Inside a DP transmitter
A DP transmitter measures the difference between two pressures: the high side (H) and the low side (L). The process never touches the sensor directly:
- Isolating diaphragms (thin stainless or alloy discs) take the process pressure on each side.
- A sealed fill fluid (usually silicone oil) carries that pressure into the sensor.
- The sensing element deflects with the difference. In a capacitance cell, a centre diaphragm moves between two plates and the capacitance change is measured. In a piezoresistive cell, a silicon chip's resistance changes with strain.
- The electronics compensate for temperature and static pressure, scale the reading to the configured range and drive the 4–20 mA output (with HART on top).
Sensor types: DP/GP, AP and AP/GP
The same transmitter family is usually offered with different sensor modules. What changes is what the measuring diaphragm compares the process against:
| Type | Measures | Typical uses | Watch out for |
|---|---|---|---|
| DP (differential) | H − L | Flow across orifices/venturis, level with wet or dry legs, filter ΔP | High static (line) pressure shifts the zero slightly: trim zero at line pressure |
| GP (gauge) | Process − atmosphere | Most pump, header and vessel pressures | Reading changes with weather/altitude only by the atmospheric part; keep the vent clear |
| AP (absolute) | Process − vacuum | Vacuum columns, condensers, steam turbines, gas density compensation | Reads ~1.013 bar a with the process open to air, not zero |
| AP/GP in-line | Process vs vacuum or air, one port | Simple pressure points, compact mounting | Not for DP; the GP version's vent must not be blocked or painted over |
Coplanar vs traditional flanges. On many modern DP transmitters both process ports sit on one flat face (coplanar). That lets one integral manifold bolt directly on, which means fewer joints and fewer leak points than a traditional bi-planar flange with separate fittings.
The DP flow principle
A primary element (orifice plate, venturi, flow nozzle, averaging pitot) narrows the flow. The fluid speeds up and its pressure falls, so the upstream tap reads higher than the downstream tap. The transmitter measures that DP. By Bernoulli, flow rises with the square root of DP:
Q = K · √ΔP
- Orifice plate: cheap and easy to change, but high permanent pressure loss and a typical turndown of about 3–4:1.
- Venturi: low permanent loss, good for large lines and dirty fluids, but expensive and long.
- Averaging pitot: inserted through one hole, very low loss, suited to large ducts and air.
Square-root extraction and low-flow cutoff
Because DP rises with the square of flow, something must take the square root, either the transmitter (output configured as "square root") or the DCS. Do it in one place only. Taking the root twice is a classic commissioning error.
Q% = 100 · √(ΔP% / 100) e.g. 25 % DP → 50 % flow
Low-flow cutoff. Near zero the curve is very steep: 1 % DP already means 10 % flow, so a tiny zero drift shows up as a large false flow. Transmitters and DCS blocks therefore use a low-flow cutoff (often 5–10 % flow) that forces the output to zero below it.
Turndown is limited for the same reason: 10 % flow needs only 1 % of the DP span.
Check numbers with the Square-root DP Flow calculator.
Tap orientation and impulse lines
Most DP flow errors come from the impulse lines, not the transmitter. The rule is simple: keep the lines full of the right phase, so liquid lines hold no gas and gas lines hold no liquid.
- Slope impulse lines at least 1:12 (about 8 %) continuously, with no humps or sags.
- Keep H and L lines together, the same length and the same temperature, so any head errors cancel.
- Avoid taps at the bottom of the pipe (sediment) and, for liquids, the top (gas).
- Heat-trace or insulate lines that can freeze, but never overheat a water-filled steam leg, because it will boil and the leg will empty.
Manifold valves: the safe sequence
A 3-valve manifold has a high-side block, a low-side block and an equalizing valve between them. A 5-valve manifold adds two vent/test valves. The sequence matters: if both blocks are open while the equalizer is open, process flows straight through the manifold. On steam, that can also blow the water legs out.
Putting into service
- Check all three valves are closed.
- Open the equalizer.
- Open the high block slowly: both sides now see process pressure, DP = 0.
- Check or trim the zero here if required.
- Close the equalizer.
- Open the low block. The transmitter now reads the true DP.
Taking out of service
- Close the low block.
- Open the equalizer.
- Close the high block.
- Vent or drain through the vent valves (5-valve) before removing the transmitter.
Ranging and HART configuration
- LRV / URV: for flow, usually 0 to the DP at maximum flow from the orifice calculation sheet (for example 0–250 mbar).
- Transfer function: linear (DCS takes the root) or square root (transmitter takes the root), but never both.
- Damping: a little (about 0.5–2 s) calms DP noise; too much slows the loop.
- Units and tag: set the engineering unit, tag and descriptor so the HART device matches the loop drawing.
- Alarm direction: high or low failure current (NAMUR NE 43: ≤ 3.6 mA or ≥ 21 mA), as the DCS expects.
Re-ranging maths: Transmitter Re-range. Signal checks: 4–20 mA Signal Scaler.
Calibration check
- Isolate with the manifold (sequence above) and vent both sides to atmosphere.
- Connect a pressure source and reference gauge to the H side; leave L vented.
- Apply 0, 25, 50, 75, 100 % of span, rising and falling; record the mA (and HART PV) at each point.
- Compare with the expected output; tolerance is often ±0.25 % of span for the loop sheet, but use your site's procedure.
- If needed, do a sensor trim (zero and span), then an analog output trim with a calibrated meter.
- Return to service with the manifold sequence; do a final zero trim at line pressure with the equalizer open.
Troubleshooting DP flow
| Symptom | Likely cause | Check |
|---|---|---|
| Reads high or low by a steady amount | Gas trapped in a liquid leg, or liquid in a gas leg; unequal steam legs | Vent or drain the legs; refill steam legs to the same level |
| Reading stuck or very sluggish | Plugged tap or impulse line, frozen line | Blow down the lines (with permission), check tracing |
| Flow shows when the line is stopped | Zero drift amplified by square root; no low-flow cutoff | Equalize and check zero; set a low-flow cutoff |
| Flow about √ too low or too high | Square root applied twice, or not at all | Compare transmitter transfer function with the DCS block |
| Reads negative / reverse | H and L impulse lines swapped | Trace lines from the orifice taps |
| Noisy reading | Pulsation, two-phase flow, damping too low | Check straight runs and process; add a little damping |
| Wrong flow after a plate change | Different bore or a plate installed backwards | Check the plate tab stamp and the bevel faces downstream |
Q&A: test yourself
Pick an answer to check it. Each one explains why.
Manufacturer manuals
For a specific model, always use the manufacturer's own reference and installation manuals: they list approved fill fluids, torque values, hazardous-area wiring and the exact HART menus. For example, the reference manual for the 3051S series is freely available from the manufacturer's documentation library (search "3051S reference manual" on emerson.com). This page explains the principles in general terms and is not a substitute for that documentation.
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