Twenty 10,000 kg load cells, 0.1 kg or 0.01 kg displayed precision, and a P3000 with 16-bit analog cards is a mismatch you cannot scale or filter your way out of. Put a weighing instrument (indicator/controller) on each vessel and packout station, let it handle excitation, conversion, corner trim and calibration, and read finished weight into the PAC over Modbus. Get it running with that architecture, then tune it properly.
Size the Counts Before You Buy Anything
Work out how many counts the requested precision needs. The source gives 20 cells, 4 vessels and 4 packout stations. The split below assumes 4 cells per vessel (16) and 1 cell per packout station (4). Confirm it against the mechanical drawings.
| Scale | Capacity (assumed) | Counts for 0.1 kg | Counts for 0.01 kg | Best case, 16-bit (65,536 counts) |
|---|---|---|---|---|
| Packout, 1 cell | 10,000 kg | 100,000 | 1,000,000 | about 0.153 kg per count |
| Vessel, 4 cells sharing load | 40,000 kg | 400,000 | 4,000,000 | about 0.61 kg per count |
The 16-bit figure is a ceiling. A 4-20 mA input loses the codes below 4 mA, and module noise eats the bottom bits. Real usable resolution is worse than the table shows.
Calculate the signal at the cell too. Use the rated output (mV/V) and excitation voltage from the load cell calibration certificate:
Full-scale signal (mV) = rated output (mV/V) x excitation (V)
Signal per division (uV) = full-scale signal (mV) x 1000 / divisions requiredWith common bridge outputs, 1,000,000 divisions puts each division in the tens of nanovolts. That is below plant EMI and thermal noise. Check: compare the divisions you need against the accuracy class and maximum number of intervals on the load cell datasheet. Agree a displayed increment with the customer that the cells can actually hold. The request for 0.01 kg is display precision, not accuracy, and everyone should sign off on that before commissioning.
Drop the Analog Quick Fixes
These are the usual first attempts. None of them adds real resolution.
- Signal conditioner to 4-20 mA into an analog card. The conversion chain caps you at the card's counts, as shown in the table above.
- Scaling the raw value to a REAL with more decimals. This prints digits that change in 0.15 kg or 0.6 kg steps. It is false precision.
- Averaging or oversampling in the PAC. This smooths noise. It does not create codes the converter never produced, and it adds lag to fill and dosing cutoffs.
- One analog channel per cell, summed in logic. This multiplies I/O, and you still inherit 16-bit limits per channel. You also end up doing corner correction in ladder with no calibration tooling.
Check: if you have an analog card already wired to a test scale, load and unload a known weight. Watch the smallest step in the raw count. That step is your real resolution.
Wire Each Vessel Through a Summing Junction Box
The four cells on a vessel act as one scale. Their signals are paralleled in a summing junction box, and the box feeds one instrument channel.
- Mount the cells, then check that every one carries load. Look for level mounting plates, free check rods, and no binding hardware.
- Make piping, conduit and chutes flexible at the vessel boundary. A rigid pipe carries load around the cells and shows up as nonlinearity and hysteresis.
- Do not shorten load cell cables. Cable length is part of the cell's temperature compensation. Coil the excess instead.
- Run the home run from the junction box to the instrument as six-wire, with sense lines if the instrument supports them. This compensates for voltage drop on long runs.
- Land the shields at one end only, following the instrument manual.
Check (corner test): place the same test weight over each cell in turn and record each reading. Adjust the junction box trim, or use the instrument's digital corner correction, until the four readings agree within the tolerance in the instrument manual. The source's worry about trimming four cells per vessel goes away here. A weighing instrument with digital corner correction turns an hour of pot-twiddling into a menu routine.
Put the Conversion in a Weighing Instrument
A dedicated weight indicator or controller has a high-resolution bridge converter, stable excitation, filtering, zero/tare, calibration routines, and a digital output. This is where the resolution comes from. The PAC then only reads a finished number.
| Selection point | What to look for |
|---|---|
| Channels | One per vessel. Some instruments handle several scales, which can cover the packout stations. |
| Internal resolution | Datasheet internal counts well above the displayed divisions you agreed. |
| Excitation capacity | Enough current to drive 4 cells in parallel at the rated bridge resistance. |
| Communications | Modbus RTU or Modbus TCP, matching a free P3000 port. |
| Calibration | Deadweight and span calibration, digital corner correction, stored calibration. |
| Weight data format | 32-bit integer or float register map, plus status bits (motion, overload, zero error). |
Weigh the cost against the alternative. One analog channel plus a conditioner per scale looks cheaper, but you get 0.15 kg to 0.6 kg steps and no calibration tooling.
Check: zero the empty vessel. Calibrate span with certified test weights, or a build-up method for large vessels. Confirm return-to-zero after the weight is removed. Do not touch the PAC until the instrument display reads correctly on its own.
Connect the Instrument to the P3000 Over Modbus
- Set the instrument's station address, baud/parity or IP settings to match the P3000 port configuration.
- Configure the P3000 as Modbus client and read the gross/net weight and status registers from the instrument's register map.
- Map the 32-bit weight into a matching tag type. If the value reads wildly wrong but moves with load, swap the word order. Word order is the most common Modbus weight fault.
- Apply scaling only if the register is an implied-decimal integer. Take the decimal position from the register map.
- Use the motion and error bits to gate logic. Do not latch a fill complete or record a packout weight while the motion bit is set.
- Add a comm-loss timer. A stale weight on a filling vessel must stop the fill.
Check: put a known weight on the scale. The P3000 tag must match the instrument display to the last displayed digit, update at the poll rate, and flag an alarm when you pull the comm cable.
Verify the Whole Chain End to End
- Run a zero-to-span-to-zero sequence on each vessel and each packout station. Log readings at the display and in the PAC.
- Repeat the corner test with product-level loading if the process allows.
- Fill with piping connected and agitators or feeders running. Compare against the static test to expose mechanical restraint or vibration.
- Check stability at the agreed display increment. If the last digit hunts at rest, increase the instrument filter or coarsen the increment. Do not average in the PAC.
- Record calibration data, junction box trim positions and instrument parameter backups for every scale.
FAQ
Why does my PLC weight reading jump in large steps even though I scaled it to two decimals?
The analog card's count resolution sets the step size, not the tag's decimals. A 16-bit input spread over 10,000 kg is about 0.153 kg per count at best, and over 40,000 kg it is about 0.61 kg.
Why does a 4-20 mA load cell transmitter lose resolution compared to a weight indicator?
The transmitter output is re-digitized by the PLC analog card, which caps you at its counts minus the unused 0-4 mA span. A weight indicator digitizes the millivolt bridge signal directly with a much higher-resolution converter and sends the result digitally.
Why does my vessel read differently depending on where the load sits?
The cells are not matched at the summing point, or piping or a check rod is carrying load. Run a corner test with one test weight, trim at the junction box or with digital corner correction, and free any rigid connections.
Why does the Modbus weight value in the PLC show a huge wrong number?
The 32-bit value is being read with the wrong word order or the wrong data type. Swap the word order, confirm integer versus float from the instrument register map, and apply the implied decimal only for integer registers.
When should I stop and call the weighing instrument or load cell manufacturer?
Stop if the corner test will not converge after mechanical checks, if zero drifts with the vessel empty and disconnected from process, or if a bridge resistance check shows a cell out of spec. Contact the instrument or load cell manufacturer's official technical support with the calibration log and parameter backup, because a damaged cell or wrong excitation setup cannot be corrected in PAC logic.