PC-02Process measurement · PDT
Differential Pressure Transmitters
One measurement, several jobs: differential pressure infers flow across a primary element, level in open and closed vessels, and the condition of filters, strainers and heat exchangers.
01Scope
What's included
DP transmitters for flow, level, filter and pressure-drop measurement.
- DP transmittersHigh and low process connections with a single differential output, usually 4–20 mA / HART or fieldbus.
- DP transmitters with remote sealsCapillary-connected diaphragm seals for closed-vessel level and hot, viscous or corrosive media.
- Primary flow elementsOrifice plates, venturi tubes, flow nozzles and averaging Pitot tubes paired with a DP transmitter.
- Manifolds3- and 5-valve manifolds to isolate, equalize, vent and zero-check the transmitter.
- DP switches and gaugesLocal indication and switching for filters, HVAC, cleanroom and fan monitoring.
02Compare
Measurement methods
The transmitter is the same; the method around it decides accuracy, pressure loss and what has to be engineered.
| Principle | How it works | Suited for | Considerations |
|---|---|---|---|
| DP flow — orifice plateMedia: LiquidGasSteam | How it worksA restriction creates a pressure drop proportional to the square of the flow rate; flow is calculated from the square root of the DP. | Suited forClean liquids, gases and steam over a wide range of line sizes; well standardized (ISO 5167). | ConsiderationsPermanent pressure loss, limited turndown because of the square-root relationship, and upstream and downstream straight-run requirements. |
| DP flow — venturi and nozzleMedia: LiquidGasSteam | How it worksA venturi's gradual contraction and diffuser recover most of the differential, giving much lower permanent pressure loss than an orifice; a flow nozzle's contoured inlet gives a loss only moderately lower than an orifice. | Suited forVenturis for large lines where pumping or compression energy matters; nozzles for high-velocity steam and feedwater, where they resist erosion better than an orifice edge. | ConsiderationsLonger, heavier and more costly to install than an orifice plate. |
| DP flow — averaging PitotMedia: LiquidGasSteam | How it worksMultiple ports on an insertion probe sense the average velocity pressure across the pipe. | Suited forLarge pipes and ducts where low pressure loss and insertion (sometimes hot-tap) installation matter. | ConsiderationsLow DP at low velocities; ports need a reasonably clean fluid to stay open. |
| DP levelMedia: Liquid | How it worksThe DP between the vessel bottom and the vapour space is proportional to liquid height multiplied by density. | Suited forClosed and pressurized vessels, boiler drums, interface between liquids of known density. | ConsiderationsDensity changes shift the reading; wet legs, dry legs and remote seals each change the calibration and temperature behaviour. |
| Pressure-drop monitoringMedia: LiquidGas | How it worksThe DP across a filter, strainer, heat exchanger or fan indicates fouling, blockage or flow condition. | Suited forFilters and bag houses, HVAC and cleanroom pressure cascades, pump and exchanger condition. | ConsiderationsVery low ranges (mbar or Pa) need attention to zero stability and the static pressure rating. |
General engineering guidance. Limits and performance depend on the specific instrument and variant — confirm them against the manufacturer's datasheet for the selected model.
03Specify
What to specify
The parameters that decide the selection. State each with its unit; where a value is not known yet, say so — it becomes part of the engineering review.
Measurement task
Flow, level, interface or pressure drop — it decides the primary element, seals and whether the output is linear or square root.
Differential range and unit
Calculated from the flow element or vessel data; the normal operating point should sit well inside the span.
Static (line) pressure
The DP is small compared with line pressure — the transmitter must be rated for the maximum static and one-sided overpressure.
Primary element data (flow)
Line size, beta ratio, fluid density and viscosity, and the flow range together set the DP at maximum flow.
Medium and process temperature
Hot, viscous, crystallizing or hygienic media call for remote diaphragm seals rather than impulse lines.
Process connection
Transmitter flange with manifold, direct-mount seals or capillary seals — it must match the tappings and vessel nozzles.
Wetted materials and fill fluid
Diaphragm material and seal fill fluid must suit temperature, vacuum and — in food or pharma — hygiene requirements.
Accuracy and stability
At low DP ranges the static-pressure and ambient-temperature effects on zero matter as much as reference accuracy.
Output and communication
4–20 mA / HART is typical; square-root extraction can be done in the transmitter or the control system — decide which.
Area classification and enclosure
Certification for the hazardous zone, IP rating and housing material for the installed location.
04Integrate
Outputs & communication
Interfaces typically offered in this category, and what each means for the control system. Availability depends on the specific model.
- 4–20 mAAnalog
- Current loop with a live zero: 4 mA is the bottom of the range, so a broken wire (0 mA) is distinguishable from a zero reading. Two-wire (loop-powered) on most transmitters; four-wire with an active output on line-powered instruments such as analyzers — the I/O channel must match. Accepted by practically every PLC and DCS analog input.
- HARTDigital over analog
- A digital signal (Bell 202 FSK, 1200/2200 Hz) superimposed on the 4–20 mA loop — configuration, diagnostics and secondary variables over the existing two wires.
- PROFIBUS PAFieldbus
- Two-wire, bus-powered process fieldbus at 31.25 kbit/s, coupled to PROFIBUS DP — several instruments share one cable, with intrinsically safe variants for hazardous areas.
- FOUNDATION Fieldbus H1Fieldbus
- Two-wire, bus-powered digital fieldbus at 31.25 kbit/s with function blocks in the field devices — used mainly where the DCS architecture is built around it.
- Relay / discreteDiscrete
- Switched contact or transistor output for alarms, interlocks and pump or valve control — it carries a state, not a value.
05Install
Installation practice
- Keep impulse lines short and continuously sloped so gas vents back (liquid service) or condensate drains back (gas service).
- For steam flow, condensate pots or filled impulse lines at equal height keep both legs at the same liquid head.
- Use a 3- or 5-valve manifold to equalize, isolate and vent for zero checks without stopping the process.
- For closed-vessel level, choose between a dry leg, a wet leg or remote seals before calibration — each changes the zero and span.
06Products
Differential Pressure products
Status · no models listed
Specific models are not listed on the site.
Our engineers can recommend instruments against your process data. The selection checklist on this page is the information that decides it — send what you have.
How a review runs
PDT01Share the process data
Medium, range, conditions, connection and the system it reports to.
02Engineering review
The measuring principle and the installation are checked against the application.
03Instrument options
Suitable devices are proposed with the data needed to confirm the selection.
Next steps
Where to go next
- 01SolutionInstrumentationProcess measurement and final control elements — selected for the process conditions, installed, calibrated and loop-checked.
- 02ResourceFlow Measurement Technology SelectionElectromagnetic, vortex, Coriolis, ultrasonic, DP and thermal mass — matched to the fluid.
- 03Related categoryFlowFlowmeters for liquids, gases and steam across measuring principles.
Engineering enquiry
Specifying a differential-pressure measurement?
Share the medium, the range, the process conditions and the system it has to connect to. Our engineers can help match the principle and the instrument to the application.