Servicing and Troubleshooting Process and Inventory Weighing Systems

A field guide to keeping tank, hopper, reactor, mixer, and silo weigh systems accurate: how to inspect the mechanics and wiring, balance the load across cells, read the signal, localize faults at the cell and the instrument, and maintain the system over its long service life.

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What Servicing a Weigh System Involves

A process or inventory weigh system measures the mass of a vessel resting on several load cells. It has effectively no moving parts to wear out, so service is less about repair and more about protecting two conditions the measurement depends on: the vessel must be free to move, and the signal must reach the instrument clean. This guide covers BLH Nobel systems and tools and walks through start-up, calibration, mechanical and electrical troubleshooting, and routine maintenance.

Free to move. A load cell or weigh module must deflect 0.008 to 0.10 inch vertically without binding. Through-floor mounts, piping, conduit, and vent lines all have to leave the vessel free, and yokes or slide plates must clear their motion stops at operating temperature.
Protected from interference. Load cell cables run in metallic conduit, away from AC power, motor starters, and relays, with a drain at the lowest point of moist runs. Clean routing and proper grounding keep RFI and EMI out of a low-level signal.

The Service Sequence

Servicing follows a logical order. Each stage confirms the one before it, so a problem found late usually points back to a step that was skipped.

Stage	What happens
1 · Inspect	Check the mechanics — mounts, piping, conduit, vent lines, and module orientation — then the wiring, grounding, and instrument voltage selection.
2 · Set up & balance	Hook up the FSk-40 and 325 test gear, read each cell's signal, and shim the supports until the load is shared evenly.
3 · Calibrate	Choose a calibration method by accuracy and vessel type, then verify the signal path end to end.
4 · Troubleshoot	If readings are wrong, use the symptom at the cell or the instrument to localize the fault before replacing anything.

Before you Measure: The Inspection Checklist

Most measurement problems are mechanical or wiring faults present from the start. A disciplined walk-through of both domains catches them before they reach the display.

Mechanical checks — before loading

Deflection	Vessel free to move about 0.10 inch vertically without binding.
Orientation	Weigh modules arranged per the manual to accommodate thermal expansion; a steel vessel expands 6.5 × 10⁻⁶ in/in/°F.
Yoke / slide	Clear of the built-in motion stops at operating temperature; attachment level within ½°.
Force arrow	On cylindrical-beam cells (KIS, KDH-3), pointing downward and as plumb as possible.
Piping	Horizontal where possible, supported on the same structure as the vessel, as far from it as practical.

Electrical checks — before power-up

Cable routing	Metallic conduit away from AC power, motor starters, and relays; a short flexible run at the cell eases replacement.
Conduit drain	At the lowest point of any moist run; replace rubber plugs in unused openings with steel fittings.
Summing box	Correct terminations, with remote sense lines in use on the cable to a remote indicator.
I.S. barrier	On Division 1 systems, the summing-box shield ties to the barrier ground bus; the instrument shield ties to instrument ground, not the barrier.
Instrument	Verify voltage selection, polarity, and options; connect only non-powered switch contacts to digital inputs.

Balancing the Load and Checking the Signal

Each cell should carry an approximately equal share of the load, so no single cell is overloaded and the structure proves itself stable. A three-point support tends to balance naturally because three points define a plane; four or more supports usually need shimming. With the FSk-40 in shim mode, read each channel as a percentage of load and compare the spread between the lowest and highest readings.

Support	No shimming needed when the spread is	If greater
Three-point	about 30% to 36%	Add shim under the lowest point(s)
Four-point	about 22.5% to 27.5%	Add shim under the lowest point(s)

To shim, jack the vessel slightly, loosen the attachment bolts, and add stock — usually a few thousandths to a few tenths of an inch — between the cell top plate or yoke and the vessel support, then lower and re-read. Hanging vessels have no plates to shim, so balance is set with threaded tension-rod adjustments. If balance is impossible to reach, suspect a piping attachment problem.

Finally, prove the signal path. Connect the 325 calibrator to the instrument and step it through 0, 1.0, 2.0, and 3.0 mV/V; the displayed value should track each setting, and a deviation beyond about 0.01% of reading points to a wiring or equipment fault. A model 308A summing unit shows a small 2% to 4% output shift from its guard circuit, which the model 306 does not have. DXp-10/15 units have no display or mV/V output and cannot take this initial test.

Troubleshooting Load Cell Faults

The symptom usually names the fault. Resistance and leakage checks are made with the cell wired into the summing unit.

Symptom	Points to	Diagnosis and action
Zero shift (unloaded output)	Overload	A non-zero output with no load means the cell was overloaded. Correct the cause, then replace it. Not field-repairable — a shift near 20% of rated capacity.
Drifting output	Leakage	Drift loaded or unloaded signals electrical leakage, often moisture. Measure any excitation or signal lead to case ground: expect at least 2000 megohms on a megohm meter applying under 20 V (use clips). The factory can often recondition moisture-affected cells.
Negative output	Orientation	The transducer is loaded backwards. Reorient so the force arrow points toward the applied force — downward on a tank. An installation fix, not a cell fault.
No output	Open circuit	With excitation and load present, no output means an open cell circuit (welding current, lightning, fatigue). Check across the excitation and signal leads: 350 or 700 ohms, within about 1 ohm. Not field-repairable; the factory may rebuild it.
Overload display in shim mode	Overrange	FSk-40 overload means the output exceeds the 3.5 mV/V input range — the element has yielded. Confirm by reading mV/V at zero load against the zero-balance spec. Not repairable.
Long-term drift	Leakage or structure	Output cycling over hours. The FSk-40 drift function names the affected channel. Check piping and conduit at that cell and that supports share the vessel's structure; disconnect pipes one at a time. Often a structural fix.
Non-repeatability	Interference	Readings do not repeat, usually from mechanical interference or stiff piping. Record each channel up and down the range and plot it; the poor channel sits next to the problem. Free the mechanics rather than replacing the cell.

Troubleshooting Instruments and Communication

Most instrument and transmitter problems come down to wiring or configuration. Isolate the indicator by connecting the 325 calibrator directly to it; a steady reading there clears the instrument and points back upstream.

Symptom	Likely cause	What to do
Drifting display or output	Load cell or wiring, rarely the instrument	Connect the 325 directly to the instrument; if steady, the fault is upstream. If not, check terminals and return the unit for repair.
Overload / overrange / underrange	Overloaded cells, open or incorrect wiring, or setup	An open or 35 mV overrange input sets the diagnostic bits; double-check the security of the sense and signal lead connections.
Excitation fault	Excitation-circuit wiring	Recheck the excitation connections and the voltage at the green and black terminals of the 325. If correct and still faulting, return for repair.
Serial transmit / receive	Wiring or polarity (RS-232/485/422)	Bridge LEDs across the transmit and receive pairs, cathode to the negative conductor; flashing confirms digital pulses. No flash in continuous mode means return for repair.
Analog output fault	Open current-output loop	Verify loop integrity by checking wiring and continuity with an ohm meter. If intact and persisting, return for repair.

Calibration in Brief

Servicing and calibration go together, but the methods themselves are covered in depth elsewhere. There are eight options, from quick electronic simulation that checks only the signal path to full-scale deadweight that proves the whole mechanical system. The right one depends on the accuracy you need and what the vessel allows.

Method	Typical accuracy	Proves mechanics
Push-button / PROM	0.05 to 0.5%	No
mV simulation	0.25 to 1.0%	No
mV/V simulation	0.10 to 1.0%	No
Partial deadweight	0.5 to 2.0%	Yes
Mass or volumetric flow	0.25 to 1.0%	Yes
Applied force	0.25 to 1.0%	Yes
Full build-up / substitution	0.05 to 0.2%	Yes
Full-scale deadweight	0.02 to 0.1%	Yes

Each method's procedure, equipment, and trade-offs are detailed in the companion handbook, Calibration Methods for Process and Inventory Weigh Systems.

Routine Maintenance

Strain-gage load cells have no moving parts, so a mature system stays stable for years on a light, regular routine.

Mechanical Check for anything that blocks the roughly 0.10 inch of vertical movement — dirt, ice, or build-up under the transducer and keep cells out of standing water for long periods.
Electrical Look for chafed or damaged cable jackets and repair them at once to keep moisture out. Check the summing box for condensation; if it recurs, add desiccant or drains.
Calibration Quality programs usually set the interval. Stability is good, but verify calibration at least once a year.
Corrosion Most cells are stainless and resist corrosion. Inspect painted cells for breaks in the coating and touch them up as needed.

The complete TC0008 handbook also includes the wiring diagrams, FSk-40 and 325 hook-up details, and shimming figures.

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