SITRANS P DSIII Flow Configuration: DP-to-Flow Parameterization

1. Overview

The Siemens SITRANS P DSIII series (catalog family 7MF4433-* and related 7MF4x33 variants) is a high-accuracy HART and PROFIBUS-PA differential pressure transmitter commonly used as the secondary element in primary flow installations. Although the device is fundamentally a DP cell, the DSIII firmware includes a dedicated flow application mode that internally linearizes the square-root relationship between differential pressure (ΔP) and volumetric flow (Q). This document is a working reference for instrumentation engineers, commissioning technicians, and process control specialists who must parameterize the transmitter for fan, air, steam, water, or gas flow measurement and reconcile its output against theoretical Bernoulli-based calculations.

2. Underlying Physics: Bernoulli and the Square-Root Relationship

For an incompressible fluid passing through a primary element (orifice plate, averaging Pitot, Venturi, V-cone, or wedge meter), Bernoulli's equation combined with continuity yields the classical volumetric flow equation:

Q = C · A · √(2 · ΔP / ρ)

Where:

• Q = volumetric flow rate (m³/s)
• C = discharge coefficient (dimensionless, typically 0.60 – 0.65 for a sharp-edged orifice per ISO 5167)
• A = cross-sectional area of the bore at the primary element (m²)
• ΔP = differential pressure measured by the transmitter (Pa)
• ρ = fluid density (kg/m³)

For compressible flow (gas, steam), an expansibility factor ε is applied, and density must be evaluated at upstream conditions:

Q = C · A · ε · √(2 · ΔP / (ρ_up))

Because Q is proportional to √ΔP, the DSIII firmware's Square Root characterization is the exact mathematical inverse required to produce a linear 4–20 mA output that is directly proportional to flow. If the transmitter's current output transfer function is also set to square root, the square root is applied twice and the output is grossly non-linear. See Siemens Industry Online Support for the DSIII operating instructions (A5E00047090 series).

3. SITRANS P DSIII Operating Modes Relevant to Flow

The DSIII supports several Transmitter Type / application switch positions. The three relevant to flow work are:

4. Parameter Reference

The following parameters appear in SIMATIC PDM (Process Device Manager), the HART handheld (e.g., 375/475), and the device DTM. They are the most frequent sources of commissioning confusion.

5. Measuring Range vs. Working Range

These two parameters are the single largest source of commissioning errors on DSIII flow installations.

Measuring Range is the differential pressure span of the sensor itself. It is dictated by the maximum ΔP the primary element produces at the maximum design flow, plus any static line pressure offset if the DSIII is in a bi-directional or elevated-static-pressure application. Example: an orifice sized for 10 mbar DP at maximum flow → Measuring Range 0 – 10 mbar.

Working Range is the equivalent engineering flow scale (and unit) that the user wants to see at 4 mA (0 %) and 20 mA (100 %). It does not change the physics; it only sets the output scaling. Example: 0 – 10 m³/s → Working Range 0 – 10 m³/s.

6. Pressure Raw Value vs. Measured Value (PV)

When monitoring the DSIII online via SIMATIC PDM, two DP-related values are visible:

• Pressure Raw Value — the digitized, temperature-compensated sensor output expressed in the configured pressure unit. It has been linearized against the calibrated sensor characteristics but has not been passed through the square-root block that produces the flow PV.
• Measured Value / PV — the final output variable, which in Flow mode is √(Pressure Raw Value), rescaled to the Working Range. In Pressure mode the PV equals the Pressure Raw Value directly.

This distinction is essential when validating an installation against a Bernoulli calculation: use the Pressure Raw Value to compare DP-to-DP, and use the PV to compare flow-to-flow. Cross-comparing a raw DP from one transmitter to a flow PV from another (or vice versa) is a guaranteed source of dispute.

6.1 To obtain a "raw" DP reading from a transmitter currently in Flow mode

1. In PDM, change Transmitter Type from Flow to Pressure.
2. Adjust Measuring Range to span the expected DP at the test point.
3. Set Characteristics to Linear on the output.
4. Re-zero the transmitter (LRV) at the actual line static pressure with no flow.
5. Read the Pressure Raw Value or PV — they will be identical in Pressure mode.
6. Restore the original Flow configuration once the test is complete.

7. Primary Flow Element Compatibility

A DSIII in flow mode is a generic DP-to-flow converter. The discharge coefficient C, the bore area A, and the fluid density ρ are characteristics of the primary element, not the transmitter. Compatible primary elements include:

For fan and air-flow applications, the same equations apply with ρ = 1.2 kg/m³ at standard conditions. A multi-tap piezometer ring (a circular array of static-pressure tappings around the fan inlet or duct cross-section) is a recognized averaging arrangement per ISO 3966; the DSIII sees only the difference between the high- and low-pressure manifolds, not the ring itself.

8. Step-by-Step Commissioning Procedure

Prerequisites: SIMATIC PDM V9.x or later installed, HART modem (e.g., SITRANS TH200/TH300, MACTek Viator, or 475/375 handheld), the DSIII datasheet and the primary element calculation sheet (orifice β-ratio, design ΔP, C, A, design Q).

1. Verify mechanical installation. Confirm impulse lines are filled (for liquid service, no gas pockets; for gas service, no liquid slugs), high side is on the upstream tap, low side on the downstream tap, and both manifolds are equal-length.
2. Connect PDM. Power the loop at 24 VDC minimum, attach HART modem across the test points or at the terminal block. Open the DSIII DTM in PDM and upload the device configuration.
3. Set Transmitter Type to Flow. Navigate to Device → Transmitter Type and select Flow. Confirm the application switch prompt and accept.
4. Enter the Measuring Range. Set LRV and URV to the primary element's calibrated DP span. Example: LRV = 0 mbar, URV = 10 mbar. The DSIII will internally assign 0 – 100 % to this span.
5. Enter the Working Range. Set the flow LRV and URV in the desired engineering unit. Example: 0 m³/s to 10 m³/s. This drives the 4 mA and 20 mA current points.
6. Confirm Output Characteristics = Linear. The square-root function is already inside the device from Step 3. The output to the current loop must be linear, so PV ∝ flow.
7. Perform a Lower Range Value (LRV) trim with both manifolds at equal static pressure and zero flow. PDM path: Device → Calibration → Lower Trim. This sets 4 mA at 0 flow.
8. Verify with a known DP source. Apply a calibrated test pressure to the high side equal to 25 %, 50 %, 75 %, and 100 % of URV. Read the PV (flow) and current; expect √(DP/URV) × URV_flow for flow and 4 mA + (16 mA × percentage) for current.
9. Verify with a known DP source on the live process. Cross-check the DSIII flow against the DCS Bernoulli calculation. Differences within ±2 % are typical; larger deltas indicate either wrong C, wrong A, wrong ρ, or an impulse-line problem.
10. Download the configuration back to the device and write-protect if the installation requires it.

9. Verification and Acceptance Test

A documented acceptance test protects both the commissioning team and operations. Recommended checks:

The intermediate points must follow the square-root curve within the DSIII's reference accuracy of ±0.075 % of the calibrated span (typical for the DSIII HART at 25 °C, 10:1 turndown). Confirm with the device's individual test certificate (ITS-90 or factory calibration report) shipped with the unit.

10. Troubleshooting Matrix

11. Reconciling DSIII Flow with Bernoulli Calculations

The most common audit finding in DSIII flow installations is a difference between the transmitter's reported flow and a hand-calculated Bernoulli flow. The decision tree below localizes the discrepancy.

1. At a known, stable flow point, read the Pressure Raw Value from the DSIII in PDM.
2. At the same physical point, read the reference DP from an independent calibrated gauge or from a temporary second DSIII in Pressure mode.
3. If the two DPs agree within ±0.1 % of span, the DSIII is reading DP correctly. The disagreement with Bernoulli is therefore in the constants: C, A, ρ, ε, or the unit conversion (Nm³/h vs. m³/s, etc.). Re-verify the primary element calculation sheet from the manufacturer.
4. If the two DPs disagree, the DSIII needs a zero or span trim, or its impulse lines have a problem (gas in liquid line, liquid in gas line, blockage, leak).

12. Safety, SIL, and Maintenance Considerations

For safety-instrumented flow loops (e.g., burner management, vent flow, leak detection), the DSIII is available in IEC 61508 SIL 2 variants. Confirm the SIL rating on the nameplate and the FMEDA report before using the transmitter in a SIF. For non-SIL installations, plan a 5-year calibration interval with annual visual inspection of the impulse manifolds and a zero check at line pressure.

For DP flow installations on dirty or condensing services, install:

• Two-valve manifolds with drain/vent ports for liquid service.
• Three-valve manifolds for gas/steam service with condensate pots.
• Chemical seals (SITRANS P series P-seal) where process fluid is corrosive, viscous, or above 250 °C.

13. Field Commissioning Checklist

• [ ] Manifold equalized at zero flow; PV reads 0.000 m³/s; current reads 4.000 mA.
• [ ] LRV and URV match the primary element calculation sheet.
• [ ] Working Range unit string matches DCS scaling.
• [ ] Output Characteristics = Linear (verified in PDM and at the DCS).
• [ ] HART tag, descriptor, and message fields populated.
• [ ] Loop voltage ≥ 12.5 VDC at the transmitter terminals.
• [ ] 25 / 50 / 75 / 100 % DP test points pass the square-root verification.
• [ ] Pressure Raw Value matches independent reference DP within 0.1 % of span.
• [ ] Configuration downloaded to device and written to project archive.
• [ ] Burst mode disabled if multi-drop HART bus is used.