If you can see the bubble in the compass, this is an indication that a leak has developed and a repair is required. Rather than repairing the compass, I decided to replace it with a KVH 1000 digital Fluxgate compass.
One reason I went with a digital compass is the KVH 1000 model has a NMEA-0183 output, which can be connected to the chartplotter.
A Fluxgate compass is typically more accurate than the GPS in terms of bearing. I am sure those of you having a GPS receiver know that GPS cannot tell what direction you are headed until you actually begin to move. This is one example of the error within the GPS. While GPS provides a very accurate position, it typically does not provide a very accurate bearing. KVH claims 0.5 degree accuracy, while the typical GPS receiver is in the order of 5 degrees of accuracy. RayMarine even recommends the use of a Fast Heading sensor with their chartplotters to improve MARPA and radar overlay functions.
Fortunately, the new compass fit perfectly over the hole left by removing the old one. The only modification I had to do is to drill new mounting holes as the hole pattern was different. I could simply connect the power leads and the compass would begin to function (after a short compensation sequence).
However, its my intent to connect the compass to the chartplotter, so there are a few extra steps that need to be taken.
Since I already have a multiplexer setup, I can connect the compass to an unused port. However, the Brookhouse AIS-C multiplexer has a special input ports reserved for a Fluxgate compass, and for the best performance, one of these ports should be used.
As I stated, the compass outputs positioning data via NMEA-0183. However, this is almost too much of a good thing. For some incredible reason, the positioning data is output 10 times per second. This can overwhelm a NMEA-0183 network, especially at 4800 baud, especially when you consider NMEA recommends no more than a one signal-per-second transmission rate. When connected to the correct port and properly configured, the Brookhouse multiplexer employs a technique called "Data Pacing". In data pacing, the data stream is interrupted once per second, or once per two seconds, depending on which port is used. In most situations, losing compass data for one second at a time will not appreciably effect the accuracy, and it will go a long way in preventing overload of the NMEA port.
Should the built-in Data Pacing algorithms not be sufficient, Brookhouse can program a custom script for you. Other popular schemes are to filter out 8 out of every 10 packets. However, some applications, such as MARPA work better with high data streams, so check the performance of these funcions after performing Data Pacing.
Our navigation project has now grown to the configuration shown below.
KVH Digital Fluxgate Compass Technology
Brookhouse AIS-C Installation