Overview
This technical reference covers the integration of a Siemens SIWAREX WL280 compression load cell with an OMEGA TXDIN1600S DIN-rail transmitter to produce a 4-20 mA analog signal proportional to applied weight. The document addresses load cell selection rules, mV/V sensitivity matching, excitation voltage requirements, wiring practice (4-wire and 6-wire), calibration (tare and span), resolution and overdimensioning, noise mitigation, and field troubleshooting. It is written for system integrators, instrumentation engineers, and commissioning technicians who must connect a strain-gauge load cell to a PLC, DCS, SCADA RTU, or panel indicator that accepts only current-loop analog input.
Reference is made throughout to the published specifications of the Siemens SIWAREX WL280 load cell and the OMEGA TXDIN1600S transmitter. Accuracy classes and overload limits follow OIML R60 for strain gauge load cells and NIST Handbook 44 for industrial weighing practice.
Prerequisites
- Siemens SIWAREX WL280 load cell, 500 kg rated (the reference application; other Emax values available)
- OMEGA TXDIN1600S strain-gauge to 4-20 mA DIN-rail transmitter
- 24 VDC regulated power supply, 100 mA minimum (transmitter loop excitation)
- Shielded 4-conductor or 6-conductor load cell cable, gauge sized for ≤ 5 Ω loop resistance
- Precision reference masses, NIST-traceable, ≥ 50% of Emax for span calibration
- Digital multimeter, 0-30 mA DC range and 0-100 mV DC range, 0.05% accuracy
- PLC or DCS analog input module rated for 4-20 mA, 16-bit minimum
- Insulated screwdriver set, cable gland, ferrules, and DIN-rail terminal blocks
Load Cell Selection Principles
Overdimensioning
A load cell should be selected so the maximum expected process load (vessel tare weight plus product) does not exceed 60-75% of the load cell's rated capacity (Emax). This is called overdimensioning and provides three benefits: accuracy stability over the operating range, headroom for transient peak forces during filling or impact, and extended service life by avoiding operation near the rated limit.
For the reference application — a 500 kg SIWAREX WL280 measuring a nominal 300 kg process load — the Emax utilization is 60%, which is the recommended operating point for reliable long-term performance.
Overload Tolerance
Strain-gauge load cells have two distinct overload limits:
- Safe overload (Emax): Rated capacity. Continuous operation beyond this point degrades accuracy and causes permanent zero shift.
- Ultimate overload (Eu): Mechanical destruction limit. Per OIML R60 and manufacturer datasheets, typical values are 150% Emax for general-purpose cells, 200% Emax for ruggedized industrial cells, and 300-500% Emax for hermetically sealed heavy-duty compression cells.
The SIWAREX WL280 is specified with a 300% Emax ultimate overload rating — substantially above the IEC 200% baseline — making it suitable for environments with shock loading, side loads, or vibration. When the application involves such conditions, select the cell size so that even transient peak forces stay below Eu and that steady-state forces stay below Emax.
Application: 500 kg Cell Measuring 300 kg
For a 500 kg cell with 300% Eu rating, the ultimate load is 1500 kg. The steady-state process load of 300 kg (vessel + product) sits at 60% Emax, leaving a 200 kg margin before the cell reaches its rated capacity and a 1200 kg margin before mechanical destruction. This margin accommodates:
- Fill surges during hopper charging
- Impact forces from falling material
- Vessel swing in suspension-mount installations
- Thermal expansion of the supporting structure
- Fouling buildup between maintenance cycles
SIWAREX WL280 Specifications
The SIWAREX WL280 is a stainless-steel, hermetically sealed, compression load cell from the Siemens SIWAREX weighing portfolio. Key parameters per the Siemens datasheet:
| Parameter | Value |
|---|---|
| Accuracy class | C3 per OIML R60 |
| Maximum capacity (Emax) | 500 kg (also available in 1000 kg, 2000 kg, 5000 kg, 10 t, 20 t, 50 t) |
| Sensitivity (Cn) | 2.85 mV/V ± 0.1% |
| Combined error | ≤ 0.023% of Cn |
| Repeatability | ≤ 0.005% of Cn |
| Creep (30 min) | ≤ 0.025% of Cn |
| Temperature effect on zero | ≤ 0.012% of Cn / 10 K |
| Temperature effect on span | ≤ 0.009% of Cn / 10 K |
| Excitation voltage (recommended) | 5-12 VDC |
| Excitation voltage (maximum) | 18 VDC |
| Excitation voltage (nominal) | 10 VDC |
| Bridge resistance | 350 Ω ± 5 Ω |
| Operating temperature | -30 °C to +80 °C |
| Protection class | IP68 / IP69K (hermetic) |
| Safe overload | 150% Emax |
| Ultimate overload | 300% Emax |
| Construction | Stainless steel 17-4 PH, welded hermetic seal |
| Cable | 4-conductor, shielded, PUR jacket, 5 m standard |
The Cn = 2.85 mV/V value is the critical parameter for amplifier matching. With 10 V excitation, the full-scale output is:
Full-scale output = Cn × Vex = 2.85 mV/V × 10 V = 28.5 mV at Emax (500 kg)
mV/V Sensitivity and Excitation Voltage
Sensitivity Matching
Load cells are specified by their rated output in mV/V: the millivolts of bridge output per volt of excitation at full rated load. Common industrial values are 1 mV/V, 1.5 mV/V, 2 mV/V, 2.85 mV/V (Siemens SIWAREX default), and 3 mV/V.
The amplifier's input range must encompass the cell's full-scale mV output. If the amplifier expects a 4 mV/V signal but receives a 2.85 mV/V cell, the amplifier output at full load will be only 71% of its 4-20 mA span (approximately 15.4 mA). This can be corrected in the PLC's input scaling, but you lose the headroom that the amplifier provides for overload detection and you also lose 29% of the ADC resolution.
For the SIWAREX WL280 paired with the TXDIN1600S, the sensitivity must be configured in the transmitter's DIP switches (or via software, depending on the TXDIN1600S variant). Set the transmitter's input sensitivity to 3 mV/V or the nearest available setting that is ≥ 2.85 mV/V.
Excitation Voltage Rules
- Lower excitation is acceptable. Excitation below the rated maximum is acceptable and often preferred for thermal stability. A 5 V excitation on a 2.85 mV/V cell produces 14.25 mV full scale instead of 28.5 mV.
- Higher excitation is NOT acceptable beyond the limit. Excitation above 18 VDC on the SIWAREX WL280 will cause self-heating of the strain gauges, drift, and possible permanent damage.
- 10 V is the convention. Most instrumentation amplifiers, including the OMEGA TXDIN1600S, supply 10 VDC nominal excitation.
The TXDIN1600S supplies a regulated 10 VDC excitation to the load cell bridge, which matches the SIWAREX WL280 specification exactly.
OMEGA TXDIN1600S Transmitter
The OMEGA TXDIN1600S is a DIN-rail mounted strain-gauge-to-current-loop transmitter. It accepts a low-level mV input from a full-bridge strain-gauge sensor and outputs a proportional 4-20 mA current on a 2-wire current loop.
| Parameter | Value |
|---|---|
| Input range | ±30 mV, ±60 mV, or ±120 mV (DIP-selectable) |
| Input sensitivity | 1-3 mV/V typical (DIP-selectable) |
| Excitation to load cell | 10 VDC regulated, 30 mA max |
| Output | 4-20 mA, 2-wire loop-powered |
| Loop supply voltage | 12-36 VDC |
| Accuracy | ±0.05% of span (typical) |
| Linearity | ±0.02% of span |
| Thermal drift | ±0.005% of span / °C |
| Operating temperature | -40 °C to +85 °C |
| Response time | 250 ms default (1 s, 3 s options) |
| Configuration | DIP switches (zero, span, range, filter) |
| Enclosure | DIN-rail mount, IP20 |
The TXDIN1600S is loop-powered: the 24 VDC supply connects in series with the transmitter and the PLC's analog input. The transmitter draws 4 mA at zero load and 20 mA at full load, regulating the loop current to represent the measured weight.
DIP Switch Configuration for SIWAREX WL280
For a 500 kg WL280 cell driving a 4-20 mA signal representing 0-500 kg:
- Set input range DIP switch to ±30 mV (covers the 28.5 mV full-scale output).
- Set sensitivity DIP switch to 3 mV/V (or nearest ≥ 2.85 mV/V setting).
- Set response time (filter) DIP switch to 250 ms for stable weight readings.
- Set output to 4-20 mA (default).
- Set excitation voltage DIP switch to 10 V (default).
Wiring and Electrical Connections
Cable Selection
Use shielded cable with the following conductor count:
- 4-wire cable: Excitation +, Excitation -, Signal +, Signal -. Acceptable for cable runs under 15 m where lead resistance compensation is not required.
- 6-wire cable: Adds Sense + and Sense - lines that feed the actual bridge voltage back to the amplifier. Required for cable runs > 15 m or for high-accuracy applications to compensate for voltage drop in the excitation leads.
For the reference application (load cell mounted adjacent to the TXDIN1600S panel), the supplied 5 m SIWAREX cable is sufficient and uses 4-wire connection.
SIWAREX WL280 Color Code (Standard)
| Wire Color | Function | TXDIN1600S Terminal |
|---|---|---|
| Red | Excitation + | EX+ |
| Black | Excitation - | EX- |
| Green | Signal + | IN+ |
| White | Signal - | IN- |
| Drain / Shield | Shield | GND (transmitter end only) |
Transmitter to PLC Loop Wiring
- Connect +24 VDC to the TXDIN1600S "PWR+" terminal.
- Connect the TXDIN1600S "Iout" terminal to the PLC's analog input + terminal.
- Connect the PLC's analog input - terminal to the 24 VDC return (common).
- The PLC analog input must be configured for 4-20 mA input mode (not 0-10 V or 0-20 mA).
Shielding Practice
- Ground the cable shield at the transmitter end only. Do not ground at the load cell end.
- Use a separate cable tray or conduit for the load cell cable. Do not run it parallel to VFD output cables, motor leads, or relay control wiring. Maintain minimum 30 cm separation, crossing at 90°.
- If the cable must cross power wiring, cross at right angles.
- Use metal cable glands at both ends to bond the cable shield to the enclosure 360°.
Calibration Procedure
Calibration Principles
A load cell + amplifier system requires two adjustments:
- Zero (tare) calibration: The output current when no weight is applied must be exactly 4 mA.
- Span calibration: The output current at a known reference weight must be exactly 4 + (16 × W_ref / W_full_scale) mA.
For a 500 kg cell with 4-20 mA = 0-500 kg:
| Applied Weight (kg) | Output (mA) |
|---|---|
| 0 | 4.00 |
| 100 | 7.20 |
| 200 | 10.40 |
| 250 | 12.00 |
| 300 | 13.60 |
| 500 | 20.00 |
The reference weight should be at least 50% of full scale for the calibration to be representative. Use calibrated test masses (NIST-traceable) or, in process applications, apply a known fill (e.g., a calibrated hopper of known weight) and accept the reduced accuracy.
Step-by-Step Calibration
- Mechanical installation check. Verify that the load cell is mounted per Siemens instructions: no side loads, no torsion, no thermal gradients. The load must be applied through the central loading axis only.
- Power-up. Apply 24 VDC to the transmitter. Allow a 5-minute warm-up.
- Zero adjustment. With no load on the cell, measure the current loop with a multimeter in series. Adjust the ZERO trim pot on the TXDIN1600S until the meter reads exactly 4.00 mA.
- Apply reference weight. Place a calibrated 250 kg mass (50% of Emax) on the load cell.
- Span adjustment. Adjust the SPAN trim pot until the meter reads 4 + (16 × 250/500) = 12.00 mA.
- Iterate. Repeat steps 3-5 because zero and span adjustments interact on most transmitters. Two or three iterations typically converge.
- Verify linearity. Apply 100 kg, 250 kg, and 500 kg test weights and confirm the output matches the calculated current within ± 0.05% (the TXDIN1600S specified accuracy).
- Record calibration data. Document the as-found and as-left readings, the reference masses used, the ambient temperature, and the date.
Negative mV Offset at No Load
A small negative offset (e.g., -0.07 mV) at the transmitter's mV input with no load is normal. It originates from:
- Bridge resistor tolerance mismatch (typically ± 0.1% per resistor)
- Mechanical pre-load from installation bolts or vessel weight on the cell before zeroing
- Thermal EMF at the terminal connections
This offset is eliminated by the zero calibration step. Do not attempt to correct it by mechanically pre-loading the cell or by adjusting the transmitter's excitation voltage.
Resolution, Accuracy, and Overdimensioning
Resolution Calculation
Resolution is the smallest weight increment the system can reliably display. It is governed by the cell's repeatability, the amplifier's noise floor, and the ADC resolution in the PLC.
For a 500 kg SIWAREX WL280 with 0.005% repeatability:
Absolute repeatability = 0.005% × 500 kg = 0.025 kg = 25 g
This is the smallest weight change that produces a deterministic output. The PLC's 16-bit ADC over a 4-20 mA range resolves:
ADC resolution = (500 kg / 65535 counts) × 16 mA = 0.122 kg / count
The amplifier's noise floor (typical 0.5 µV rms at the mV input) over the 28.5 mV full scale equals:
Noise equivalent = (0.5 µV / 28.5 mV) × 500 kg = 8.8 g
The system's effective resolution is the largest of these three numbers: approximately 122 g, limited by the PLC ADC.
Measuring Small Loads on Large Systems
When the application requires measuring a small incremental weight (e.g., 10 kg) on a system sized for a large total (e.g., 500 kg), the small weight represents only 2% of span. The PLC's 4-20 mA input changes by:
ΔI = (16 mA × 10 kg / 500 kg) = 0.32 mA
ΔADC = 0.32 mA / 16 mA × 65535 = 1310 counts
This is measurable but sits in the noise band of a typical industrial installation. The recommended technique is pre-load taring:
- Install a permanent dead-weight of 50 kg on the weighing structure.
- Tare the transmitter at this pre-load (output = 4 mA).
- Apply the process weight (10 kg). The output becomes 4 + (16 × 10/450) = 4.36 mA.
This reduces the measured weight to 2.2% of span, but the dead-weight stabilizes the mechanical system and reduces the influence of structural compliance, side-load drift, and zero instability. Pre-load taring is the standard technique for low-resolution systems where the absolute accuracy requirement is moderate but the repeatability must be high.
Noise Mitigation and Signal Integrity
Sources of Noise
- Conducted noise from VFDs and motor drives. Common-mode and differential-mode noise on the load cell cable.
- RFI/EMI. Radio-frequency interference from wireless devices, walkie-talkies, or switching power supplies.
- Ground loops. Differences in ground potential between the load cell ground and the transmitter ground.
- Thermal EMF. Temperature gradients across terminal connections generate microvolt-level voltages.
- Mechanical vibration. Particularly in process environments with mixers, agitators, or conveyor impacts.
Mitigation Techniques
| Noise Source | Mitigation |
|---|---|
| VFD / motor noise | Maintain 30 cm separation from VFD cables; cross at 90°; use shielded cable with shield grounded at transmitter end only |
| RFI / EMI | Use metal cable gland at transmitter enclosure; ensure enclosure is bonded to plant ground; use ferrite bead on load cell cable if RFI persists |
| Ground loops | Use single-point ground at transmitter; disconnect shield at load cell end; verify no alternate ground path through mounting hardware |
| Thermal EMF | Keep cable length short; avoid temperature gradients across terminal block; use same-metal terminals (avoid copper-aluminum junctions) |
| Mechanical vibration | Increase transmitter filter time constant (1 s instead of 250 ms); install vibration isolators under vessel feet; use multiple load cells with junction box for averaging |
Cable Length Limits
| Connection Type | Maximum Length | Notes |
|---|---|---|
| 4-wire | 15 m | Lead resistance causes calibration shift with temperature |
| 6-wire (with sense) | 100 m | Sense leads compensate for lead resistance |
| 4-wire + remote sense at transmitter | 50 m | Requires transmitter with remote sense input |
Troubleshooting Matrix
| Symptom | Probable Cause | Action |
|---|---|---|
| Output stuck at 4 mA regardless of load | Open bridge (broken wire); no excitation; transmitter in fault mode | Check excitation voltage at cell terminals (should be 10 V); check continuity of all 4 bridge wires; cycle power |
| Output stuck at 20 mA regardless of load | Input polarity reversed; cell shorted; span mis-configured | Reverse Signal + / Signal -; inspect cell for damage; verify DIP switch range setting |
| Output drifts slowly with temperature | Excitation voltage too high; cell self-heating; ambient temperature gradient | Reduce excitation to 5 V if permissible; shield cell from drafts; check for thermal sources |
| Output noisy (jitters ± 1-2% of span) | Electrical noise pickup; mechanical vibration; insufficient filtering | Check cable routing; increase TXDIN1600S filter to 1 s; verify shield grounding; isolate from vibration |
| Output reads zero but PLC shows 4 mA as 0 kg | PLC scaling error; range mismatch | Verify PLC analog input range is 4-20 mA (not 0-20 mA); check engineering units scaling |
| Negative output at no load (e.g., -0.07 mV) | Bridge offset; installation pre-load | Normal, correct with ZERO calibration |
| Output accurate at zero but wrong at full scale | Span mis-calibrated; sensitivity mismatch | Recalibrate span; verify amplifier sensitivity setting matches cell mV/V rating |
| Output overshoots or oscillates | Filter time constant too short for application; mechanical resonance | Increase filter to 1-3 s; check for resonant mounting |
| Output loses 1-2% accuracy over time | Cell overload; mechanical drift; calibration drift | Inspect installation for side loads; verify cell not near Eu limit; recalibrate |
| Reading shifts when AC powered equipment cycles on/off | Ground loop; inadequate filtering | Install signal isolator between transmitter and PLC; verify single-point grounding |
| Output correct at low loads, drifts at high loads | Cell over-loaded; mechanical binding; installation geometry | Verify total load (vessel + product) < 60% Emax; check mounting hardware for binding |
| Output stable but PLC reading jumps in steps | PLC ADC resolution insufficient; electrical noise on AI module | Use 16-bit AI module; check AI common mode voltage; install signal isolator |
Verification Checklist
Before declaring the system operational, confirm the following:
- Excitation voltage at the cell terminals is 9.5-10.5 VDC.
- Output at no load is 4.00 mA ± 0.01 mA after warm-up.
- Output at 50% of Emax matches calculated value ± 0.05 mA.
- Output at 100% of Emax is 20.00 mA ± 0.05 mA.
- Output stability over 10 minutes is within ± 0.05 mA (with stable load).
- Shield is grounded at transmitter end only, with continuity verified.
- Cable routing maintains 30 cm separation from VFD / motor cables.
- PLC analog input is configured for 4-20 mA mode (not 0-10 V or 0-20 mA).
- PLC engineering unit scaling is correct (e.g., 4 mA = 0 kg, 20 mA = 500 kg).
- Tare function in the PLC is operational and removes vessel weight at startup.
- No cold-junction errors from temperature gradients across the transmitter terminals.
- Calibration certificate and commissioning record filed in plant documentation system.
Field-Commissioning Sequence
- Inspect mechanical mounting per Siemens WL280 installation drawing.
- Verify load path passes through cell central axis; verify no side-load constraints.
- Megger bridge to ground: ≥ 5000 MΩ at 500 VDC.
- Connect load cell cable to TXDIN1600S following the color code table.
- Connect 24 VDC loop supply.
- Verify excitation at cell terminals = 10 VDC ± 0.5 V.
- Power-cycle transmitter and wait 5 minutes for thermal stabilization.
- Apply zero calibration with no load.
- Apply 250 kg reference mass; adjust span.
- Verify linearity at 100 kg and 500 kg.
- Record calibration data and environmental conditions.
- Hand off to operations with commissioning sheet signed.
How do I verify the mV/V sensitivity of an unmarked load cell?
Apply a known load at a known excitation voltage and measure the bridge output. For a 100 kg reference weight at 10 V excitation on a 2 mV/V cell, the output is 2 × 10 × (100/Emax) mV. With Emax = 500 kg, the output is 4 mV. Confirm by linearity at 200 kg (8 mV) and 300 kg (12 mV). A portable mV/V calibrator or a precision strain-gauge indicator can also be used.
Why is my output drifting slowly even though nothing is changing?
The most common causes are thermal gradients (drafts, sunlight on one side of the vessel), self-heating from excessive excitation voltage, or a slowly leaking bridge resistor. Verify by covering the cell to eliminate drafts, reducing excitation to 5 V, and measuring bridge resistance to ground with a megohmmeter (should be > 5000 MΩ).
Can I connect multiple SIWAREX WL280 cells in parallel to one TXDIN1600S?
Yes, with a junction box that sums the parallel signals. Use a summing box with individual trim resistors for each cell so the cells are matched within 0.1%. Without trimming, paralleled cells will each carry an unknown portion of the load and the system accuracy degrades to that of the least-trimmed cell. The TXDIN1600S excitation can typically drive up to four 350 Ω cells in parallel.
How long can the cable between the load cell and transmitter be?
With 4-wire connection, keep the cable under 15 m to avoid excitation lead voltage drop affecting calibration. With 6-wire (sense lead) connection, cable runs up to 100 m are acceptable if the cable gauge is sized so that the loop resistance does not exceed the transmitter's drive capability (typically 30 mA at 10 V = 333 Ω total loop).
What is the difference between accuracy and repeatability in load cell specifications?
Accuracy is the closeness of the indicated weight to the true weight across the entire range, including linearity, hysteresis, and creep errors. Repeatability is the ability to produce the same reading when the same load is applied repeatedly under identical conditions. Repeatability is always better than accuracy. For a process where you tare before each batch (e.g., a batching scale), repeatability is the relevant metric; you can achieve batch-to-batch consistency even if absolute accuracy is moderate.
Do I need a separate 24 VDC supply for the transmitter or can I use the PLC analog input power?
The transmitter is loop-powered, meaning the PLC's analog input module typically provides the loop power. Verify your PLC analog input module supplies the loop voltage (most do, but some require an external supply). If the PLC does not supply loop power, install a dedicated 24 VDC regulated supply. The current loop must be powered from one source only to avoid ground loops.
What is the impact of using a load cell rated for much higher weight than needed?
Larger cells produce a smaller mV signal at the same load, reducing the signal-to-noise ratio and effective resolution. For a 10 kg measurement on a 5000 kg cell at 2.85 mV/V, the output is only 0.057 mV (57 µV) — easily lost in the noise floor of most industrial installations. Always match the cell size to the application within a 2-3× margin.