Fish-hold refrigeration controls — what failure looks like.

Refrigerated seawater is the cold heart of any commercial scallop, herring, or squid trip. The boards that run it are usually one of three controllers — the Carel pCO5+, the Danfoss MCX, or the older Honeywell Falcon — and they are usually wired into the engine-room mains and the compressor in fairly standard ways. The failures, when they come, look more dramatic than they are. The four below are 90 percent of the service calls.

§ 01 · The temperature probe drifts

The Pt100 RTD probe in the hold is a thin steel-sheathed pencil, hard-mounted with a stainless gland and run on twisted-pair shielded cable to the controller. It is also the cheapest part of the system. Pt100 probes age by drifting low — reading a colder temperature than the actual hold — at a rate that depends on the probe's class and how often the hold is washed down with hot water.^[1]

The signature: the controller reports the hold is at −1.5 °C and is comfortable; the captain's analog gauge reads −0.2 °C; the catch comes off softer than the buyer expects. The cause is almost always the probe.

The fix: replace with a Class A Pt100 (±0.15 + 0.002·t °C accuracy) and re-calibrate against an Onset HOBO MX2202 reference for an hour at known cold. Cost: $180 of part, two hours of labor.

§ 02 · The low-pressure cutoff trips

The compressor's suction-line low-pressure cutoff is a safety. It opens the compressor relay if the suction pressure drops below the saturation pressure for the design temperature — on R-507A systems, that is around 8 psig at −25 °C. The cause of an unexpected trip is almost always something mechanical: a clogged suction-line strainer, a closed valve, or a low-charge condition.

This is mechanical, not electrical. The reason it lands as our service call is that the symptom looks electrical — the controller flashes "LP CUTOFF" and the captain reaches for the electrician first. The fix is to call Fish Ex Marine Refrigeration or the boat's regular refrigeration shop, who pulls the strainer or addresses the charge. Our role is to verify the controller's logic before we hand it off — sometimes the controller has a faulty input and the cutoff isn't actually tripping.

§ 03 · The contactor pits

The compressor contactor — usually a 50 A definite-purpose unit — is rated for a million cycles in catalogue, more like 100,000 in practice when it is sized correctly. When it has been undersized (say, a 25 A contactor on a 22 A continuous load with a 100 A locked-rotor inrush), it pits faster — the running surface gets a black coating that introduces resistance, the resistance heats the contact, the heat melts a small pit, the pit chatters under closing, and the chatter accelerates the failure.

The signature: the compressor starts but takes 4–6 seconds to reach full current draw, and you can hear a faint chatter at the contactor box during that startup. The fix: oversize the replacement to a properly-rated definite-purpose contactor, with the right HP rating for the LRA. We have a stock of Eaton C25 series in three sizes on the truck.

§ 04 · The control transformer eats itself

The 24 V control transformer is fed from the engine-room 240 V (or 480 V on larger boats) panel. It has primary taps for 208, 220, 240, 277, and 480; secondary is 24 V. The failure mode is universal: someone fed it 240 V into the 208 V tap, which causes a fast saturation of the core, drives the secondary to about 28 V, and overloads everything downstream — including the controller's input, which can also fail.

The signature: the controller LCD blinks erratically, downstream relays start chattering, and the transformer itself sometimes hums noticeably louder than it used to. The fix is straightforward: re-strap to the correct primary tap, replace the secondary fuse, and verify the controller's input is still healthy. Cost: $90 in parts, one to two hours.

§ 05 · Prevention

For a working scalloper or herring boat, the discipline that prevents most of the above:

1. Replace the Pt100 probe every two years on a calendar schedule, regardless of how it reads. They are cheap and the cost of a soft trip is not.
2. Keep an Onset HOBO reference logger in the hold every trip. The buyer will sometimes ask for the trip log; you want to have one. The data is also a tripwire on Pt100 drift — if the log diverges from the controller by > 1 K, it is time to swap.
3. Size every contactor with a margin: HP rating at or above the compressor's, and 1.5x the running current as a minimum.
4. Label the control-transformer primary tap on the cabinet door. Add a sticker that reads, in JetBrains Mono, "PRIMARY = 240 V — DO NOT MOVE."

The bulk of these prevention items live on the Plate P-02 page. If you have a board you'd like us to walk through proactively, the rate sheet is on rates; we can do a Pt100 swap and a controller-firmware check in two hours.

Cross-references: Plate P-02, Cape Cod Tradition's Carel install.