Wiring the SITRANS P MPS 7MF1570 2-Wire Pressure Transmitter

Device Overview

The SITRANS P, MPS series with catalog number 7MF1570 is a compact hydrostatic level transmitter in the Siemens process instrumentation portfolio. The device was released for use in open and closed vessels, sumps, and well casings where the measurement medium can reach the probe through a submerged protective cap. The single-piece stainless steel housing and the corrosion-resistant sensor cable allow the probe to be lowered into a slurry or chemically aggressive liquid without the need for a separate protective cage.

The MPS belongs to the hydrostatic measurement family; it does not measure head pressure indirectly with a bubbler or with a remote seal diaphragm. It senses the actual pressure at the bottom of the submerged probe. Microcontroller-based signal conditioning inside the probe converts the millivolt output of the piezoresistive cell into the standardized 4 to 20 mA current loop output.

Two physical wires exit the probe and land inside the supplied junction box that must be mounted close to the measurement point. From the junction box the same brown/blue conductors are extended to the controller. No auxiliary device power, internal battery, or 4-wire scheme is involved — the unit is strictly 2-wire, loop-powered.

Operating Principle

The hydrostatic principle states that the pressure at the bottom of a static column of liquid is proportional to the height of the column: P = ρ · g · h + P_atm where ρ is the liquid density, g is gravitational acceleration, h is the liquid level above the probe, and P_atm is the atmospheric pressure acting on the liquid surface. The sensor diaphragm, mounted inside the bottom protective cap, deflects in proportion to this head pressure. A piezoresistive bridge bonded to the diaphragm changes its resistance values with strain, producing a small differential voltage. Onboard electronics linearize, temperature-compensate, and amplify that voltage, then drive a current proportional to the pressure into the 4-20 mA loop.

Because the transmitter reports pressure, level must be derived in the receiving controller by the formula: I_out = 4 mA + (16 mA · ρ · g · h) / (P_max − P_min) For water at 4 °C (ρ = 1000 kg/m³) and g = 9.81 m/s², the head pressure equals approximately 9.81 kPa per metre of water column, or 1 mH₂O ≈ 0.098 bar.

Technical Specifications

2-Wire Loop-Powered Theory

A 2-wire transmitter has no independent power terminal. The DC supply, the transmitter electronics, and the receiver load are all connected in a single series loop. Current is the same everywhere in the loop; therefore a single standard analog input (4-20 mA) simultaneously powers the device and reports the process variable.

For wiring to work, the closed loop must obey Kirchhoff's voltage law: V_supply = V_transmitter_drop + I_loop · (R_wire + R_load) Substituting the worst-case design values V_transmitter_drop = 10 V (minimum supply rating), I_loop = 20 mA (full-scale current): R_loop_max = (V_supply − 10 V) / 0.020 A With a 24 V dc supply the maximum tolerated loop resistance is 700 Ω. With a 30 V dc supply it rises to 1 000 Ω. A 250 Ω analog input plus 50 Ω of cable pair falls well inside the budget; an 800 Ω indicator loop does not.

Lower the supply toward 10 V and headroom collapses fast. The transmitter will still turn on but the residual overhead can no longer overcome any meaningful line drop; observed current will saturate below 20 mA and the controller will report a stuck-full reading. Keep 24 V dc as the standard reference, and derate line length accordingly for long cable runs.

Prerequisites

• Power supply: a regulated 24 V dc source sized for at least the controller's analog-input card requirements plus the transmitter current at full scale. The 7MF1570 draws at most 20 mA, so a 100 mA, 24 V dc power supply is typically more than sufficient.
• Analog input card: any current-mode analog input rated for 4-20 mA with on-board 24 V sensor supply, or a passive current input fed from an external source. Common Siemens candidates: SM 1231 AI 4 × 13 bit on the S7-1200, SM 531 AI on the S7-1500, or SM 331 on the S7-300/ET 200.
• Loop wiring: shielded twisted pair, e.g., LiYCY 2 × 0.34 mm² or instrument tray cable. The shield grounds at the junction box, not at the controller end, to drain induced low-frequency noise along the cable run.
• Junction box: the supplied Siemens junction box (IP 67) is mandatory; the sensor cable must terminate inside it, with the field side feeding the cable pair into the controller.
• Multimeter: bench meters must be inserted in series with the loop; voltage measurements are taken in parallel across the analog input terminals.

Wiring Procedure

1. Isolate the analog input by opening the controller fuse or switching the analog card off. Verify absence of voltage with a multimeter on the analog channel.
2. Mount the junction box on a vertical surface within ~5 m of the measurement point. Open the cover and identify the four-position terminal strip: + , −, +T, −T, where +T / −T are the sensor-side conductors and + / − are the field-loop terminals going to the controller.
3. Land the brown sensor conductor on the +T terminal and the blue sensor conductor on the −T terminal. Polarity is critical; reverse polarity leaves the device un-powered but undamaged because the transmitter is reverse-polarity protected — yet the analog input will read close to 4 mA or below.
4. Bond the cable shield to the in-box shield clamp. Do not extend the shield to the controller; cut it back to the insulation inside the junction box.
5. Land the field loop conductors on + / −. Brown (or red) typically routes the 24 V dc positive, and blue (or black) returns the 4-20 mA signal.
6. Route the loop conductors to the analog input card. Wire the analog input as a current loop:

24 V dc (+) ──► junction box (+) ──► transmitter brown (T+) ──►
  internal sensor circuit ──► transmitter blue (T−) ──► junction box (−) ──►
  analog input (M+) ──► internal sensing resistor ──► analog input (M−) ──► 24 V dc (−)

Notice that the device has only the brown and blue leads emerging from the probe. There is no dedicated power terminal: the 24 V dc that powers the electronics is the same current that returns the measurement. The plumbing is: positive supply → +T → probe internals → −T → channel M+ → internal sense resistor → channel M− → negative supply return.

7. Torque the field terminals to 0.5 to 0.6 N·m to prevent the loop from drifting open due to vibration. Close and seal the junction box before energising.
8. Energise the loop and observe the analog input reading. With the probe atmosphere vented (probe left in air with the head dry), the reading should be near 4.000 mA (zero level). Submersion lifts the reading toward 20 mA in proportion to head.

PLC Interface Reference

Siemens S7-1200 / S7-1500

The SM 1231 AI 4 × 13-bit module (article 6ES7231-4HD32-0XB0) and the SM 531 AI 8 × 16-bit module on the S7-1500 (article 6ES7531-7KF00-0AB0) accept 4-20 mA when the measurement-type selector is set to 2-wire current. The SM 1231 supplies the loop from its integral 24 V sensor bus, so the cabinet landing becomes:

SM 1231 channel 0 (M+) ← blue sensor wire
24 V (PSU) → brown sensor wire
Channel M− (terminal at AI ⊥) → 24 V (PSU negative)

Inside TIA Portal, set Measurement type = Current (4-wire transmitter) in the AI module configuration, choose 0 to 20 mA, and the cycle time default of 2.5 ms per channel. The raw count of 27648 represents 20 mA and 0 represents 0 mA. A 4 mA zero reading therefore sits at 5530 decimal counts.

Siemens S7-300 / ET 200

The 6ES7331-1KF02-0AB0 and successor SM 331 AI modules ship with sensor-supply terminals L+ and M that feed the loop directly. The transmitter becomes:

SM 331 L+  ──► probe brown
SM 331 M+  ◄── probe blue
SM 331 M   ──► common ground

Configure channel diagnostics with STEP 7 or TIA, enable "Wire break" detection on the AI module, and read the LWL SFC 59 diagnostics for a quick checkout of the loop.

Third-party controllers

Any current-mode analog input with internal or external 24 V supply works as well. Allen-Bradley 1734-IE2C, WAGO 750-455, Phoenix Contact Axioline AI, and Schneider Electric BMX AMI 0410 are confirmed by their manufacturer manuals to accept loop-powered 4-20 mA devices when wired in the sequence + supply → transmitter → input → ground. Always verify the channel's input burden: the voltage drop across it at 20 mA must fit inside the budget calculated earlier (typically 4 to 5 V).

Verification and Commissioning

1. Open-loop check: with the controller powered and the transmitter disconnected at the junction box, the analog input should read 0 mA on a current-type channel (raw count 0) and the AI module should flag a wire break if that diagnostic is enabled.
2. Current injection: insert a milliamp source set to 4.000 mA across the analog input terminals. The controller reading should equal 0 % level. Repeat at 12.000 mA and 20.000 mA — these should yield 50 % and 100 % level respectively.
3. Transmitter check at 4 mA: connect the loop and read current at the junction box with a series milliamp meter. With the probe vented the value should be 3.95 to 4.05 mA. Outside this window the transmitter itself is suspect.
4. Transmitter check at full span: lower the probe by a known distance, or apply a known hydrostatic pressure equivalent of, for example, 5 m of water (≈ 49.05 kPa) and confirm the loop climbs to the corresponding fraction of span — for a 10 m range device, 5 m equals 12.000 mA.
5. Voltage verification: read the voltage at the transmitter terminals. With 24 V dc supply and 20 mA flowing the voltage across the brown-blue pair should sit between 10 V dc and 22 V dc depending on total loop resistance. A reading below 10 V indicates insufficient supply headroom — check the supply and the burden resistor of the analog channel.

Troubleshooting Matrix

Field Practices

Mount the junction box within ~5 m of the measurement point and leave a small drip loop in the sensor cable so that any moisture tracks away from the cable gland. Never route the brown/blue pair alongside line-voltage conductors in the same conduit — separate raceways keep capacitive coupling below the controller's noise rejection curve. When designing for density compensation, place a separate density sensor or sample loop upstream so that the controller's mass balance remains accurate as density drifts.

Document the calibration at start-up. A 7MF1570 unit is factory linearized to the customer-specified span, but the controller should convert raw counts to engineering units with the formula: Level [m] = (I_loop − 4 mA) · SPAN_m / 16 mA where SPAN_m is the meter's full-scale water column in metres. For water-only applications the formula simplifies to Level [m] = (I_loop − 4 mA) · SPAN_m / 16 mA directly. For other liquids multiply by ρ_water / ρ_actual.

FAQ