DECISION PROCESS
Engineering selection logic
For flow measurement, the transmitter reads differential pressure generated by a primary element such as an orifice or another engineered restriction. The relationship between differential pressure and flow is not a universal transmitter constant. Pipe dimensions, fluid properties, pressure, temperature, primary-element geometry and the intended flow range belong in the sizing record. Square-root extraction and compensation must be assigned explicitly to the transmitter, flow computer or control system.
For closed-vessel level, the high side normally responds to liquid head while the low side references vessel vapour pressure. Density, tapping elevations, wet or dry reference leg, condensate behaviour and remote-seal fill systems determine the calculated zero and span. Changing density can change the inferred level. Capillary lengths, seal sizes and mounting elevations should be defined as part of one matched assembly, especially where ambient or process temperatures vary.
For filter, pump or equipment monitoring, the decision variable may be pressure loss rather than flow or level. The expected clean and dirty values, static line pressure, allowable overload and alarm logic should be recorded. In every DP application, manifolds, equalising procedures, impulse-line slopes, phase management and mounting location affect commissioning. The proposed range and static-pressure rating must be verified for the selected SKU, not inferred from a similar housing.
Selection factors
- Application calculation
- Identify flow, closed-tank level or equipment differential duty and provide the sizing or elevation calculation inputs.
- Differential and static pressure
- Give minimum, normal and maximum differential pressure together with maximum common line pressure and upset conditions.
- Process interface
- Define impulse lines, manifold, direct mount or remote seals, tapping elevations, fill fluids and orientation.
- Fluid and environment
- State phase, density, viscosity, temperature, ambient gradients, condensation, freezing, plugging and corrosion risks.
- Signal and documentation
- Confirm square-root or linear output, compensation location, power, communication and required calculation or calibration records.
Suitable scenarios for evaluation
- Flow measurement with a documented primary element and sizing calculation for the actual pipe, fluid and operating envelope.
- Closed pressurised vessel level where reference-leg or remote-seal geometry and liquid density are controlled.
- Filter, strainer, pump or heat-exchanger monitoring where clean, normal and alarm differential values are defined.
Limitations and confirmation points
- A DP transmitter does not by itself validate a flow element, convert pressure to level without density assumptions or correct poor impulse-line design.
- Static-pressure limits, seal effects, turndown, accuracy and approval scope must be verified for the complete selected configuration.
RFQ INPUTS
Information to send for configuration review
- Measurement duty: flow, level or differential monitoring
- Minimum, normal and maximum differential pressure
- Maximum static line pressure
- Fluid, density, viscosity and phase
- Process and ambient temperature
- Pipe or vessel geometry and tapping elevations
- Primary-element sizing data or level calculation
- Impulse lines, manifold or remote-seal arrangement
- Output and compensation location
- Required calculations, certificates and quantity
Request configuration-specific selection support View Differential Pressure Transmitters
ENGINEERING FAQ
Frequently asked questions
Can the same differential-pressure transmitter be used for both flow and level?
A transmitter family may support both duties, but the configured range, process connections, accessories, calculation and installation are application-specific. Flow usually requires a primary-element sizing record and may use square-root extraction. Closed-vessel level requires density, tapping elevations and reference-leg or seal details. The two applications should therefore have separate datasheets and order checks, even if the proposed sensing platform or enclosure appears similar.
Why are static pressure and differential range specified separately?
The sensor measures the difference between its high and low sides, which may be a small value, while both sides can simultaneously see a much larger line pressure. The required DP span therefore does not describe the pressure boundary or allowable common pressure. Supply minimum and maximum DP, maximum line pressure and credible one-sided events. The selected model documentation must confirm both measuring and mechanical limits for the proposed configuration.
What installation information is needed for closed-tank level?
Provide vessel dimensions, upper and lower tapping elevations, minimum and maximum level, liquid density and its variation, vapour condition, process temperature and ambient exposure. State whether the low side uses a dry leg, wet leg or remote seal, and identify fill fluid, capillary arrangement and transmitter elevation. These inputs establish the expected zero and span. Final settings should be recorded during engineering and checked during commissioning.