Installing a Marine Scale on Deck or in the Fish Hold

A marine scale is only as good as its installation. The same scale can give steady, repeatable readings when it is rigidly mounted near the centre of the vessel, and erratic ones when it hangs on a flexible bracket at the stern with a hose resting on the platform. This article sets out general good practice for positioning, mounting, cabling, power, drainage and earthing of marine scales on deck, in the fish hold or in a processing area. It does not replace the manufacturer's installation manual, the vessel's class and flag requirements, or the judgement of a qualified marine electrician.

Choosing the position

The ideal position minimises vessel motion at the scale while fitting the workflow of the crew. Vertical motion at any point on board is the sum of heave and the contributions of pitch and roll, which grow with distance from the centre of rotation, as shown in the Delft University of Technology textbook Offshore Hydromechanics. In practice, weigh these factors:

Factor	Prefer	Avoid
Vessel motion	Close to amidships and the centreline, low in the hull	Bow, stern, far outboard, high superstructure
Vibration	Stiff structure away from engines, pumps and winches	Bulkheads next to machinery, thin plating
Workflow	Between sorting and the next step (labelling, packing, hold)	Positions that force carrying boxes across walkways
Water and cleaning	Free drainage, access for hosing under and around the scale	Low points where water pools, direct green-water exposure
Temperature	Stable conditions	Positions that alternate between direct sun and freezing spray
Cabling	Short, protected route to indicator, printer and power	Routes crossing walkways, hatches or moving equipment

Motion considerations with worked numbers are covered in Getting Accurate Weights in Rough Seas.

Wall mounting and column mounting

Bench scales on vessels are usually fixed to a wall or a column rather than standing loose, because a loose scale moves and a moving scale cannot weigh correctly or safely. A wall mount fixes the scale to a bulkhead or frame; a column mount places it on a pedestal fixed to the deck, which is useful where there is no suitable wall or where the scale must be reachable from several sides.

1. Fix to structure. Bolt or weld the mount to a frame, stiffener or deck structure, not to thin lining panels. Press firmly on the installed platform: no visible flexing should occur.
2. Set the height. Choose a working height that avoids repeated lifting from floor level, typically so that the platform is close to the height at which boxes arrive.
3. Level the platform in port, in calm water, with the vessel at normal trim. The US requirements for scales that weigh catch at sea (50 CFR 679, Appendix A) require platform scales to be rigidly installed in a level condition.
4. Keep clearance around the platform so that it can move freely under load and nothing but the load touches it.
5. Protect against impact from swinging gear, forklifts or pallet trucks with guards where needed, without touching the platform.

Platform scales are installed on or into the deck. A fold-up platform frees deck space and makes cleaning underneath easier; a ramp makes rolling loads on and off safer.

Welding precautions and force shunts

Two installation mistakes damage accuracy more than any other: welding current through the load cells and connections that carry part of the load past them.

• Welding. Disconnect or remove the load cells before welding on or near the installation. The Alfa Laval load cell installation guidelines instruct installers to disconnect load cells from the instrumentation whenever welding is performed. Welding current that passes through a load cell can damage it.
• Force shunts. The same guidelines state that connections from the weighed structure to its surroundings, such as pipes, cables and bellows, must be as flexible as possible, and that anything resisting vertical movement of the load causes errors. On a vessel, typical culprits are stiff cables pulled tight across the platform, hoses, ice chutes, drip trays and guards that touch the platform edge.

Cable routing

Load cell and data cables carry small signals and live in a wet, vibrating, salt-laden environment, so the route matters as much as the connections. General good practice:

• Route the cable so it cannot be cut, crushed or pulled during installation or use; the Alfa Laval guidelines stress this explicitly.
• Keep the open ends of load cell cables protected against humidity during installation; moisture inside a cable can creep towards the load cell.
• Separate signal cables from power cables and from high-current equipment such as winch and pump motors, and cross them at right angles where they must cross.
• Use cable glands suited to washdown, form drip loops below entry points so water runs off rather than into the enclosure, and avoid entries on top of enclosures.
• Fix cables at short intervals so that vibration does not fatigue conductors or loosen connectors.
• Do not shorten or extend load cell cables unless the manufacturer permits it; cable length can be part of the calibration.

Power supply and earthing

Power and earthing on a vessel must follow the vessel's electrical installation rules; the IEC 60092 series covers electrical installations in ships, and class and flag state requirements apply on top. Leave the connection to the vessel's supply to a qualified marine electrician. From the weighing side, the points to check are:

• Supply quality. Generators, winches and refrigeration compressors cause voltage dips and spikes. Ask the scale supplier for the permitted supply range and consider a dedicated, protected circuit.
• Battery option. A battery-powered scale avoids supply problems and cable runs for portable or temporary use.
• Single, defined earth. Follow the indicator manual on where the cable screen and protective earth are connected. Multiple earth connections along a signal cable can form ground loops that inject noise.
• Isolation for washdown. All mains connections must be protected for the wet environment and positioned so that washdown hoses are not aimed at them.

Corrosion: mounting materials matter

A stainless steel scale can be undermined by the hardware used to fix it. Euro Inox explains that galvanic corrosion needs three conditions: metals with different corrosion potentials, an electrical connection between them and an electrolyte film connecting both. Seawater and salty spray provide the electrolyte, and in contact with stainless steel it is usually the other metal that corrodes, for example aluminium superstructure, galvanised steel brackets or zinc-plated bolts.

• Use stainless steel fasteners and brackets of a compatible grade with an AISI 316 scale.
• Where contact with aluminium or carbon steel is unavoidable, electrically insulate the joint with plastic bushes or polyamide washers, as Euro Inox recommends.
• Do not rely on coating only the less noble metal: a small scratch then concentrates corrosion at one spot.
• Avoid crevices where salt water stands; seal or design them out.

More on material choice and hygiene is in the hygiene and materials hub.

Washdown and drainage

Every weighing station on a fishing vessel is hosed down daily, so installation must let water leave and let the cleaner reach every surface.

1. Position the scale so that water drains away from it, never towards the indicator or cable entries.
2. Leave enough space under and behind the platform to clean; slime and scales that collect there shift the zero and become a hygiene risk.
3. Avoid mounting over a low point where water pools around the base or feet.
4. Check that cleaning does not require removing guards that could be refitted touching the platform.
5. After cleaning, let the platform drain, then zero the empty scale.

Commissioning checklist

Before the scale goes into service, run through a structured commissioning check and keep the record on board.

1. Mount rigid, platform level, nothing touching the platform.
2. Cables fixed, glands tight, drip loops formed, no signal cable bundled with power cables.
3. Supply voltage checked; earth and screen connections as per the manual.
4. Gravity setting or compensation checked for the area of operation; see Gravity, Latitude and Why Marine Scales Need Gravity Compensation.
5. Zero, then test with known weights at low, middle and high load, and at the corners of the platform.
6. Filter and stability settings recorded.
7. Printer, labeller or network connection tested end to end.
8. Crew briefed on zeroing, loading and daily checks.

How WPL approaches this

WPL scales are designed for fixed installation on board. The M2 Series comes with a wall mount as standard and a column mount as an option, and can run on an optional battery. The M5 Series platform folds up for cleaning and deck space, and the M6 Series is fully sealed and available with an access ramp; both include a wall mount as standard. Settings can be backed up and restored with the R10 Configuration Panel, which helps when a scale is serviced or replaced. See the buyer's guide for selection, and request a quote to discuss a specific installation.