistive designs diffuse resistors into a silicon die, giving high sensitivity in a compact, low-power package that suits OEM and low-range work.
The instrument connects to the process through impulse lines, small-bore tubing from the process taps to the ports, usually through a three- or five-valve manifold that allows isolation, equalization and venting without shutting the process down. Equalizing the two ports is also how you check and reset zero at line pressure. Where impulse lines cannot be used, remote diaphragm seals take their place.
Two datasheet concepts complete the picture. Accuracy is usually quoted as a percentage of calibrated span, so range selection directly affects real-world error, and turndown describes how far a configurable transmitter's span can be reduced below its upper range limit before performance degrades. Zero and span adjustments map the measured range onto the output: zero can be elevated or suppressed for installations where the resting DP is not zero, such as wet-leg level measurements.
The lineup
Four families cover the overwhelming majority of DP orders. They share the same measuring principle; what changes is the media the ports accept, the range, and how much intelligence sits on top of the sensor.
Both ports rated for liquid service
Air and non-condensing gases, inches WC
HART / digital, configurable, diagnostic
DP + static pressure + temperature
When impulse lines will not do
Impulse lines assume the process fluid can safely sit in small-bore tubing. Viscous, crystallizing, corrosive or sanitary media break that assumption, and lines exposed to weather can freeze or accumulate condensate that shifts the reading. Remote diaphragm seals solve this by placing a flush diaphragm at the process connection and transmitting pressure to the DP cell through a sealed, fluid-filled capillary.
Seals are their own discipline: the fill fluid expands and contracts with ambient temperature, capillary length affects response time, and the seal pair should be matched and installed symmetrically to keep those effects balanced. If your media rules out impulse lines, raise seals at the enquiry stage, because they change the instrument, the installation and the price.
Where they work
Wherever the quantity that matters is a difference between two pressures, a DP instrument is usually the most direct way to measure it. These six applications account for most of the DP transducers in service.
Orifice plates, venturis and averaging pitot tubes create a DP proportional to the square of flow. The transducer reads the drop and flow is recovered by square-root extraction, in the transmitter or the control system. The classic DP application.
Hydrostatic head on the high port, vapor space on the low port, and the difference tracks liquid level independent of vessel pressure. Wet-leg and dry-leg installations, and remote seals where the media demands them.
A loading filter drops more pressure at the same flow. Monitoring DP across the element turns filter change-out from a calendar guess into a measured decision, in hydraulic systems, process lines and air handlers alike.
Building static, duct static and room-to-room differentials measured in inches of water column. Cleanrooms, isolation rooms and laboratories hold specified positive or negative offsets, and low-range DP sensors are how those offsets are proven.
DP across a pump is its developed head; a falling reading flags wear, recirculation or blockage. Across a heat exchanger, rising DP is the standard early warning of fouling, prompting cleaning before performance collapses.
Pressure-decay and differential-decay test rigs compare a test part against a reference volume, where a small DP resolved at a common line pressure reveals leaks far too small to see as a change in gauge pressure alone.
Specifying an instrument
A DP quote is only comparable to another DP quote when both describe the same instrument. These are the specification points that define what you are actually buying, and the trap that goes with each one. None of them favors any particular brand; all of them come straight off the datasheet.
| Specification | What it governs | What to watch for |
|---|---|---|
| DP range, uni- or bidirectional | The span of differential the instrument resolves, and whether it reads one direction or both sides of zero. | Size the span so normal operation sits well inside it, and go bidirectional wherever the differential can legitimately reverse. A unidirectional unit driven backwards just reads zero. |
| Static (line) pressure rating | The common-mode pressure both ports can ride at while the instrument measures a small difference. | A small DP span and a high static rating are independent numbers. Check both, and check the published line-pressure effect on zero when the DP is small relative to the line. |
| Process media & wetted materials | Which fluids may touch each port, and what the diaphragm, ports and seals are made of. | Wet/wet vs dry-only is the first split; material compatibility with the actual process fluid is the second. Liquid on a dry-only sensor is a dead instrument, not a degraded one. |
| Output signal | How the measurement leaves the instrument: 4-20 mA loop, 0-5/0-10 V, millivolt, or digital protocols. | Match the receiving system before anything else. Loop-powered 4-20 mA travels far on two wires; voltage outputs want short runs; digital protocols need a system that speaks them. |
| Accuracy class & stability | Reference accuracy as a percentage of span, plus long-term drift per year on the datasheet. | Accuracy quoted at reference conditions is the best case. For a wide-turndown transmitter, check accuracy at your actual span, and weigh stability if calibration access is difficult. |
| Overpressure protection | How much one-sided pressure the sensor tolerates when the other port sees none, without damage or shift. | Startups, valve slams and blocked lines routinely apply full line pressure to one port. Verify the one-sided rating against that event, not against normal operation. |
| Temperature effects | Compensated temperature range, plus the stated zero and span shift per degree outside reference conditions. | An instrument that is accurate at 20 °C can be far less so on a hot roof or a cold pipe rack. Read the temperature coefficient at your installation conditions, not the headline accuracy. |
| Process connections | How the instrument meets the process: threaded ports, flanged connections, manifolds, or remote seals. | Match connections to your taps and your maintenance practice. A three- or five-valve manifold is what makes zero checks and removal possible without a shutdown. |
| Hazardous-area approvals | Certification for classified areas: intrinsic safety, explosion-proof or non-incendive ratings for the zone. | The approval must match your area classification and your wiring method, and intrinsically safe loops need the barriers designed in. Retrofit certification is not a thing; specify it up front. |
Ranges, ratings, materials and approvals vary between manufacturers and models. Work from the datasheet of the exact instrument being quoted rather than from generic figures, and state your process conditions in full at the enquiry stage.
Common questions
It comes down to the reference the measurement is made against. A gauge instrument measures pressure relative to the local atmosphere, an absolute instrument measures relative to a full vacuum, and a differential instrument measures the difference between two pressures applied to its own high and low ports. Neither reference is atmosphere or vacuum unless you plumb it that way. Choose differential when the quantity you actually care about is a difference: the drop across a filter or orifice plate, the head of liquid in a vessel, or the pressure of one room relative to another.
Strictly, a transducer converts pressure into a raw or amplified electrical output, typically millivolts or a voltage such as 0-5 V or 0-10 V, while a transmitter adds signal conditioning and drives a 4-20 mA current loop that can run long cable distances without degradation, often with a digital protocol such as HART on top. In everyday industrial usage the words are used almost interchangeably, and plenty of catalogs mix them. The reliable habit is to ignore the product title and read the output specification on the datasheet, because that is what your control system actually connects to.
A wet/wet DP transducer is built so that both pressure ports can be exposed to liquid, with wetted materials on both sides rated for the process media. That is what you need for liquid flow across an orifice plate, filter monitoring in liquid service, or pump differential head. Dry or dry/dry units are intended for air and non-condensing gases only, usually at low ranges for air handling and building pressure work. Putting liquid on a dry-only sensor is a common and avoidable way to kill an instrument.
Start from the normal operating DP and choose a span so that it sits comfortably inside the range rather than at the bottom edge, then check the maximum DP the process can produce during upsets, startups and cleaning cycles. Accuracy is usually stated as a percentage of span, so a range that is far too wide makes small readings proportionally less accurate, while a range that is too narrow saturates during excursions. Remember that the DP span, the one-sided overpressure rating and the static line pressure rating are three separate numbers, and all three have to fit your process.
Whenever the differential can legitimately change sign. Room-to-room pressurization can flip when doors open or fans stage, some duct and filter arrangements see reverse flow during backwash or purge, and test benches often need to read both directions. A bidirectional instrument has a range such as plus-or-minus a stated value and reports which side is higher. A unidirectional unit driven backwards simply reads at or below zero and tells you nothing useful about how far backwards you are.
It is the common-mode pressure both ports can sit at while the instrument measures a small difference between them. The two are wildly different in many real installations: a transducer may resolve a DP of a few PSI, or even inches of water, while both of its ports ride on a line at hundreds or thousands of PSI. The static rating tells you the sensor survives and performs at that line pressure. Related specifications quantify how much the zero and span shift as line pressure changes, which matters when you are measuring a small DP at high static pressure.
Yes, and DP flow measurement is one of the oldest and most widely installed flow techniques. A primary element such as an orifice plate, venturi or averaging pitot tube creates a pressure drop that is proportional to the square of the flow rate. The transducer measures that drop, and the flow is recovered by taking the square root, either in the transmitter itself or in the control system. Because of the square relationship, usable turndown is limited compared with some other technologies, and multivariable transmitters add static pressure and temperature measurement to compute compensated mass flow for gases and steam.
There is no single interval, because drift depends on the sensor technology, the range, the temperature and pressure cycling the instrument sees, and how far it operates from its reference conditions. Manufacturers publish long-term stability figures on the datasheet, and those are the honest starting point. In practice, critical and custody measurements get scheduled calibration checks, while non-critical HVAC instruments often receive only periodic zero checks. Two habits pay for themselves everywhere: re-zero the instrument at line pressure after installation, and trend as-found calibration data so the interval is set by evidence rather than by guesswork.
Contact
Send the basics of your application and we will point you in the right direction: the DP range and static line pressure, the process media on each port, the output signal your system needs, and quantities and timeline.
Range & span sizing
Matching the span to normal operating DP with honest headroom for upsets, and deciding when bidirectional is worth it.
Media & materials
Wet/wet vs dry-only, wetted-material compatibility, and when remote seals replace impulse lines entirely.
Signal & integration
4-20 mA, voltage or digital into your PLC, DCS or building system, plus manifolds, mounting and approvals.