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Troubleshooting a load cell

Alas, the plight of the ignoble load cell: doomed to ride for eternity at the point of a knife, dunked in salt water ever and anon, but expected to work flawlessly.

Load cells are a tremendous help when tuning the rig for different conditions. They are especially important on fractional rigs, where it can be hard to judge the headstay tension based on the backstay (unless you have done your math ahead of time). Load cells are also used frequently on larger yachts to judge rig tension all around, as it would be pretty difficult to use your typical Loos gauge on 3/4″ PBO.

On land, load cells are pretty much indestructible. They are used widely in industrial and transportation applications with nary a problem. However, put them in the marine environment, and forget about that eternal reliability, especially when put on a yacht. Load cells, especially load pins, have some important considerations that are frequently ignored, leading to failure.

The load cell is basically a Wheatstone bridge. The actual configuration of the measured circuit can vary, but they are all essentially the same. Bending stress on the special printed circuit causes a change in resistance in one or more legs of the circuit. This change in resistance unbalances the Wheatstone bridge, and this is measured by the amplifier. The amplifier then converts the measurement into something usable by external devices, typically in RS-232 or NMEA 0183 format.

One of the most frequent failures I have seen involves the cabling. A well-sealed cable keeps out salt water and prevents infiltration into the electronics. Once this cable is damaged, ether through a nicked sheathing or a pulled-away seal, failure is sure to follow in short order. Salt water will wick throughout the wire by capillary action, and will also seep into the actual sensing portion of the load cell. As we all know, salt water and electronics don’t mix. I have received items back where the entire cable has corroded inside the sheathing due to one nick in the rubber.

Another common failure has to do with the way that the load pin is constructed. Many of them have a preferential strain direction due to the internal configuration of the individual strain gauge circuits. The preferential direction is usually indicated by an arrow etched on one end of the pin. If the pin is mounted in such a way that the strain is applied in a different direction than indicated, the printed circuit foil inside is either ripped off its mounting, or simply tears and breaks continuity. Either way, you end up with a non-functioning load pin.

Load cells are also susceptible to lightning damage. They are mounted to the mast and rigging, which are almost always made of a conductive material. This forms a large antenna on the boat, which can lead to damage to the load pin through induced current by a nearby “splash” strike – the lightning doesn’t even have to directly hit the boat! It’s usually pretty obvious when this happens, as there are a few other things on the boat that stop working as well. However, I have been on at least one boat that had a load pin fail due to lightning, but left everything else intact.

There are also the typical bug-a-boos of electronics: weak connections, bad power, et cetera. If the amplifier isn’t powered, or if wires aren’t connected, chances are that the load pin won’t work.

A load pin can be connected to the Ockam system through a variety of interfaces: the older 066, the newer 067, or the T2 (among others). Read the individual manual sections in these links for some
troubleshooting information specific to the interface; some of these
troubleshooting items are repeated below.

If a load pin is connected to an Ockam system, and it doesn’t seem to work, here are a few things to try:

  • Check that the interface is sending data on the expected tag. It is possible to change the data tag on many Ockam load cell interfaces. If the display is set to look at the wrong data tag, then you will not see data. Also check that the data tag is not used by another interface on the system (for instance, a position interface).
  • If connected to a T2, check that switch S3 is set to accept load cell data.
  • On the T2 and e-Series (square box) interfaces, check that the data input light is blinking as though data is being periodically transmitted. If not, the signal cable or power cable for the amplifier box may be loose or disconnected.
  • Check that the amplifier box is configured correctly. The Diverse Yacht Services load pin amplifier (the yachting world’s de facto standard for many years) has two internal rotary switches that need to be configured correctly. Refer to the documentation included with the amplifier for the correct settings.
  • Check that the load pin is zeroed. With no tension on the backstay, the forestay load pin should read right around zero. If not, try re-zeroing the amplifier.
  • Check the resistance on the unloaded pin. There are five wires from the load pin. One wire is connected to the shield and can be disregarded. The resistance between any two of the remaining wires should be 350 Ohms. The resistance should be the same for all possible pair combinations. If it is significantly different than 350 Ohms, then the pin is damaged and must be sent in for repair.

Beyond this, it can get complicated. Don’t hesitate to contact us if you have a load pin problem on your boat that you can’t resolve.

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